How to Add an Air Duct to a Room

By |Last updated on |Ductwork|119 Comments

This tutorial explains how to add a room air duct for heating and cooling by installing a flexible air duct and vent for better air flow.

Room Heating and Cooling Problem

My daughter’s bedroom was too warm in the summer and too cold in the winter because it had just a single air duct from the central heating and air system. By comparison, my son’s bedroom is about the same square footage and has two supply air ducts that keeps it comfortable year round.

I solved the problem by installing flexible duct branch line and second air vent in my daughter’s bedroom:

HVAC Flexible Air Duct Branch Line

HVAC Flexible Air Duct Branch Line

Air Handler and Duct Work Problems

A large number of residential Heating Ventilation and Air Conditioning (HVAC) systems are poorly designed and/or installed. The problems I’ve found in my home are:

  • Blown fiberglass insulation (the white fluffy stuff that looks like cotton) was piled up near the scuttle hole entrance in the attic over my daughter’s bedroom and the far side of the attic had little or no insulation. Proper insulation is essential for climate control.
  • Dirty AC evaporator coils. Dirty air handler evaporator coils will block the air flow, decrease system efficiency and run up your electric bill.
  • Dirty or clogged air filter. Change the air filter regularly because a dirty filter will block the air flow.
  • Pinched, loose or disconnected flexible air ducts have a huge impact and waste energy.

If you have any of the above problems with your central air system, try fixing those problems first.

Residential Ductwork Projects

I’ve made extensive improvements to my home’s ductwork – adding new air vents and branch lines, replacing flex duct with sheet metal and installing return air pathways:

  1. How to Add an Air Duct to a Room (you are here).
    Ductwork materials and cut an opening in the drywall ceiling.
  2. How to Add an Air Duct to a Room – Part 2
    Install the vent boot and start collar.
  3. How to Add a Room Air Duct with Speedi-Boot
    Easy to install, self-sealing vent boot.
  4. How to Add a Room Air Duct with Speedi-Boot – Part 2
    Plan the air duct branch line and saw an opening in the drywall ceiling.
  5. How to Add a Room Air Duct with Speedi-Boot – Part 3
    Mount the Speedi-Boot and connect the new air duct.
  6. How to Install an Air Duct in a Suspended Drywall Ceiling
    Speedi-Boot installation in a finished basement ceiling.
  7. How to Install an Air Duct in a Suspended Drywall Ceiling – Part 2
    Connect the flexible duct branch line.
  8. How to Install Round Sheet Metal Duct
    Replace central AC flexible trunk ducts with round sheet metal duct for better efficiency.
  9. How to Install Round Sheet Metal Duct – Part 2
    Plenum start collar and adjustable elbow.
  10. How to Install Round Sheet Metal Duct – Part 3
    Metal support straps and snap-lock duct pipe.
  11. How to Install Round Sheet Metal Duct – Part 4
    Adjustable elbows and rigid duct board distribution box connection.
  12. How to Cut and Assemble Snap Lock Duct Pipe
  13. How to Seal Sheet Metal Duct with Mastic
  14. How to Insulate Round Sheet Metal Air Duct
  15. How to Insulate Round Sheet Metal Air Duct – Part 2
  16. Bryant Evolution AC Condenser and Ductwork Installation
    Replace rigid duct board distribution boxes with round sheet metal duct.
  17. Tamarack Perfect Balance In-Door Return Air Pathway Review
    Easy to install return air grill in a bedroom door.

How to Add an Air Duct to a Room

Speedi-BootA Faster and Better Way to install a new air vent. Please see this newer project for a faster and better way to install a room air vent using Speedi-Boot.

The remainder of this article explains how to install an air vent using a conventional vent boot.

Flexible Air Duct Branch Line Installation Materials

Flexible Air Duct Branch Line Installation Materials

The materials for the six inch diameter branch line cost about $60 at Home Depot:

  1. Starting Collar – 6 inch diameter
  2. Insulated Flexible Duct – 6 inch diameter, 25 feet length
  3. Register Box (a.k.a. “vent boot”) – 6 in by 10 in
  4. Register Grille – 6 in by 10 in
  5. Metal Foil Tape – one roll
  6. Nylon Zip Ties – 36 inch

Air Duct Installation Tools

The tools needed for this project are:

  • Handsaw
  • Carpenter’s square
  • Tape Measure
  • Step Ladder
  • Cordless Drill/Driver
  • 3 inch Wood Screws
  • 1/2 inch self-tapping sheet metal screws
  • Flashlight
  • Cordless Fluorescent Light (very helpful)
  • Rotozip
  • Sabre Saw (or Jigsaw)
  • 15 amp Heavy Extension Cord
  • Hammer
  • Small Finishing Nail – 4d (penny) size
  • Pencil

Duct Work Installation Guide and Tips

The Duct Work chapter from Heating and Cooling Essentials by Jerry Killinger is an excellent introduction to duct work operation, materials and installation.

How to Install a Ceiling Air Vent

The first task is to locate the new vent boot on the ceiling. I measured the existing ceiling register and plotted the new register in an identical position on the right side of the room. A carpenters square is used to draw a perfect 6″ by 10″ outline on the ceiling where the new hole will be cut for the air vent.

I relied on the regular spacing of the ceiling joists such that the register box would be in the gap between the ceiling joists. Before cutting the drywall, I drove a finishing nail into the ceiling to verify there were no joists, making a series of small holes as shown below. If I made a mistake in the position of the register box, it’s easy to patch the small hole made by a finishing nail. A stud finder could be used.

Install an Air Vent: Ceiling Register Outline 6" x 10"

Install an Air Vent: Ceiling Register Outline 6″ x 10″

A Rotozip spiral saw is used to cut the drywall for the ceiling register. Tip: Have a helper hold a vacuum cleaner hose up to the saw to catch most of the dust.

Install an Air Vent: Drywall Ceiling Cutout

Install an Air Vent: Drywall Ceiling Cutout

I left the drywall panel in place to prevent the attic insulation from falling into the room. I’ll remove the panel after the insulation is cleared away in the attic.

Ceiling Drywall Cutout for the New Air Vent

Ceiling Drywall Cutout for the New Air Vent

Now it’s time to go into the attic.

Attic Access for Ductwork

Be extremely careful while working in the attic. Step only on the joists because the drywall under the insulation will not support your body weight – you can fall through the ceiling – resulting in injury or at least a messy drywall repair.

The attic gets very hot in the summer and you can quickly over heat. It’s best to work very early in the morning before it gets hot.

I needed to go into the attic to install the register box and flexible air duct. But I found there was a problem – I couldn’t get to the attic above the bedroom because the roof deck over the main house didn’t leave enough room to squeeze my 215 lb frame through the scuttle hole. Crawling wasn’t an option due to the risk of falling through the ceiling.

Flexible Duct to Attic above the Bedroom

Flexible Duct to Attic above the Bedroom

To solve the problem, I cut the roof deck with a handsaw and sabre saw as shown in the photo below. This allowed me to crouch and slide through with ample handholds while walking on the ceiling joists.

Roof Deck Scuttle Hole Cutout

Roof Deck Scuttle Hole Cutout

View of the attic scuttle hole looking back to the main house:

Attic Scuttle Hole

Attic Scuttle Hole

Here’s the insulation problem I found – mounds of it were on the left, but little or no insulation was over the rear areas of the attic above the bedroom. I redistributed the insulation evenly over the attic.

Blown-in Insulation Mounded in the Attic

Blown-in Insulation Mounded in the Attic

This project is continued in How to Add an Air Duct to a Room – Part 2.

Take care,

Bob Jackson

Copyright © 2018   Reproduction strictly prohibited.


  1. Lucy July 12, 2009 at 11:35 pm - Reply

    Can this be done with a first story room? Our Master bedroom is big and only has two air vents on one end and it stays hot during the summer and cold in winter since the air doesn’t circulate. Would it be possible to do this? can we run the air from the other two ducts?

    Thank you,


  2. Bob Jackson July 13, 2009 at 7:06 am - Reply

    I need to know more about your house and the arrangement of the central air system to say with certainty.

    Are the A/C registers on the floor or ceiling? If you have a basement and the ducts are routed under the floor, you should be able to run a new branch line and install a register. You’ll have to cut a hole in the floor of course for the register.

    Lucy wrote:
    > can we run the air from the other two ducts?
    I don’t think you’d be happy splitting an existing branch line as it won’t increase the overall air volume significantly. A new branch line from the main trunk will increase the volume of air and make a real difference in comfort.

  3. David July 16, 2009 at 10:45 am - Reply

    Do I need to check my AC unit to determine if it can handle another air duct???

  4. Bob Jackson July 16, 2009 at 11:37 am - Reply

    Central A/C units are sized for the *square footage* of the house and adding a new duct shouldn’t be a problem. The important thing is to run the new branch extension from the main trunk line as I’ve shown here.

    Take a look at the GAF Installation Tips at this link (also referenced in the main article above):

  5. Keyosha July 20, 2009 at 12:35 pm - Reply

    I bought a home where the previous owners put an addition over the garage which is now a large extra bedroom in the house. They had a wall unit heater installed which is extremely ancient, and running it increased our electricity bill by alot. There are no existing vents going to this room, but rather than replacing the wall unit, I would like to install air duct vents into this room, is that possible?

  6. Bob Jackson July 20, 2009 at 5:01 pm - Reply

    You should be able to work something out. It’s difficult to give specific advice without knowing the details of the house construction. Do you have an attic over this new addition? Some pictures of your A/C system ductwork and attic areas would help – post them on

  7. josh hart July 22, 2009 at 3:14 pm - Reply

    I have a room similar to yours in the l wing of my house in Florida, this room seems to be much warmer (5-10 degrees) especially if there is no fans running. I have been told that a return air vent would work better than adding a register. I have also heard that reducing the size of the duct running to that room with help as it is a thicker diameter than other bedrooms in the house. Supposedly the smaller size keeps the air flow higher and thus cooler the room better. What do you suppose is the best solution to getting this room as cool as the rest of the house?

  8. Bob Jackson July 22, 2009 at 6:06 pm - Reply

    To determine if the room needs a new return vent, open the door about 1/2 inch. Is the room still warm after 30 minutes? If it’s warm, you don’t need an additional return vent (assuming you already have at least one), but a new cold air supply vent.

    The effect of opening the door about 1/2 inch is to mimic the effect of a new return vent by giving the air an escape route; at the same time the door is *almost* closed such that cold air from the main house doesn’t spill into the room.

    > I have also heard that reducing the size of the duct
    > running to that room with help as it is a thicker
    > diameter than other bedrooms in the house.
    Sounds like the installer knew the room needed a larger air supply and therefore used a larger diameter (“thicker”) duct.

    > Supposedly the smaller size keeps the air flow higher and
    > thus cooler the room better.
    You don’t want to do this – a smaller duct means lower air volume. The air velocity may be a bit faster through a small duct pipe, but friction losses in a small duct will defeat your goal. Bernoulli’s Principle explains the physics of air flow. See the pipe animation at

    To get more air use the same or larger size flexible duct pipe and be sure to tap off the main trunk line. It wouldn’t hurt to consult with a HVAC technician.

    Living in Florida means hot summers and lots of solar heating of the roof over this room. Have you checked the attic insulation? Adding the R-30 fiberglass insulation as I’ve shown here really improved the thermal stability of the room. Less heat coming in means less A/C is needed to keep it comfortable.

  9. Dale Wilbanks December 5, 2009 at 8:36 am - Reply

    Thanks Bob, just what I needed to get my daughter’s new bedroom done this morning!

    • Bob Jackson December 5, 2009 at 10:08 am - Reply

      Wonderful! Thanks for letting me know. – Bob Jackson

  10. bill February 28, 2010 at 8:27 pm - Reply

    If installing a flexible duct, can I squeeze it to get it through a tight space and still get adequate airflow? I need to get a 6″ diameter through at 4″ opening and an 8 inch diameter through a 5 1/2″ opening?

    • Bob Jackson February 28, 2010 at 9:26 pm - Reply

      That’s not a good idea because a 6 inch duct has 28.27 square inches of capacity versus a 12.5 square inches for a 4 inch duct – that’s more than a 50% reduction in duct capacity. Same issue for pinching an 8″ duct through a 5-1/2″ opening. The area of a circle (the duct capacity) varies with the square of the radius by the formula: Area = 3.14 * Radius^2.

      What is the obstacle that requires the smaller opening? Is there no way to route around the obstacle or modify the opening to accommodate the larger duct?

  11. Nick April 20, 2010 at 6:09 pm - Reply

    I am buying a home with central air and a heat pump. it is three bedrooms and i am converting a family room into the fourth bedroom we need on the first floor. The issue is when I put up the wall I will be closing off the heating and air source. Can I cut into the duct in the roof/floor between the first and second floor? If so can you show me a blow by blow demo on that?

    • Bob Jackson April 20, 2010 at 7:15 pm - Reply

      Hi Nick,
      Without know more details about your ductwork, vents and floorplan, it’s difficult to give a definitive answer. The surest solution is to tap into the main trunk and run a new line as I’ve shown in the article.

      > Can I cut into the duct in the roof/floor between the first and second floor?
      > If so can you show me a blow by blow demo on that?
      If I understand your situation, the ductwork for the family room is between the ceiling/floor of the 1st and 2nd levels of the home. This would make it expensive and messy to tear out the ceiling drywall to install new ductwork; I don’t have a demo for this.

      You might consider a different approach – running new a supply duct down from the attic, hopefully through a closet on the 2nd floor, to the 1st floor family room.

      I think your situation is challenging enough to seek on an on-site assessment/estimate by a professional HVAC technician. Check for professionals in your area.

      Thanks for reading,

  12. Peter January 4, 2011 at 5:43 pm - Reply

    I wonder if just redistributing the insulation might have gone a long way to fixing this problem, without adding the second duct. I think I would have given that a try first.

    Also, where you advise having someone hold a vacuum cleaner to catch the falling dust as you cut the hole in the drywall, I’ve found that fine dust quickly destroys vacuum cleaner motor bearings. They still work, but become unbearably noisy. I use an old one for jobs like that.

    • Bob Jackson January 4, 2011 at 6:08 pm - Reply

      Improper insulation was a definitely contributing factor, however when I compared the air flow to my daughter’s room at the end of that long run of duct to the other bedrooms by holding my hand next to the ceiling vent, the air flow was significantly less strong. That bedroom above the also garage receives a lot of sun loading from the east and south roof faces. In the summer, the garage gets very hot from outdoor temperatures in the 90’s F to low 100’s F plus the heat from the car engines which doesn’t help.

      I grabbed the house vacuum cleaner for small jobs like this. I use the yellow shop-vac for real messes and big jobs. Thanks for the tip!

  13. Glenn June 15, 2011 at 4:08 pm - Reply

    I’ve got a persisting problem with my computer room upstairs, esp. being too hot in summer. The attic AC/HT unit sits between the upstairs bedrooms (3). Theo BR on my side of house is the coldest room in house and the BR next to it is functioning very well. I’ve partically closed the other 2 BR vents and opened my BR (home office) all the way and it makes no difference. They’re still cold and I’m still hot.
    The AC service we have says the connection of the main return does not allow room to replace with a connector that allows 2 returns. My thought is to place a small return, the size of AC vents and ducts, right above the computers in the ceiling and run that return duct into the duct work of the main return just a couple of feet before it enters the unit.
    Does this sound reasonable solution? If so, can I just cut into the main duct and tape and AC putty the smaller duct into it – making sure it seals good? Or is there a “T” connection to allow the main large return to flow unimpeded and a entry for smaller duct to fit into the side?
    What may help to know is that the AC/H unit has no room to move or add any larger attachments due to how the attic is built, BUT there is room to install a small return vent and its duct work to run into the main return.

    Advice is greatly appreciated!

    • Bob Jackson June 15, 2011 at 5:55 pm - Reply

      Instead an AC return duct, have you thought about installing a new fresh air supply duct and vent? This will be much more effective compared to an air return duct. How is the office bedroom temperature when you leave the door open? Leaving the door open is about the same as installing a new return duct.

      You’ve already tried to balance the AC by closing vents/registers in the other bedrooms. What’s needed is more cold supply air to make the office bedroom comfortable. Is there something in the system layout that prevents running a new fresh air duct to the ceiling? A couple of photos of the attic air handler and ductwork would help. You can send pics to me at this address: bob (at sign)

  14. Becky June 21, 2011 at 2:14 pm - Reply

    Hi, I have a similar project we are working on: We have an upstairs that does not have any ventilation what so ever. Plenty of windows, but no venting to push heat or cool air from the central unit (in basement) into the upstairs. Need to know how to go about getting this done to make this into the kids’ room. We do have three openings that lead into the attic, looks like the same as your pics posted, with the joists and insulation all over the place (although ours looks like old newspapers, should we replace?). We are unsure how to get started with this: where to put the vents, how many vents…

    • Bob Jackson June 21, 2011 at 9:31 pm - Reply

      The first thing to do is verify your AC unit can handle the extra cooling load of the 2nd floor. See this central AC sizing chart for guidance. If your AC unit is too small to handle the 2nd floor living space, it’ll run all the time and you won’t be comfortable. My AC unit was properly sized to handle the total living area, so my problem was limited to improving the air supply.

      The other question is ductwork sizing, which is not a simple question that I can answer here because it requires specialized knowledge, room measurements, design and calculations. You might try this online Ductwork Design Service or better, hire a local HVAC company for an onsite assessment.

  15. daniel June 22, 2011 at 2:14 pm - Reply


    I am finishing my basement and the contractor added a piece of wood to seal the space between the metal box (the vent) and the drywall.

    Should i be worried that the wood will get moldy due to the AC?

    thank you!

    • Bob Jackson June 23, 2011 at 1:25 pm - Reply

      It’s normal to build a wood frame behind the drywall to mount the vent boot. The wood shouldn’t get moldy because as the air conditioner cools the air, the excess water vapor condenses on the evaporator coils inside the air handler and drains away… so you’ve got cool dry air blowing out the vent. This is a key purpose of air conditioning, to both cool and dry the air to lower the relative humidity.

  16. Blake August 1, 2011 at 1:56 pm - Reply

    Hi Bob,

    I have a slab garden home with a storage room off the back inside the screened in patio. The storage room is insulated therefore I put a dog door in the door so my dog can go in and out to get out of the heat. All duct work is in the attic with vents in the ceiling. I would like to run a vent into that room to keep him cool during the hot summers. The master bathroom shares the wall with the storage room. I have two options. Tap into the vent that feeds into the masterbath and run at best five more feet to the storage room. OR Come off the main trunk which is at the front of the house in the attic and make a long run across the entire attic. The storage room is tiny and wont take much to cool but I am extremely concerned with messing up the balance of the rest of the house. What would your advice be?


    • Bob Jackson August 1, 2011 at 5:22 pm - Reply

      I normally don’t recommend splitting a branch line, but since it’s a “tiny” storage room go ahead and do it. Take care to install a ceiling register with adjustable louvers that will fully close. Open the louvers just enough to make the storage room comfortable and limit the impact on the master bathroom. During the winter, close the vent louvers if you don’t want to heat the storage room.

      Have you thought about a return air duct/vent? Otherwise the air will be forced outdoors around the dog door. If there’s an interior door to the storage room without a bottom weatherstrip or sweep, that could be your air return. (If it has a bottom weatherstrip/sweep, leave it alone.) Might need to shorten the door so there’s a 5/8″ clearance above the floor for the return air. Or you could install a pass-through vent in the common wall shared by the master bedroom and storage room; this might not be desirable though because the dog will hear you and bark and whine.

      Should splitting the branch line prove unsatisfactory, you can always run a 3″ duct line from the main trunk at the front of the house.

  17. scott November 9, 2011 at 2:41 pm - Reply

    Bob, It appears that you used R6 flex. I am getting ready to replace all of my attic flex and was told that R8 had to be used in attics (any area without climate control) due to a code change. I was told that I would have to wrap or blow insulation over all R6. Please help.

  18. paul November 16, 2011 at 11:18 pm - Reply

    i added an extra duct but the air flow has stop where i tapped into and both have stop working any suggestions?

    • Bob Jackson November 17, 2011 at 5:30 am - Reply

      I need the particulars of how to you installed the branch line. Did you tap into the rigid trunk ductwork or wyed into a flexible line? Are you sure you tapped into an air supply line and not an air return line? Pictures will help a great deal – send them to bob (at)

  19. dan March 28, 2012 at 2:35 pm - Reply

    Thanks – very nicely done.

    GAF web link has changed to:


  20. bill clagget November 24, 2012 at 12:16 pm - Reply

    is a 12 inch duct too big for a standard room

    • Bob Jackson November 25, 2012 at 1:00 pm - Reply

      Hi Bill,
      The short answer is “Yes – a 12 inch duct is probably too large for a standard size room.’, but determining the correct duct size requires some thought and engineering as discussed in my reply dated June 21, 2011 at 9:31 pm in the comments section of this project. Also see my discussion of AC Sizing in the comment dated December 14, 2011 at 11:00 am in this related project.

      Let’s assume a ‘standard size room’ is 150 square feet. According to “How Contractors Really Size Air Conditioning Systems” the typical cubic feet per minute (CFM) of airflow required is 1.04 CFM/square foot. You will therefore need at least 156 CFM of airflow and possibly more (1.5 CFM/square foot = 225 CFM) if the room is not well insulated or you live in a very hot or cold climate.

      Let’s assume 1.04 CFM will be sufficient air flow. Plugging the following values into the ResDuct™ – Residential Air Duct Calculator:
      * Duct Type for Supply & Return: Flexible
      * Airflow CFM for Supply & Return: 156

      Results in a 9 inch diameter flexible duct.

      Re-running the Air Duct Calculator with 1.5 CFM/square foot for 225 CFM total airflow estimates the flexible duct size at 10 inches in diameter.

      Please run the duct size calculator with actual square footage and duct type (sheet metal, fiber board, flexible) for your room to get a more accurate estimate.

      If in doubt, you could install a 10 inch diameter duct with a louvered vent to regulate and better balance the airflow.

  21. Tony N January 23, 2013 at 9:44 pm - Reply

    am i able to cut a hole in my air duct in my basement to heat and keep a section of my water pipes warm when the temp drops well below freezing

    • BobJackson January 24, 2013 at 7:43 pm - Reply

      You want to install a new air vent in the main trunk line to keep the basement water pipes from freezing? If you have a rigid board or sheet metal trunk line, a Speedi-Collar with built-in damper to regular the air flow should do well. It’s fast and easy to install. The damper has a rachet system to lock it in place and closes air tight when you don’t need it in warmer weather.

      If the exterior basement walls aren’t insulated, then insulating the walls will minimize heat loss and cold intrusion.

      I also recommend wrapping the water pipes with foam pipe insulation if you haven’t done that already. Foam pipe insulation is available at home improvement stores.

  22. John Keenan February 27, 2013 at 6:56 pm - Reply

    I want to remove a duct from the main supply trunk and move it back, but there is another branch in the bay I want to use. Can I run the two vents out of the same tap on the supply trunk, using a wye branch?

    • BobJackson February 27, 2013 at 8:21 pm - Reply

      The best way would be disconnect the old branch line and cap off the trunk tap, then install a new starting collar at the bay trunk line for the new branch line.

      I’m guessing there’s no room to install a starting collar on the bay trunk line? What size is the branch line that you want to relocate? In the follow example, less assume it’s a 6 inch branch line.

      1. Cap off the old 6 inch branch duct at the main supply trunk starting collar. Remove the old starting collar and install a cap plate.
      2. Remove the 6 inch starting collar at bay trunk line that you want to wye.
      3. Install a new starting collar that is 2 inches larger in diameter on the bay trunk line. In this example, the new starting collar is 8 inches in diameter.
      4. Install an 8 inch inlet to 6 inch outlet wye at the bay trunk line. You may need to run a short section of 8 inch duct to connect the 8 inch starting collar to the 8 inch wye inlet.
      5. Connect the old and new 6 inch bay branch lines to the wye.
      6. Install louvered air registers in the rooms to balance the air flow.

      The reason for an 8 inch inlet on the reducing wye with two 6 inch outlets is to provide sufficient air volume for the two branch lines. An 8 inch duct has 201 square inches of area. A 6 inch duct has 113 sq. in. of area. 201/113*100% = 178%, meaning the 8 inch wye inlet has 78% greater capacity than the 6 inch wye outlets. This is should be enough to make the rooms comfortable assuming no special heating or cooling needs, for example, a room with lot’s of windows and sun exposure. In my home, most of the louvered vent registers are about 2/3rds open, therefore I have more than sufficient HVAC capacity.

      If you believe an 8 inch to 6 inch wye won’t be large enough, use a 10 inch inlet wye with 6 inch outlets. A 10 inch duct has 277% greater capacity (314 sq. in.) compared to a 6 inch duct.

      * Best method is to install a new starting collar in the trunk duct for the relocated bay branch line.
      * If that’s not possible, start with an 8 in to 6 in reducing wye.
      * Should the 8 in wye not be sufficient, you can always cut a larger hole in the trunk line for a 10 in wye.

      The risk of installing too large a wye is your vent registers could get noisy from too much air flow whistling past the louvers.

  23. Daniel March 2, 2013 at 11:13 pm - Reply

    I am finishing/soundproofing a studio/music room over garage and it will be sealed as airtight as humanly possible. I thought I should install a return vent so we don’t choke on our own CO2. I would install an a/c vent as well but don’t think the system can handle it. Will this help?

    Also, and I can’t seem to find this out anywhere, how do you cut a start collar hole in flexible duct? The room is unfinished and the large return air trunk is right there. I would like to tap into it. (Much easier than trying to thread a new duct back to the main unit). Are there special take-off collars for flex? How do I cut a neat hole with that wire running through it?


    • BobJackson March 3, 2013 at 11:13 am - Reply

      > I would install an a/c vent as well but don’t think the system can handle it. Will this help?
      A return air vent without a fresh air supply vent will not have much, if any, affect on the rate of air exchange in the room over the garage.

      > I can’t seem to find this out anywhere, how do you cut a start collar hole in flexible duct?
      > Are there special take-off collars for flex? How do I cut a neat hole with that wire running through it?
      Starting collars are for rigid duct board and sheet metal ducts. An air duct wye connector is used to tap into flexible duct. The wye is installed by cutting flex duct in two, slide the two ends on the wye and seal with HVAC tape and a zip tie. Connect the new branch duct to the 3rd leg of the wye the same way.

      Take care to correctly orient the wye. For a fresh air supply line, the air flows in the bottom and out the top of the Y in a diverging path. With a return air duct, the air enters the top of the Y and exits the bottom in a converging path.

  24. John Keenan March 24, 2013 at 11:24 am - Reply

    I should have gotten back to you sooner. I closed off the one branch. I put in an 8″ whye on the top of the main trunk where I wanted to relocate the branch. That location had a branch line already. Then I fed off in two directions. I reduced the branches to 6″. Damper in each(wasn’t a need for them, but they’re there). Worked out great. Thank you. Due to previous renovations I assume, someone found it easier to run flex tube across bays in basement when changing heat register locations, than to change location on the main trunk allowing to run in the bay. I hated having flex tube hanging all over, not to mention that I eliminated over 10 ft of supply. I was just concerned that running two branches off one location on the main trunk would be a problem. It wasn’t, as far as I can tell.

  25. Anita May 31, 2013 at 2:59 am - Reply

    My heating and cooling runs straight through my basement ceiling but there is no air flow or vent in my basement to keep from moisture building up. So I want to add a vent along it. I also have my Husband helping, what should we do?

    • BobJackson May 31, 2013 at 9:17 am - Reply

      My finished basement shares the same trunk ductwork with the 1st floor of my home. This project shows how to install a new air vent in the basement suspended drywall ceiling.

      Let me know if you have more questions.

  26. Joe Schaerer July 3, 2013 at 1:55 pm - Reply

    Will this Speedi-Boot from Home Depot work just as well installing a new register in the floor. All my bedrooms have two register except the one in the front of the house which seems to get very warm in the summer. The vents are all in a line so finding the main ducting shouldn’t be a problem. I live in a re-manufactured home and everything is accessible under the unit.

    • BobJackson July 3, 2013 at 6:56 pm - Reply

      Speedi-Boot is great for floor installations. Consider using a 90-degree register vent model which is often needed to avoid a sharp bend in the flexible duct connection.

      Floor Vent Installation Methods YouTube video

      Speedi-Boot Installation Instructions on page 8:
      Floor Installations
      There are four pre-piloted holes in the four corners of the Speedi-Boot to fasten to the bottom of the floor decking with sheet metal or wood screws. You may also use the trim register pilot holes and the hanger arms to secure to the floor decking. This creates a tight fit and compresses the rubber gasket to the floor deck eliminating air leakage. The hole in the floor decking must be cut 3/8″ larger than the outer mud-ring dimension.

  27. Rick Hester July 20, 2013 at 6:11 am - Reply

    I have a one story home in NC with HVAC ductwork running in the crawl space. We have recently converted a screen porch to a sun room and want to install vents in the sunroom to provide HVAC. However, the floor is poured concrete covered with tile. I asked our HVAC technician about the capacity of our unit and says that it is large enough to handle the extra square footage. When I asked if he could run a vent through the ceiling, he stated that air doesn’t like to “flow up hill.” He had no suggestions for a solution.

    Do you have any suggestions on getting cool/warm air to flow from the main trunk under the house, up over an eight foot ceiling, and out into our sunroom? The total length of the duct from main trunk to outlet would be about 30-35 feet.

    The sunroom is only 12 X 15, so would I need two ceiling ducts?

    Thanks for you time.

    • BobJackson July 22, 2013 at 12:49 pm - Reply

      The 8 feet ceiling height isn’t an issue, the 30 to 35 feet of total duct length and guessing two 90 degree bends will have significant friction losses. 6 inch duct would normally be sufficient for a 12 x 15 room, but due to the long duct length install a duct booster fan or use 8 inch duct without a booster fan.

      Two ceiling ducts will be needed (supply & return ducts), especially if you plan to close the door between the sun room and house which would block the door air return. Use louvered vent registers to balance the air flow and shutoff the air when you’re not using the room to save on heating/cooling costs.

  28. Ashley October 24, 2013 at 8:35 am - Reply

    We bought a home and there is a vent in the master bedroom but its not connected to any duct work (there is no heat or air). Everything is ran under our house though. Would it be a big deal to tap into the main line and go ahead and run it? Is it a difficult task?

    • BobJackson October 24, 2013 at 5:53 pm - Reply

      > vent in the master bedroom but its not connected to any duct work
      Very odd! Is the attic above the master bedroom? Is there any ductwork in the attic? I’d be concerned there’s a disconnected duct somewhere in the attic.

      Is the master bedroom on the first or second floor? Where is the air handler located?

      Installing ductwork is mainly an issue of figuring out the best path from the trunk duct to the new vent given the layout of your home. For example, if you have a single story home with ductwork in the crawl space it would be easiest to install a branch duct to a vent in the bedroom floor.

  29. Ashley October 29, 2013 at 8:57 am - Reply

    Ok, I’m sorry. I should have given you a bit more info. The house had been foreclosed on when we bought it and it was very apparent that the first owner was in the middle of reconstructing it. There were a lot of “loose ends”, unfinished rooms and such. There’s no “attic” as the roof on the second floor are the A ceilings? So the duct work starts right under the master bedroom (which is on the first floor) and runs throughout the house from there. So I was thinking just tapping into the mainline right under our bedroom. I mean it’s obvious he just didn’t finish it because all the pipe that’s needed is already under the house. :) thanks.

    • BobJackson October 29, 2013 at 6:50 pm - Reply

      The home was being remodeled by the prior owner before foreclosure interrupted the job, leaving you with significant unfinished work. There’s no attic because the home has cathedral ceilings. The trunk duct runs below the main floor.

      A new branch duct can be installed from the trunk to the new vent in the bedroom. Are there existing supply and return vents in the bedroom? What are the dimensions of the vents and ducts? How large is the master bedroom – width and length? What is the distance from the trunk duct to new bedroom vent? Do you have a ceiling fan? Is there a crawlspace or basement? Is the trunk duct made of sheet metal, rigid ductboard or flexible duct?

      > it’s obvious he just didn’t finish it because all the pipe that’s needed is already under the house.
      Given the incomplete ductwork, I recommend inspecting all the ductwork for:
      * Open ductwork – a huge energy waste
      * Separated ductwork, broken seams, air leaks

      This Ductwork Guide will be helpful to understand your HVAC system and proper ductwork installation practices.

  30. bob November 1, 2013 at 11:21 am - Reply

    Hey Bo..I also have a basement. They installed a new Trane system a few years back. They gave me one duct off the main trunk in what was then the finished side. They said they’d come back and add a few more ducts and a return when I completed finishing the basement.

    We just about have the remaining 12 x 18 section finished. Had the guy back out. He said he’d add as many ducts as I wanted but it likely would not warm the basement….same old story, stat upstairs etc etc. He suggested I could add a single duct in the 12 x 18 area and a return…all for some air flow but I should also add some electric radiators.

    After reading this thread I think I could handle the job myself. I am a bit concerned about the house needing to be rebalanced though. The tech did not seem to be. Thoughts???

    Anyway…the main duct and a return runs against the long (back) side of the room. Should I just pop a vent thru the drywall into the duct for the vent or should I bring the duct line off the trunk to the room center…or does it matter….or maybe adding the vent accomplishes nothing! Thoughts????

    Similarly, what do I do about the return…right off the trunk for the return or run something the center of the room. How much space should there be between the vent and the return? Again it’s a 12 x 18 room and the main duct / main return runn along the long back wall.

    Thanks Bob!

    • BobJackson November 2, 2013 at 12:32 pm - Reply

      > He suggested I could add a single duct in the 12 x 18 area and a return…
      > all for some air flow but I should also add some electric radiators.
      I agree with the HVAC technician’s assessment. Basements tend to be cooler than the main house. Because the thermostat is upstairs, there’s no way to signal for heating without installing a remote temperature sensor in the basement, which would make the main floors too warm. An electric radiator is easiest.

      > After reading this thread I think I could handle the job myself. I am a bit
      > concerned about the house needing to be rebalanced though. The tech did not
      > seem to be.
      I concur with the tech.

      I was talking to my local building inspector recently and he said a building permit with a load calculation (a complex engineering task best left to the pros) is called for when adding new ductwork. I think most people would think that excessive for minor branch line additions but I’m not going to argue with regulations. Do install a louvered supply vent register so you can adjust the room airflow. Return vents must be the fixed permanently open type (non-louvered).

      > the main duct and a return runs against the long (back) side of the room.
      > Should I just pop a vent thru the drywall into the duct for the vent or
      > should I bring the duct line off the trunk to the room center
      The supply and return branch lines must be installed with a start collar, duct, vent boot and register. Ideally, place the supply vent at the far side of the room away from the door and locate the return a couple of feet away from the door. If the basement room has windows, the supply vent should near the windows. If access limits the placement, separate the two vents as much as possible so the air fully mixes in the room.

      Also see my advice dated December 14, 2011 at 11:00 am in the comments section of this post.


  31. bob November 2, 2013 at 2:53 pm - Reply

    Thanks Bob. I don’t see a Dec 2011 post but i will look again. As far as a permit to install a duct….that officially crosses the line into ‘in it for the $$$$’. What safety issue could there be with an unbalanced house? Jeez.

    So in your opinion is it even worth adding a vent and return..or just add the radiators…or both.

    BTW, the vent would be towards the end on the trunk. At the the is already a vent going upstairs. Would adding this vent for basement a feet away from an existing one matter?

    • BobJackson November 2, 2013 at 6:00 pm - Reply

      > I don’t see a Dec 2011 post but i will look again.
      Scroll down about 4/5ths of that web page. Comments are listed in chronological order.

      > So in your opinion is it even worth adding a vent and return…
      > or just add the radiators… or both.
      Definitely add the supply and return ducts for year round humidity and temperature control. The electric radiator is portable can be used as needed, but it won’t do anything for fresh air exchange.

      If you have several people in the finished basement room addition without air vents, you’ll quickly notice it’s getting stuffy (humidity is going up) due to lack of fresh air exchange. I have a similar issue with my basement bedroom… it only has a supply air duct and no air return – an error by previous homeowner’s who hired a contractor to finish the basement without a building permit. Close the door, there’s no air exchange and it gets stuffy. I’ll install either a return duct or jump duct soon.

      > BTW, the vent would be towards the end on the trunk. At the the is already a vent going
      > upstairs. Would adding this vent for basement a feet away from an existing one matter?
      It will matter, the question is “how much”? It depends on the size and length of the trunk duct, new branch line, etc. That’s why a louvered supply vent is recommended to balance the air flow by trial & error until all rooms are comfortable. One or two louver adjustments is usually sufficient.

  32. bob November 4, 2013 at 3:05 pm - Reply

    ok, The return will work well as its right at the opening to this area. The vent will be a about 12 feet away from the return, in the back of the room. My only issue is that I can see how I can craft a collar to go thru the drywall and into the trunk and affix a say 14 x 6 louvered grill to it.

    I can also see atttaching a collar and some insulated duct and attaching that to a speedy vent but…. I have a dropped ceiling and this becomes problematic. Should I attempt to find a way to get the vent in the center of the room, again 12 foot wide room, vent would be 6 feet from wall…or just hang it off the trunk which already petrudes about 2 feet into the room. I hope this makes sense.

    • BobJackson November 4, 2013 at 8:51 pm - Reply

      Is your dropped ceiling drywall or removable acoustic panels? Speedi-Boot is compatible with both. If your suspended basement ceiling is drywall like mine, then physical access and vent placement is limited.

      > or just hang it off the trunk which already petrudes about 2 feet into
      > the room. I hope this makes sense.
      The supply air trunk duct is exposed inside the room? Is it rigid duct board, sheet metal or flexible duct? Pictures would help – send to bob (at)

  33. Bob November 5, 2013 at 9:37 am - Reply

    >Is your dropped ceiling drywall or removable acoustic panels? Speedi-Boot is compatible with both. If your suspended basement ceiling is drywall like mine, then physical access and vent placement is limited.

    Armstrong Ceiling Panels. I do not recall why I had an issue on the other side of the basement. I recall purchasing a speedy boot and returning it for a round 8″ vent with diffuser…it may have something to do with the location or a tight fit.

    > or just hang it off the trunk which already petrudes about 2 feet into
    The supply air trunk duct is exposed inside the room? Is it rigid duct board, sheet metal or flexible duct?

    I could not get a picture that covered the whole area. The basement ‘L’ shaped. Down the stairs is the main area, finished and about 27 x 27…but has a 10 x 12 work room in the back corner. There is on supply line in this area over a sitting area.

    The area in question is being finished..its almost done. It’s a 12 x 18 area that sort of completed the’L’. The air handler sits between these two areas. Ridged 12 x 12 (?) silver covered duct flows from the air handler to this area. It runs about 15 into the 12 x 18 room. Similar sized return duct runs in back of the supply and abuts the back wall. I can not tell what area(s) above feed the return. I was able to see that the supply feeds two vents in my family room at about each end of the room. If its feed something else, I can not tell but it could be; perhaps not. My vent being added to the basement would sit at the far end this supply/return run, a foot from one supply deed the room above and 2 feet from the physical end of the run.

    The supply is positioned for easy access. The duct is faced with a 12 x 12 already painted board but the bottom is still exposed. I can easily fashion a collar thru the board into the ridgid ductboard and affix a 14 x 6 (?) louvered vent. As mentioned, the bottom is still exposed, so I can get in there and in sure a tight seal around my makeshift collar.

    It seems less work to me to install in this fashion and would sit 2 foor from the back wall…but if moving the vent essentially another 4 foot into the room and having it blow down rather than out makes a difference I am up for it I guess. Wheeew! I appreciate your patience and expertise!

    The neighbor question doing the whole thing…he noted the door at the top of the stairs is cut an inch from the floor and acts as a return pulling air from the basement upstairs…not to mention the doggie door that is there as well!

    • BobJackson November 5, 2013 at 7:02 pm - Reply

      > I can easily fashion a collar thru the board into the ridgid ductboard and affix a
      > 14 x 6 (?) louvered vent. As mentioned, the bottom is still exposed, so I can get in there
      > and in sure a tight seal around my makeshift collar.
      I believe that is the best plan.

      > he noted the door at the top of the stairs is cut an inch from the floor and
      > acts as a return pulling air from the basement upstairs
      A 1 inch gap along the door bottom wouldn’t provide area for return air from the 12ft x 18ft finished basement. Most doggie doors are weighted or have a magnet to keep it closed blocking the return air path. A return vent and duct in the basement will pressure balance the room (air flow in = air flow out) and be the most comfortable solution. You might try a transfer grill over the basement stairway door – this works only if there are no closed doors in the finished basement.

  34. bob November 8, 2013 at 10:52 pm - Reply

    Well, I went down to measure before buying any product to install my vent. Good thing. The way the rigid duct hangs from the rafters and after dropping my ceiling will only allow me to get a 4″ high vent thru the side. The good news is they sell a 3.25 x 10 starter collar that would make the job simple but….is that opening adequate?

    The alternative is to go through the top of the rigid duct and use flex duct to bring the sucker to away from the wall dropping down into a ceiling tile. More cost, more work AND I would have to be in the rafter bay right along side and existing supply run…I was planning on staying a few feet away from the existing supply run and coming out the side. I can’t really mover over…there is wiring above the rigid duct in all other bays.

    is a 3X10 opening sufficient? Heck, I am not even sure they sell louvered grills that size….

    • BobJackson November 9, 2013 at 12:11 pm - Reply

      If I follow your description correctly, the square or rectangular air duct is made of duct board and runs between the floor joints. There’s only 4 inches vertical clearance between acoustic tile suspended basement ceiling and the floor joists. The finished basement room is 12 feet x 18 feet.

      A 12ft x 18ft room is 216 square feet. At 1 CFM of air flow per square foot of floor space you’ll need 216 CFM from the vent. Selecting fiber board supply duct and 216 CFM airflow in the ResDuct™ – Residential Air Duct Calculator results in a 9 inch diameter air duct.

      9 inch diameter round duct is not a common size. 8in and 10in diameter duct, start collars and boots are available in a wide range of configurations. Because it’s a basement with minimal exposure to the weather and your ceiling workspace is limited, let’s choose an 8 inch duct diameter.

      A critical constraint is the duct pipe and vent boot must fit the 4 inch vertical clearance between the ceiling tiles and floor joists.

      This configuration may work for you:
      * Speedi-Boot 4in x 12x Wall Stack SBH-4128 WS
      * Speedi-Products 8in Oval to Round Straight Boot SM-OTRS 08
      * 8 inch round duct board start collar

      The SBH-4128 WS is only 3.5 inches high to fit between the ceiling tiles and floor joints.

      Install the start collar in the side of the duct board trunk, attach the oval to round adapter so the offset end runs below the joist, then Speedi-Boot wall stack vent. Fasten the sheet metal duct connections with at least 3 screws, seal the joint with mastic (best) or metal foil HVAC tape and wrap it with duct insulation.

      I couldn’t find the dimensions of the SM-OTRS 08 oval, but you can contact the manufacturer to verify the oval-to-round adapter fits the Speedi-Boot wall stack.

      Let me know if this approach works for you.

  35. bob November 9, 2013 at 5:23 pm - Reply

    No, How did you get those pictures in there? The supply and return trunk / ducts run under the rafters perpendicular to them and against the back wall creating a bulkhead. A wire runs above the trunk through the rafters up until the last two rafter bays below. The end rafter bay contains the duct running upstairs. The next rafter bay in is where I could add another supply through the top but then I’d have two supplys right next to each other.

    I can come out the side anywhere but the trunk only hangs so low and fatoring in the dropped ceiling leaves me with only 3 – 4 of vertical space for a grill….though it can be a wide grille. The room is 12 x 18 and opens the lager space. I wish I could post a picture if told how I will…

    I could get a collar thru the top, flex duct to that and use a speedy duct to my ceiling tile….assuming my grid falls in the right spot when I put it up. But it seems having two supplies side by side is not a good thing.

    • BobJackson November 10, 2013 at 10:17 am - Reply

      > I can come out the side anywhere but the trunk only hangs so low and fatoring in the dropped
      > ceiling leaves me with only 3 – 4 of vertical space for a grill….though it can be a wide grille.
      I’m getting a better idea of the duct and joist layout, but not quite understanding the 3-4 inches of vertical space constraint if the trunk duct hangs below the joists. It seems there should be adequate clearance to run a collar, flex duct and vent boot between the rafter bays?

      > I wish I could post a picture if told how I will…
      You can email photos bob (at), replace the (at) with the @ symbol with no white spaces, or upload photos to a photo sharing website like Flickr and paste the link.

      > I could get a collar thru the top, flex duct to that and use a speedy duct
      > to my ceiling tile… assuming my grid falls in the right spot when I put it up.
      > But it seems having two supplies side by side is not a good thing.
      If you can some out anywhere along the trunk duct then a start collar in the side is better because it avoids an extra 90 degree turn.

      I look forward to the photos.


  36. bob November 11, 2013 at 11:24 am - Reply

    I’ll try to upload tonight. I measured. The trunk is 9″high. It abuts the rafters…or put another way, hangs 9″ below the rafters. My ceiling tiles will come about 4 inches from the rafters. That means I have about 5 inches of trunk left to tap into …from the side……and part of that is thickness of the bottom f the trunk.

  37. bob November 23, 2013 at 2:45 pm - Reply

    I am back…trying to post some pics:
    This is from the rear back corner of the 12 x 18 room..

    basement air ducts

  38. bob November 23, 2013 at 2:46 pm - Reply

    …if that works…I’ll post some more!

    • BobJackson November 23, 2013 at 5:11 pm - Reply

      Yes – I see the photos.

      The single link to the photobucket album will do. Just post the remaining pics to the album and give each one a title like “Basement Ductwork 1”, “Basement Ductwork 2”, etc. to identify it by name in our discussions.


  39. bob November 23, 2013 at 6:14 pm - Reply

    Here is looking towards the 12 x 18 room, you walk past the washer / dryer to get there. The Air handler is in back of the sliding door ..


    The first image showed the boxed in duct work…here is a shot looking up at it.

    If this works Ill describe my issue again un a latter post

  40. bob November 23, 2013 at 8:20 pm - Reply

    Well, I hope the 3 or so pics made it, One shows the main room looking into the 12 x 18 area in question. Another shows the bulk head covering the existing duct. The one photo shows the supply and return alongside each other…the return stops and the supply continues to about 3 feet from the back wall. At the end of this run is a branchoff going up stairs. I want to put a vent for the basement in this area. I could go out the top but I’d have to runs tapping off the supply duct alongside each other. I really just wanted to come out the side with a vent. I can only get something 3 to 4 inches tall through the side though since the supply duct sits up against the rafters and my dropped ceiling has to drop down 4 inches. Will something 4 x 10 or so be ok?

    • BobJackson November 25, 2013 at 4:07 pm - Reply

      Your basement ductwork photos cleared up the situation.

      The supply and return air ducts are made of fiberglass duct board concealed by an interior soffit. Because there’s only ~3 inch of clearance between the soffit fascia board and the duct board it would be difficult and unsightly to cut into the soffit and bring a branch duct out the side of the duct board trunk. A duct board start collar (or takeoff) is too long and will extend past the soffit.

      The supply duct extends past the hallway into the 12ft by 18ft room in the finished basement.

      My recommendations for installing the new branch air ducts are:
      * Install the return air duct in the center of the hallway (blue line in photo at the previous link).
      * Install the supply air branch duct at the 2nd to last floor joist bay (red line in photo).
      * Extend the supply air duct as far as possible into the 12x18ft room to maximize the air circulation. (The length of my red line was limited by the picture.)
      * Both branch air ducts will be run from the top of the trunk duct board via flexible duct to a Speedi-Boot mounted between the floor joists. Not sure if mounting to the floor joists would be too high for your acoustic tile ceiling. If the ceiling tiles are strong enough, let the boot rest on the tile and use the register vent screws to pull the boot snug against the back of the tile. Otherwise you could install blocks extending past the floor joist edge to mount the Speedi-Boot lower to match the ceiling tile.

      It would be best if you can find a No. 320 Top Takeoff for Duct Board to tap into the duct board trunks. The Top Takeoff will fit better in the floor joist bay and be more efficient compared to a standard round / straight start collar. It will also avoid a possible constriction in the flexible duct with a 90 degree bend. A similar Top Takeoff for duct board is the Heating & Cooling Products 298 EZ Turn Ductboard Take-Off.

      Check with your local HVAC supply stores (where the pro’s shop) to find a duct board top takeoff collar because it’s a specialty item and I doubt the big box home improvement stores will have it. Take care to buy a takeoff collar for fiberglass duct board because there are many similar styles of takeoff collars with mounting tabs (fingers) but those are designed for connecting to sheet metal duct.

      Please post pictures of the finished branch ducts to photobucket. I’d enjoy seeing how it turns out.


  41. bob November 25, 2013 at 6:54 pm - Reply

    Very clear now but note I would have no problem getting a 4 x 10 through the side…the interior soffit comes down enough it would not look bad…not sure if that makes a big difference.

    The start collar you describe is at my local Lowes. I had any issue on the finished section of my basemet with a speedy duct and getting it to fit and ended up using a 6 or 8 inch round thing with insulated duct connected right to it…it has a louver too..was a pain to put up but I think the way everything lined up with the ceiling tile square I could not squeeze in a boot.

    My only concern with having the retur where you say (not that there is any where else to put it is that it is by the washer…I wonder if laundry smell / dtergent smell or noise will permiate up thru the house?

    Note the pic you labelled 12 x 18 room…the one looking into the unfinished part of the basement I am working on. My hvac is behind the 2′ sliding door you see.Ther are actually two 2′ doors there…you only see one. Originally the hvak sales guy said to wall that in and put a large return there. The installer said it was better not to box the unit in…the idea being that no one knows how big the next unit would be and isntalltion might be a pain if walled in. I could still add a larger return their..the doors are an inch or two off the floor…its not a tight seal..but I am still by the laundry area and just to the right of the hvac is the stair way comng down and under it are some pets in cages! I am afraid that smell might permiate thru the house..that would be worse than the dtergent smell!

    Maybe I can load a few more pics.

    • BobJackson November 25, 2013 at 7:44 pm - Reply

      > I would have no problem getting a 4 x 10 through the side
      Do you mean the side (cut a hole in the soffit fascia board) or the bottom where the duct board is exposed in the photos? Placing a return vent at the end of the return trunk inside the new room would get you away from the clothes washer and pet cages. The supply vent should be located at the far side of the room such that the air flow circulates the full length of the room.

      > The start collar you describe is at my local Lowes.
      Ah yes! Standex Air Distribution Products, Inc. sold at Lowes is the same manufacturer for the Snappy top takeoff start collar. I checked the website and only saw the T7 takeoff which is for sheet metal duct. I’m sure your store has the duct board takeoff or similar product.

      I’m not understanding the problem fitting a Speedi-Boot. A photo of the other vent boot with the insulated connector would help. The important thing is you’ve got a solution.

  42. bob November 26, 2013 at 10:09 am - Reply

    1) thru the side….meaning horizontally thru the white board that boxes in the duct board….just pop a lovered 4 x 10 vent right onto the white board.

    2) As I recall the ceiling grid in the other portion of the basement fell in the middle of a rafter making it difficult to get the speedy boot to fit. I was able to get the flex hose around it and had enough for to afix a 6″ round vent.

    • BobJackson November 26, 2013 at 11:12 am - Reply

      > just pop a lovered 4 x 10 vent right onto the white board
      I couldn’t think of a good way to install a start collar in the side of the duct board through the soffit fascia. The problems I see are:
      * A round start collar would be too long to extend beyond the white board fascia. Tin snips will solve that.
      * The hole in the fascia board would have to be sawn to fit the wide flange of the start collar through, leaving a large gap.

      A register box might work with modifications. The box is 3.5 inches high which appears to be more than the space between the duct board trunk and the front of fascia board. Let’s assume the No. 505R or other slim profile register box will fit. The advantage is the rectangular box flanges are mounted at the front so you don’t need to cut an oversize hole in the white fascia board as would be for a round start collar. The disadvantage is the register box is designed for connecting to flex duct or sheet metal duct. It can be modified to function as a duct board start collar by:
      * Cut the rectangular hole in the white fascia board.
      * Insert the register box and mark the circular duct outline against the duct board.
      * Cut the circular hole in the duct board.
      * Insert the register box fully into the fascia and duct board.
      * Reach through the register box and mark the exterior round metal duct where it meets the interior duct board. You’ll be working blind by feel but the duct board interior face will guide your marker.
      * Cut the duct board mounting fingers / tabs with tin snips to the depth of the marked line on the outside of the round metal duct.

      Now you can insert the register box, reach inside and bend the duct board mounting finger against the duct board to hold it in place. Seal the joint with mastic or HVAC metal foil tape. The register box body may conceal the round duct joint against the duct board making it impossible to seal the joint. Since this is a return air duct inside the home’s thermal envelope (air conditioned space), an air leak here isn’t a real concern.

      Another method for creating a return air duct between the soffit fascia and duct board trunk would be to fabricate a rectangular box out of fiberglass duct board. A 4″ x 10″ return duct box can cut from a single strip of 28″ long duct board that’s 3- or 4 inches wide measured fit exactly in the rectangular hole from the face of the soffit board into the trunk duct. Use a serrated knife to miter the interior corners at 45 degrees like V so the duct board corners will fold a 90 degree angle. Fold it into a rectangle and tape the single corner joint closed. Fasten the short jump duct to the trunk duct with HVAC tape. The return vent register can be mounted to the white fascia board with screws. Now that I’ve written up this idea, I like this option the best – inexpensive and simple.

      > As I recall the ceiling grid in the other portion of the basement fell in the middle of a rafter making it difficult to get the speedy boot to fit.
      Make sense. It looks like a Speedi-Boot supply vent should fit between the 2×10 or 2×12 joist bays.

  43. bob November 26, 2013 at 12:24 pm - Reply

    I may still go the ‘thru the top route’ if there is an advantage…. but my proposed ‘through the side’ route would use this starting collar from HD:

    1) cut the hole through the white board.
    2) Mark and cut the duct
    3) Slide the starting collar thru the side board and into the duct and flip the little flanges 90 degrees insuring the collar cant pull out. Tape around the duct…I can get my hands in there.
    4) I will now have some of the starting collar sticking out thru my white board..I’d have to trim that and add a grill whose louvers would fit inside the collar. so I could control air flow.
    5) Repeat on the ‘return’ end but using a non louvered return grill.

    I end up with 4 x 10 or so grills blowing out….as opposed to 6 x 12 blowing down I guess…not sure the impact of that….

    Feasable..any downside?

    • BobJackson November 26, 2013 at 8:44 pm - Reply

      I think “through the top” would be less work and a better installation because it avoids sawing the white fascia board and would use a top take off for duct board.

      My concerns about the Master Flow Rectangular Stack Duct Start Collar are:
      * Although the product descriptions says “Use to transition from ductboard plenums” the tabs/fingers appear to be too short to make a 90 degree bend against the interior face of the duct board. Your duct board trunk is probably 1 inch thick. Best to measure the two and confirm… I could be wrong. Most duct board start collars either have extra long mounting tabs and/or a straight neck to fit through the duct board so the base of the tabs extend slightly past the inside face of the duct board (compare to the tabs for the top takeoff at the above link). I checked my local Home Depot but they didn’t stock that Master Flow 4×10 Start Collar. Lowes had similar wall stack take-off with maybe 1 inch tabs that aren’t long enough to bend back against the duct board… forcing the tabs would just crush the duct board.

      * Duct fittings must be mechanically fastened via metal tabs (e.g. start collars), draw bands, screws or clamps. HVAC metal foil tape shouldn’t be considered mechanical fastener because it’s really just a sealant. I don’t doubt the rectangular start collar flange could be successfully held in place with several layers of HVAC foil tape but the tape can fail over time and wouldn’t be up to Code.

      * You’d need to saw the rectangular hole in the fascia board large enough to fit the base flanges through. Not a big deal but may require a larger register vent to cover the hole.

      Supply and Return Air Vent Placement:
      The essential issue isn’t how you make the new duct work connections, rather where the new vents are located: Were the supply and return vents located along the interior soffit, the airflow will tend to “short circuit” and flow between the two vents resulting in poor circulation at the far end of the room.

      The return vent should be located near the room entrance or close by in the hallway which is OK because the room does not have a door.

      The supply vent should be placed at the far end of the new room to avoid dead zones and maximize the air flow. The concept is illustrated in Figure 28-2 on page 2 of Ductwork. Also see Are your ducts placed properly? by Kelly’s Heating and Air Conditioning.

      Hope this helps.

  44. bob November 29, 2013 at 1:47 am - Reply

    through the top it is…thanks!

  45. Ed February 2, 2014 at 11:18 am - Reply

    Hi, Bob.
    Is it legal to have a heat vent/register in the middle of the floor? I’m removing a wall between the kitchen and the dining room and I would have to install a flat register covering if I leave the heat vent where it is. People would be walking over the register. Is that okay as long as it doesn’t bother me aesthetically?


    • Bob Jackson February 2, 2014 at 8:46 pm - Reply

      Hi Ed,
      I would relocate the floor register to an out of the way location close to an exterior wall and away from the return vents for better circulation. Keep in mind that if you sell the house a floor register in the middle of the room will raise questions.

      I’m not aware of any Code requirements that says you can’t have a floor register in an odd location. The 2012 International Mechanical Code upon which local Building Codes are based states:

      603.18 Registers, grilles and diffusers.
      Duct registers, grilles and diffusers shall be installed in accordance with the manufacturer’s installation instructions. Volume dampers or other means of supply air adjustment shall be provided in the branch ducts or at each individual duct register, grille or diffuser. Each volume damper or other means of supply air adjustment used in balancing shall be provided with access.

      603.18.1 Floor registers.
      Floor registers shall resist, without structural failure, a 200-pound (90.8 kg) concentrated load on a 2-inch-diameter (51 mm) disc applied to the most critical area of the exposed face.

      Best to call your Building Dept. and ask if there’s anything in the local Building Codes that would prohibit the register in your proposed location.

      Since people would be walking over the floor register, consider using a vent filter to keep dirt out of the duct:

      Let me know what your local Building Dept. says.


  46. Owen April 15, 2014 at 1:14 pm - Reply

    Hi Bob,

    I bought a home in North Idaho last year that included a detached shop- about 30×40. It has 16 ft ceilings and above that is a 1000 sq ft apartment.

    The apartment is heated by an 80% efficient propane fired furnace that is in the attic. The hot water heater (propane) is in a closet in the apartment also. Other propane appliances are the stove in the apt, and a “hot dawg” ceiling mounted furnace in the shop.

    I’ve been going through (what feels like) an inordinate amount of propane. My theory is there are 3 major problems 1) Not quite enough insulation in the attic 2) since the furnace is in the attic (and not in the heating “envelope”) it has to work harder to get the air warm and therefore uses more propane and 3) the hot dawg must run in very cold weather to keep the water pipes (located in an interior wall of the shop) from freezing.

    I am contemplating what it would take to retrofit the apartment- and put the heating registers at the base of the interior walls- and run the duct work on the ceiling of the shop. There is a bathroom at the back end, and I was thinking I would put the furnace right on top of that.

    Is this a crazy plan? I’m just wondering if it sounds feasible, and wondering how much it would actually reduce my propane usage.

    I’m really interested in what you think.



    • Bob Jackson April 15, 2014 at 9:45 pm - Reply

      Hi Owen,
      Nice setup! I like your detached workshop!

      > My theory is there are 3 major problems
      > 1) Not quite enough insulation in the attic
      Proper insulation would be the #1 priority for minimizing heating losses. See the ENERGY STAR Recommended Levels of Insulation R-Value map to determine how much additional insulation is needed.

      Northern Idaho is located in Zone 6 on the map which calls for attics to have R49 to R60 insulation.

      Now that you know the recommend insulation R-Value, the Owens Corning Insulation Calculator will estimate the amount of additional insulation needed for batts/rolls and blown loosefill insulation in a Northern Idaho 838xx zip code. Guessing your attic already has 6 inches of insulation the additional insulation needed for a 1200 sq. ft (= 30 ft x 40 ft) is 16 inches of batt insulation for a recommended 60 R-Value total.

      > 2) since the furnace is in the attic (and not in the heating “envelope”)
      > it has to work harder to get the air warm and therefore uses more propane
      My air handler with a natural gas furnace is also in the attic. Yes, it’s not the ideal location but common for the reason you cited. The most important improvement here is fixing any duct work problems and R-8 rated duct insulation.

      You should also crawl around the attic and seal all electrical wiring and plumbing penetrations in the walls and ceiling with something like GREAT STUFF joint and crack minimally expanding foam. It’s available at the hardware store in an aerosol can. Caulk around the bathroom exhaust fan, too.

      Take care to leave an air gap around recessed lighting fixtures that generate a lot of heat.

      Check your electrical wall outlets for air leaks and install a foam gasket (available at the hardware stores) behind the cover plate because the outlet box may not have insulation behind it or has compressed the insulation to the point it’s lost most of it’s insulation effectiveness.

      > Is this a crazy plan?
      It’s a very reasonable plan… something I would do. If the walls are insulated, I’d start with the attic as that may be sufficient.

      > wondering how much it would actually reduce my propane usage
      See the ENERGY STAR Estimating the Payback Period of Additional Insulation article which has a fairly simple formula for calculating the Years to Payback.

      Let me know what you do and the results.


  47. Jeff July 11, 2014 at 9:33 pm - Reply

    Hi Bob, great walkthrough on this, however I would caution your readers on cutting through the OSB blind valley, depending on their local code.

    In California especially, the blind valleys often are actually shear walls and used to stiffen the structure for wind and earthquake loads. Cutting through them in most cases is against code, and even if it is within code, you have to keep it below a specified percentage of area of the panel, and you need to radius the cuts. Unfortunately the only way to do something like that is to put an access hatch in the ceiling on the other side of the blind valley/shear wall.

    • Bob Jackson July 12, 2014 at 5:03 pm - Reply

      Hi Jeff,
      Thank you for pointing out the shear wall issue and differences in local Building Codes.

      I live in the Atlanta, GA area which has very little seismic activity of any significance and inexpensive earthquake insurance. My home is therefore (and unfortunately) not subject to the more stringent Building Codes like California (earthquakes) and Florida (hurricanes). I remember the reinforced trusses, walls and straps at my former South Florida home.

      As always, the homeowner should consult your local Authority Having Jurisdiction (AHJ) – a.k.a. city or county Building Dept – for guidance.


  48. Randy July 21, 2014 at 9:24 pm - Reply

    Hi Bob,

    Hoping you can help. I am tearing down a wall that separates two bedrooms, in order to make a large master bedroom. The wall I am tearing down has a return duct in it. Can i cap off this return duct and not use it? I will still have one return duct that will be approximately in the middle of the new larger room.

    the measurement of the rooms are 15 x 11 and 11 x 11. The larger room has the one return duct and two floor registers. The smaller room has one return duct that I want to remove since its in the wall I will be tearing down and it also has one floor register. Thanks for any advice.


    • Bob Jackson July 22, 2014 at 7:18 pm - Reply

      Well, it depends on knowing the sizes of the supply ducts, return ducts and vent grilles.

      The easy way is to temporarily cap off the return duct during the remodeling phase and see if the room is getting enough air with the doors closed, but I doubt that will be satisfactory because “return openings will need to be 2 to 3 times the size of the supply duct depending on system design velocities” – reference page 153, Return Air Flow Considerations. The reason return ducts need to be larger than supply ducts is the supply ducts have higher pressure and air velocity, whereas return ducts are lower pressure and velocity.

      If the room is not comfortable with a single return vent you’ll need to convert the existing wall return vent into a ceiling or floor return depending on if the return duct enters wall from above or below.

      BTW – take care that you’re tearing down a non-load bearing wall.

      • Judy P December 12, 2014 at 4:06 pm - Reply

        Need suggestion to heat an enclosed 12×8 screened porch with sliding glass doors. I tried to add heat by installing a tee from a basement duct that supplies heat to the kitchen vent. This kitchen vent puts out too much heat for the kitchen,so I thought diverting some heat from the kitchen to the porch would work. A contractor put a tee on the bottom of the duct ( he could not put it on the sides because the duct sits between two joists) with two flexible round ducts blowing into the porch from two 4 inch dryer vents with dampers and no caps. We took the dampers out because very little air and heat came in. The kitchen continues to get the full force of the the air and heat, the porch minimal. Even when I close the kitchen vent the air flow into the porch does nit increase. There is jtust enough heat to keep the temp out there above freezing, around 50 degrees,too cold to make the porch comfortable. The tee is about 5feet from the kitchen vent. Is it too far away to do any good? Are the vents too small? Is the whoe concept an engineering wrong. My contractor is not a heating person but adds duct work as needed on project. I know the glass doors although double glass with e gas will always compromise the warth of the room, but if the porch gets enough heat when the heat comes on in the house it should be comfortable enough at night so my paints and plants will be okay. How can I correct the problem using my existing ducting. Electric heat is out of the question. I have a gas system

        • Bob Jackson December 12, 2014 at 6:00 pm - Reply

          Hi Judy,
          Tee ductwork connections should be avoided because the airflow has momentum and velocity, it tends to keep moving in the same direction and doesn’t like to “turn the corner” at the 90 degree Tee. I understand there isn’t much space between the floor joists and that’s why a Tee was installed. How are you routing the duct from the basement to the porch? Is it possible to install a Wye duct connector directed downward below the joists so there’s room to connect the new duct, then come up a few feet away to the porch?

          > Even when I close the kitchen vent the air flow into the porch does not increase.
          4 inch dryer duct is far too small and uninsulated. The Tee fitting makes the situation worse. Like blowing through a straw for your application. Closing the kitchen vents just forced the airflow to lower resistance vents elsewhere besides the porch. 6 inch insulated flexible duct with Wye fittings is needed.


          • Judy P December 12, 2014 at 11:21 pm - Reply

            Hi, Bob

            I understand now why the tee is not working. The kitchen duct comes vertically directly off the main trunk and vents into the far end of the kitchen right next to the porch. The contractor drilled two four inch holes from the porch about two feet apart into the crawl space and connected two separate flexible ducts from the tee to those holes running on either side of the kitchen duct. Looks like two arms coming out of the bottom of the kitchen duct. He then wrapped insulation around them, but I think he should have used the insulated ducting. Where would the wye attach? Should the wye be connected to a duct that connects directly to the trunk line rather than the kitchen duct? All the ducting in my house is square. I may have to have a ducting person come to evaluate everything but I don’t want to make another mistake. What is your thinking? Should I just start from scratch? If you were starting this job in your own home, what would you do?Thanks.

            • Bob Jackson December 12, 2014 at 11:53 pm - Reply

              The contractor likely used 4 inch dry duct because he had a widely available 4-1/4 inch hole saw. What did he drill the holes through? A floor joist or band joist to exit the crawlspace to get outdoors to the porch? If so, such large holes will greatly weaken the joist.

              The Wye for the new branch duct should be at the main trunk duct if possible. A lot depends on the size of the kitchen branch duct and proposed location of the secondary branch ducts to the porch.

              See this discussion and the Duct Systems Design Considerations link in the discussion for an introduction to ductwork.

              Yes – do call an HVAC company and ask them what’s needed to make it right.

              • Judy P December 13, 2014 at 9:23 am - Reply

                It was the band joist if that is the part that sits on top of the foundation. The concrete of the porch sits a foot below it. The cut went directly into the crawl space. No floor joists were compromised. I kept the cut outs so they can be inserted back if a better entry is needed. Next try should be direct into the trunk duct if it will work. I am going to get a pro in here. I will read the discussion as you suggested. Thanks for your help. Wish I’d found this site before, but it never Googled based on my question. At least I’ll have some informed questions to ask the HVAC person. I really have to do some homework before relying totally on what a contractor says. You’ been a great help.

                • Bob Jackson December 13, 2014 at 9:58 am

                  I have a clear picture of the crawlspace and porch arrangement. An idea for routing the branch duct from the crawlspace to the porch without damaging the band joist is:
                  * Saw 7 inch diameter round holes for the 6 inch branch duct in the kitchen floor next to the exterior wall with the porch.
                  * Locate the holes in the joist bays, i.e. between the floor joists, and at an inconspicuous place in the kitchen where it won’t be tripped over.
                  * Use a wall stack register end boot from the kitchen floor to an rectangular hole cut through the exterior wall.
                  * You’ll need to connect a short length of rectangular sheet metal duct through the exterior wall to a register on the porch. Something like a Wall Stack Start Collar.
                  * Build an interior soffit (a fancy name for a box) to conceal the ductwork from the kitchen floor to the wall.

                  Take care to locate the wall studs and any electrical wiring in the exterior wall to plan the job.

                • Judy P December 13, 2014 at 7:11 pm

                  The wall stack end boot Is already there for the heat vent in the kitchen. It butts right up to the band joist on the outside wall. The porch is right on the other side.Could it be that simple? Can I just cut into the band joist and open into the kitchen register? If I leave the 9 foot sliding glass door to the porch open, then I’ll have a warm kitchen and porch. I have a big return in the hall by the kitchen. By the way, my house is a small split level about 1500 square. I’ll ask the HVAC person if that can be done? What do you think? If you say yes, then doing all that other stuff was silly.


                • Bob Jackson December 14, 2014 at 12:22 pm

                  > Can I just cut into the band joist and open into the kitchen register?
                  Do not cut into the band joist because it’s a key structural support for the house. Small penetrations for wiring, water and gas piping through the band joist are acceptable, but an air duct is definitely too large.

                  I think what you meant to describe is extending the kitchen wall stack which rises above the kitchen floor through the exterior wall to the porch. That would work. Use a louvered vent on the porch to close the air flow when porch heating isn’t needed.


  49. Mattie_K October 13, 2014 at 12:52 pm - Reply

    Hi Bob, I’m stumped! I’ve got the vent boot installed, worked out great! However, the other end where I would connect the new branch line to the existing system doesn’t have the rigid duct board where I can cut it out and install it. Instead, It looks to be a round insulation wrapped trunk line, looks similar to the branch lines, however is around 2x to 3x bigger around. With it being round, it doesn’t appear that the starting collar would fit like it should. Is there a different type connector that I should be using? I don’t believe it’s flexible duct either, could it possibly be rigid duct board wrapped in insulation? Thanks.


    • Bob Jackson October 13, 2014 at 6:28 pm - Reply

      Hi Mattie,
      It appears from your photo the trunk duct is round sheet metal. A 90 degree saddle tee take off is needed to connect the new flexible duct branch line to the round sheet metal trunk duct. See this annotated version of your photo.

      Examples of 90 degree saddle tees are:

      and this more basic item at Home Depot.

      Do not install a sheet metal Wye saddle because your other branch duct connections are 90 degree connections and a Wye will imbalance the air flow. The air flow will tend to follow the Wye branch because it has to turn less versus the four other 90 degree take offs.

      The steps to install the saddle tee take off are:
      1 – Measure the diameter of the round sheet metal trunk duct and the flexible duct branch line. Measure only the duct itself, do not include the insulation wrap.
      2 – Buy a 90 degree saddle tee take off that matches the trunk duct and flexible duct diameters.
      3 – Hold the saddle tee against the trunk duct insulation jacket where it will be installed (e.g. the side like the other branch duct connections) and trace the round circle inside the take off.
      4 – Cut a hole in the insulation jacket following the traced circle for the takeoff.
      5 – Now slice a vertical slit long enough that you can pull apart the sheet metal trunk duct insulation jacket and slip in the flanges of the saddle tee.
      6 – Slide the saddle tee flanges under the insulation jacket and position it so the 90 degree tee is horizontal. Now trace inside the take off on the sheet metal duct.
      7 – Use tin snips to cut the hole for the take off in the sheet metal trunk duct. Tip: Drill a 1/2 inch starter hole inside the traced circle to get the tin snips started.
      8 – If the saddle tee has a self-stick sealing gasket like the Speedi-Products item at, press it against the sheet metal trunk duct and fasten the flanges with sheet metal screws. Otherwise, you’ll need to seal it with mastic – which is always recommended.
      9 – Pull the insulation wrap over the saddle fitting and seal with HVAC tape and optionally mastic.
      10 – Connect the flexible duct branch line to the saddle tee.


  50. Mattie_K October 26, 2014 at 6:42 pm - Reply

    Wow, thanks for the prompt reply Bob. it took me a while to find one of the 90 degree saddle tee’s, no Lowes or Home Depot within 30 miles of my house had one. I ended up ordering on Amazon. I must say, your write up worked like a charm. Thanks a million times over for helping out with this!

  51. Judy p December 17, 2014 at 8:03 am - Reply

    Ok, now I am stumped. There is no way to avoid the band joist to heat this porch. The heat vent to the kitchen is flat on the floor. Immediately above it is a 9 foot sliding glass door. Can’t go from the roof because the porch is a Patio Enclosure structure that was set on a concrete pad and rested against the house under the outside soffit. I’ve already drilled those two holes through the band joist which do trickle enough air into the porch to stop it dropping below freezing. Guess I am back to the wye from the trunk to those holes or some other electrical heat source with a thermostat. Already have one of those oil radiater looking heaters that can push the temp when I’m out there into the comfortable range. Unfortunately, my electric bill doubled. HVAC person is coming after the holidays to take a look. I’ll let you know how I make out. Thanks for helping out.

    Judy P

    • Bob Jackson December 18, 2014 at 9:17 pm - Reply

      What about installing a new branch duct in the kitchen floor off to the left or right of the sliding glass door, come up out of the floor then through the exterior wall to the porch? Install the Wye duct duct connector in the basement, run the branch duct under and across several floor joists to the reach the side of the glass door, then up through the kitchen floor. Conceal the kitchen duct in an interior soffit.

  52. Judit P January 2, 2015 at 2:25 pm - Reply

    Since my last post, I have had three duct companies come in to evaluate how to add heat to my porch. I had a contractor add a tee to the existing kitchen duct on the bottom and connect ducting to the two ends out through the band board on the porch. The flow of heat was a scant amount but did provide enough heat to keep the temp from going below freezing. Each duct company suggested a different solution. Co.#1- connect to the main trunk add a ” y” and connect to the two holes in the band board widening the previously drilled 4 inch holes to 6 inches. Co. #2- said they could cut back the kitchen duct add two side ducts to the porch. They also said they may be able to cut Into the back of the kitchen floor duct through the band board and make it so that I can keep both ducts open or shut either one as needed. Co.#3- offered the same as the first company with a direct connection to the trunk a tee plus a damper to control the amount of air to the porch. Which do you think is the best way to go?


    • Bob Jackson January 2, 2015 at 9:23 pm - Reply

      > Co.#1- connect to the main trunk add a ”y” (Wye duct fitting) and connect to the two holes in the
      > band board widening the previously drilled 4 inch holes to 6 inches.

      I don’t recommend drilling 6 inch large holes in the band joist (also called “rim joist”) because it weakens the joist. The International Residential Code Council states in R502.2.8.1 Sawn lumber [IRC R502.8.1]:

      The diameter of holes bored or cut into members shall not exceed one-third the depth of the member. Holes shall not be closer than 2 inches (51 mm) to the top or bottom of the member, or to any other hole located in the member.

      Suppose your band joist is a 2×12 board which has actual dimensions of 1-1/2 x 11-1/4 inches. A 6 inch hole is over 50% of the 11-1/4 inch depth of the board. The 4 inch holes already there are over 35% the depth of band joist. If your band joist is a small 2×10 (1-1/2 x 9-1/4 inches actual size) the situation is much worse. Either way, the hole size exceeds 1/3 the depth of the joist in violation of R502.8.1 and may be closer than 2 inches from the top or bottom of the board.

      The question is does IRC 502.8.1 apply to a band joist whereas the regulation was written for floor joists? A floor joist is supported at the ends of the board and is under both tension and compression. See the joist diagram on page of 8 of this American Wood Council presentation. Unlike a floor joist, the band joist rests on the sill plate with continual support and is not subjected to bending moments like a floor joist.

      I can’t definitely answer the question about R502.8.1 as it applies to rim/band joists. I wouldn’t allow a 4 or 6 inch hole to be drilled in my band joist because it would remove too much material from this structural framing member. Better to IMHO to find another route for the ductwork – i.e. through the kitchen floor, up and out the wall concealed in an interior soffit as described previously.

      Call your local building dept. and ask if a 6 inch (or even 4 inch) hole in the band joist would be code violation. They may ask if your home has a single or double band board (two boards thick).

      Of the 3 ductwork proposals, Company #1’s suggestion to install a Wye on the main trunk duct will provide more air to the porch. However, I wouldn’t go through the band joist. If the band joist fails the repairs will likely cost thousands. Also keep in mind what a home inspector might question as a code violation should you sell the house down the road. The potential buyers might insist on repairs as a condition of sale.

      Wouldn’t it be simpler to run a new 240 volt branch circuit and electrical outlet to the porch for an electric heater? The cost for an electrician will probably in the same range as the ductwork proposals with far fewer issues.

      This project explains how to install a 120 volt branch circuit to get a general idea of what’s involved. Note the garage heater requires a 240 volt circuit which can be provided by your circuit breaker panel.

      Let me know what you decide to do.


  53. Judy P January 3, 2015 at 1:53 am - Reply

    Just to be clear the last two feet of the kitchen leading to the porch is a bump out. Two 4″holes have already been cut into the joist from my former con tractor. They are connected to a tee from the kitchen duct. The kitchen duct comes up from the crawl space under the kitchen and sits flat on the floor 8 or so inches from a 9 foot sliding glass door. On either side on the door is a stove and pantry closet and no way to get to a soffit. There is no outside wall to the porch just the sliding door. The 4″ holes have already been drilled three and one half feet apart in the 2 x 12 board under the door. The duct people said these do not compromise the structure since I asked them specifically about this. If it against code , I’ll have to check. All the band board supports is the door. There is no second floor just the joists for a slightly sloping roof. I will not put electric heat out there because of the cost to run it 24/7 and danger of fire. The demand on any electric heat would neither be cost effective and could cause a fire. I already had a heater fire once years ago and don’t want another. There is a portable radiator heater out there now that I keep at 62 degrees only at night so my plants won’t freeze, and in one month, my electric bill doubled. It is just not worth it. I would rather plug the holes and use the porch only three seasons if it came to that. Thanks for your suggestions. If I continue with this project, I’ll stay with the 4″ holes.

    Judy P

  54. Eddie February 4, 2015 at 2:02 pm - Reply

    Hello, Bob..

    We are finishing our basement and need to put air ducts, one on the bathroom, one on a room that it’s being built and two or three more around the living room area.. They are like 6″ air ducts going from the center of basement to windows side, We were planning in just make a hole on the line just between the two areas and built it there.. But my question is, If we be having any trouble by doing that!

    • Bob Jackson February 5, 2015 at 7:55 pm - Reply

      Hi Eddie,
      You will have trouble if you do that because the existing 6 inch duct is far too small to serve the two new rooms and bath in finished basement.

      The proper way is to install “home run” branch ducts to your supply and return plenums – or – tap off the trunk supply and return ducts.

      See these resources for ductwork design and installation guidance:

      * Ductwork Installation Guide

      * comment

      You can e-mail photos of your trunk duct and/or air handler supply plenum to bob[at] if you’d like specific advice. Replace the [at] with the @ symbol.


  55. hvac contractor May 9, 2015 at 1:54 pm - Reply

    Alot of misinformation here. Having 6 inch flex connected to a 6×10 vent is overkill. A 4×10 is about as much airflow as a 6″ flex can handle. Also, it is important to know the static pressure and airflow of your duxt system, as adding another supply vent can unbalance the system, and/or impact other rooms.

    HVAC systems are NOT just based on sq footage. Their is a calculation called a manual J, that takes into account insulation, windows, directions and exposure to the sun, type of construction, shade from trees, sq footage of walls and doors, and even how mamy people live there. If your contractor does not do a Manual J calc, then you need a new one.

    Similarly, your ductwork is designed by a Manual d calculation, and will match the airflow needed by your system precisely.

    This is why this is not a DIY trade.

    • Bob Jackson May 10, 2015 at 12:35 pm - Reply

      The reason I installed a 6×10 vent is to match the existing 6×10 vent. The new vent is on the right in this photo. The louvered vent balances the air flow. It’s been a very successful home comfort upgrade. Walking around the house I see 6×10 inch vents served by 6 inch flex duct in the other bedrooms.

      A link to the Manual D residential duct sizing calculator is provided in an prior comment along with “How Contractors Really Size Air Conditioning Systems” by The Florida Solar Energy Center (FSEC), a research institute of the University of Central Florida.

      In an ideal world, one would:

      1. – Hire an HVAC contractor to run the Manual J load calculations to determine the heating and cooling load for the whole house and each room.
      2. – Heating and air conditioning equipment selection per Manual S.
      3. – Duct Design per Manual D.
      4. – Air Distribution design per Manual T for vent size, type and location to prevent stagnant air.
      5. – Then follow the procedures in Manual B to balance and adjust the air flow.

      I fully support the science, engineering and field experience contained in the ACCA manuals but short of new construction or major remodeling it’s too expensive and overkill for minor ductwork rearrangements. The ductwork in many homes is poorly installed, heating and air systems are aging, and people can’t afford thousands of dollars in repair or replacement costs.

      If the reader follows the advice I’ve provided here they can decide whether to do the job themselves or hire an HVAC contractor.


  56. Antoni Scott June 28, 2015 at 2:55 pm - Reply

    I live in an 1100 sq/ft condo on the 4th floor. The front ocean facing bedroom with its full floor- to -ceiling sliding glass doors gets 5 to 7 degrees hotter than any other room. There is only one supply register in this room. Can I increase the amount of cooling in that room by adding an extra register without running a separate duct ? The ducts are hidden in the ceiling. I was thinking of tapping into the existing register and adding another a few feet away.
    The air handler is located in a hallway closet (typical of Florida condo’s). The returning air is so forceful that it can suck the door shut because the closet door grill is too small. Opening the closet door increases the noise. Can the air handler speed be slowed down to reduce air noise ? The present system has a variable speed fan but the high speed is too high.


    • Bob Jackson June 29, 2015 at 9:14 pm - Reply

      Do you have infrared blocking window film on your sliding glass doors? Blocking the sun’s heat will reduce the cooling requirements.

      > Can I increase the amount of cooling in that room by adding an extra register without running a separate duct?
      It might provide some additional airflow but in general the results would be unsatisfactory because the duct is sized to support the single vent. Is the room cooler with the interior open? If so, a through-wall return vent near the ceiling between the wall studs to the interior hallway may significantly improve the air flow.

      > The returning air is so forceful that it can suck the door shut because the closet door grill is too small.
      It should be straightforward to remove the door, cut a larger return and install a new grill. That would increase the efficiency of the AC system and help that hot room – especially with an extra return vent as describe above.

      > Opening the closet door increases the noise.
      Has an HVAC technician checked your blower motor and fan wheel? A dirty fan can cause excessive turbulence and noise. The too small door return also makes the blower motor work harder and noisier.

      > Can the air handler speed be slowed down to reduce air noise?
      Reducing the fan speed also reduces the return air flow so you may solve the noise problem while making the ocean side bedroom warmer. To adjust the fan speed you’d need refer to the owner’s or service manual for configuration options. On newer systems it may be as simple as selecting options on a touch-screen thermostat or more involved by moving DIP switches on the air handler control board. I’d leave the control board configuration to an HVAC tech as a mistake can be expensive.

  57. Matthew July 22, 2015 at 4:57 pm - Reply

    Hi Bob,

    I have a 1 story 1600sft home. I was looking into adding an additional air duct in my master bedroom which is pretty large in size with 2 large windows. So in the summer it gets very hot and winter very cold. There is only 1 air duct and 1 register. I was thinking of adding an additional air duct on the opposite side of the room. My only concern is that my master bedroom is at the very back of the house and my AC is located in the attic near the front of the house. What would you recommend? I have not checked to see if my AC has the capacity to add an additional duct, does that matter since I am only adding an addition to my master bedroom? Thanks!

    • Bob Jackson July 23, 2015 at 9:14 pm - Reply

      A new “home run” air duct from the front to back of the house in a 1600 sq ft home would probably no more than 30 feet long. Less if you can tap into a trunk duct and install a new branch.

      The central AC system capacity in BTUs or “tons” (1 ton equals 12,000 BTUs) is mainly based on the square footage of the house. So you should be fine on capacity and it’s a problem of air distribution.

      The next step is to figure out the best way to install the new ductwork based on your existing duct configuration. A new home run duct connected to the air handler supply plenum will be the most effective solution. Wye-ing off an existing branch duct is the least effective because the existing branch duct is often too small to provide the need air flow for the new branch.

      See these comments for duct sizing and duct system design. The Ductwork Installation Guide is very good.

      Install the new ceiling vent near the exterior wall above the large windows assuming the existing vent is not nearby.

      You can e-mail pictures to bob[at] (replace the [at] with the @ symbol) for specific recommendations.


  58. taylor ingram May 4, 2016 at 8:03 pm - Reply

    Hello, me and my husband recently bought our first home. It has two bedrooms downstairs, buy the upstairs is considered the master bedroom. We believe it was previously the attic that they finished. Its way to big for one bedroom, so we want to divide it into two children’s rooms. It has two vents but it blows neither hot nor cold. We really want to have the rooms done by the end of this year, is there a way to go about this inexpensively and without having to tear the whole room apart?

  59. Steve July 7, 2017 at 6:11 pm - Reply

    Hi. These are some great ideas. We have a kitchen with three gas forced hot air ducts that come out of the wall through the baseboards. One of the kitchen walls it goes out to an unheated sunroom 10′ x 10′. Can we go through the baseboard in the sunroom and connect to the heat vent on the shared wall in my kitchen and have that heat be available to both rooms ? If so how to do this ? Thanks

    • Bob Jackson July 8, 2017 at 9:31 am - Reply

      Is an insulated sunroom with 2×4 walls and glass windows? Or screened-in sunroom with storm windows? I see you’re in the New England area with cold winters. While you could tap into the duct in the common wall I don’t think you’ll get enough air flow to heat the sunroom and you’d have to install a return air duct from the sunroom back to the house.

      An electric garage/workshop heater would be a better choice. It requires a 240 volt AC, 20 AMP circuit so you’ll need an electrician.

      I heat my sunroom with a natural gas patio heater. The room has a high cathedral ceiling and I run the ceiling fan to circulate the warm air. Natural gas heaters require ventilation so in addition to the fresh air coming up from the gaps between the deck boards and exiting the soffit vents I always open an Eze-Breeze window or two.

  60. shaun September 24, 2017 at 1:58 pm - Reply

    I’m adding a register to a bedroom that didn’t have one before( its an older house that used to have pass through vents to allow heat up from downstairs). I bought a floor register but was wondering if I should put a wall register in close to the floor to help spread the air better. I am also putting a return in up high on the opposite side of the room there will also be a small register in the bathroom we are putting in up there as well the bedroom is 16×16.

    • Bob Jackson September 24, 2017 at 3:33 pm - Reply

      Floor and ceiling registers are commonly used for supply air ducts due to ease of access from the crawlspace/basement for 1st floor rooms and attic for 2nd floor rooms. For better air circulation a ceiling light can be replaced with a ceiling fan/light combo. Every room in my house has a ceiling fan and we use them in all seasons.

      Retrofitting wall vents can be a real challenge because you’d have to saw through the 2×4 wall sole or top plate potentially weakening the structure, there may be electrical wiring and/or plumbing obstructions and it’s difficult to install metal duct chases without tearing off the drywall. Bare wall cavities should not be used for air pathways due to leakage.

  61. Jack January 29, 2018 at 5:19 pm - Reply

    I am closing of the back of my garage (closest to the house) for a 8×15 spare bedroom. The ceiling is already insulated well and I will insulate the two new walls well also. Would I need to add a return or just the supply?

  62. franj berlanga March 11, 2018 at 1:55 pm - Reply

    when do air ducks need to be replaced in the attic. I just replaced my ac unit.

    • Bob Jackson March 12, 2018 at 10:11 am - Reply

      It depends on the condition, type and installation quality of the existing air ducts. Some factors to consider are:

      * Is the old duct work poorly designed and/or installed (leaky joints, pinched duct, etc.)? I replaced my attic trunk ducts because of this. See How to Install Round Sheet Metal Duct and Bryant Evolution AC Condenser and Ductwork Installation.
      * Is the old duct work very dirty? I’ve seen ductwork that’s caked inside with drywall dust because the A/C system was not turned off when finishing/sanding drywall during a remodeling job. Humidity causes the drywall dust to form a hard clay.
      * Is the old duct work damaged? Maybe a rodent chewed a hole, it’s been crushed or the flex duct inner liner is torn. The flex duct outer insulation jacket will billow if the inner liner is torn.
      * Rigid duct board is a dirt catcher and most new installations use sheet metal.
      * Energy efficiency: Attic air ducts must have at least R-8 rated insulation.

      One or more of the above considerations can make the difference between personal preference versus the old duct work has to be replaced. At this moment my 1st floor central air system is being replaced with a second Bryant Evolution (my house has two units) and I opted to have 25 feet of rigid duct board trunk replaced with sheet metal for an extra $1,250. This was optional but the house is 18 years old and why not modernize everything.

  63. B September 11, 2018 at 10:04 am - Reply


    I am looking to finish our attic, and instead of using a split duct or portable ac, are we wondering if we are able to add a vent from the current ac vent system? The ac unit can heat/ cool 3500 sqft and we have less than 2000 sqft total including the attic.

    • Bob Jackson September 12, 2018 at 9:53 am - Reply

      Your central AC system should have the capacity for the new attic room. The main considerations are:
      * Supply and return air duct sizing.
      * Attic finished living space will need extra cooling capacity versus a 1st or 2nd floor room.
      * Attic living space construction and insulation.

      The attic should be ventilated with a combination of soffit vents for air intake and gable, ridge or box vents for air exhaust. The new room walls, ceiling and insulation must not block the attic airflow or it will get really hot, making the new room unlivable and maybe damage the shingles.

      The new ductwork should be sized about 20% larger than what would normally be needed for a lower level room. If the AC is too cold, throttle the airflow with a louvered supply vent. The return vent/duct should be located near the ceiling on a wall opposite the entry door to pull out the warm air. The supply vent is best located on low on a wall opposite the return vent, but well away from the entry door. The idea is to have the air flow traverse the entire room.

      You can hire an HVAC company to calculate the duct sizing and configuration. They’ll also assess your existing ductwork. If you want to DIY, see my November 25, 2012 at 1:00 pm comment about the Residential Air Duct Calculator.

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