How to Add an Air Duct to a Room – Part 2

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How to Add an Air Duct to a Room step by step instructions. Frame-in and mount the new vent boot, install the rigid duct board start collar and connect the flexible duct branch line to the duct board trunk. This project is continued from How to Add a Room Air Duct – Part 1.

Frame-in the Ceiling Vent Boot

The vent hole has been cut in the drywall ceiling and work now moves to the attic. The insulation is brushed away from the drywall ceiling cutout for the new vent boot and air register:

Install a Room Air Vent: Drywall Ceiling Cutout - Attic View

Install a Room Air Vent: Drywall Ceiling Cutout – Attic View

Here’s the same view from inside the bedroom.

Drywall Ceiling: HVAC Register Cutout - Bedroom View

Drywall Ceiling: HVAC Register Cutout – Bedroom View

A 2×4 frame is made to mount the vent boot and register box. I used 3 inch wood screws to fasten the frame to the joists. Notice the vent boot at the left side of the photo to check the fit with the wood box frame.

Air Vent Boot Box Frame between Attic Joists

Air Vent Boot Box Frame between Attic Joists

Connect the Vent Boot to the Flexible Air Duct

The insulation jacked it pulled back to reveal the flexible duct inner duct. The inner duct is then slipped over the vent boot:

Room Air Vent: Flexible Duct Inner Core and Vent Boot

Room Air Vent: Flexible Duct Inner Core and Vent Boot

The flex duct liner is secured to register boot with metal foil HVAC tape for an air-tight seal. (Do not use the cloth “duct tape” because it will not last!) The HVAC metal foil tape makes an air tight seal between the flex duct and vent boot:

Air Duct Installation: Flexible Duct Taped to the Vent Boot

Air Duct Installation: Flexible Duct Taped to the Vent Boot

The flexible duct outer insulation jacket is pulled over the vent boot and mechanically fastened with a large nylon zip tie:

Air Duct Installation: Flexible Duct Zip Tied to the Vent Boot

Air Duct Installation: Flexible Duct Zip Tied to the Vent Boot

The register box is fastened to the wood box frame with 1/2 inch self-tapping sheet metal screws. The screws are fastened from the inside:

HVAC Vent Boot Mounted in Ceiling

HVAC Vent Boot Mounted in Ceiling

Seal the Duct Boot and Drywall

The new energy efficient building codes requires the gap between the metal vent boot and the drywall be sealed with caulk (see diagram on page 14) to prevent air leaks. Caulking is easiest option because it can be applied from inside the room in a clean and precise bead.

Vent boot sealing wasn’t required by the building code when my home was built. I’ll wait until another day to seal all the vent boots while working inside the attic.

Install the Duct Board Start Collar

The flexible duct branch line will be connected to the rigid duct board trunk with a flex duct start collar (also called a “take-off”). Always connect the branch duct to a trunk duct for the best air flow. An outline is traced in the rigid trunk duct and cut out with a utility knife. The start collar and take-off hole in the duct board trunk:

Flexible Duct Branch Line: Start Collar and Hole Cut in the Duct Board Trunk

Flexible Duct Branch Line: Start Collar and Hole Cut in the Duct Board Trunk

The start collar is placed into the rigid duct board trunk. The start collar mounting tabs (or fingers) are inside the rigid duct board trunk. The fingers are bent outward against the duct board trunk to hold the starting collar:

Flexible Duct Start Collar Installed in the Duct Board Trunk

Flexible Duct Start Collar Installed in the Duct Board Trunk

The start collar fingers are bend outward, then the start collar is sealed to the rigid duct board with metal foil HVAC tape. Mastic wasn’t required when my home was built, however sealing with mastic and fiberglass mesh reinforcing tape is required by the new building code.

Starting Collar Sealed with Metal Foil Tape to the Duct Board Trunk

Starting Collar Sealed with Metal Foil Tape to the Duct Board Trunk

Connect the Flexible Duct to the Start Collar

The flexible duct inner liner is pulled over the start collar and fastened with two layers of metal foil tape as was done at the vent boot.

HVAC Flex Duct Branch Line: Duct Liner Pulled over the Start Collar

HVAC Flex Duct Branch Line: Duct Liner Pulled over the Start Collar

The outer insulation jacket is next pulled over the start collar and secured with a nylon zip tie. Notice the gentle bend in the flex duct – it is very important to avoid kinks and pinches in the flexible duct that will restrict the air flow:

New HVAC Branch Line Flexible Duct

New HVAC Branch Line Flexible Duct

Tip: The flex duct branch line and start collar can be installed in the side of the duct board trunk to avoid that extra bend by coming out the top of the trunk if you have sufficient room and access.


The compressor was failing on my 17 year old central AC system. The AC contractor recommend replacing the rigid duct board distribution boxes (above photo) with round sheet metal duct. The HVAC technician is mounting the saddle take-offs for the flex duct branch lines here:

Central Air Conditioner Installation - Round Sheet Metal Trunk Saddle Take Offs for Branch Ducts

Central Air Conditioner Installation – Round Sheet Metal Trunk Saddle Take Offs for Branch Ducts

The new flexible duct branch line into the attic above the bedroom is shown in the following photo. The flex duct should be covered with more blown insulation to reduce heating and cooling losses in the attic. The attic is the hottest and coldest part of the house and overall the worst place to run ductwork. I plan to upgrade my attic insulation later.

HVAC Flexible Duct Branch Line Installation

HVAC Flexible Duct Branch Line Installation

The new ceiling vent register is attached with two screws included with the unit:

Flex Duct Branch Line: Ceiling Air Register

Flex Duct Branch Line: Ceiling Air Register

I turned on the furnace and checked the air flow from the new register. I was pleased to have a strong air flow that matched the other ceiling register – which is expected since both branch lines are attached to the same central duct. My daughter’s room is now warm and comfortable.

Attic Fiberglass Batt Insulation

The blown-in white insulation did a poor job of covering the attic above the bedroom. Later I installed the Solatube Skylight and added a new layer of unfaced Owens Corning R-30. Unfaced insulation without a vapor barrier is required because it’s laid on top of the blown insulation and needs to “breath” to prevent condensation. R-30 insulation is quite thick as you can see by the section in the center of the photo. This made a noticeable improvement in the year-round comfort of the bedroom.

R-30 Fiberglass Insulation Batts

R-30 Fiberglass Insulation Batts

Take care,
Bob Jackson

Copyright © 2019   Reproduction strictly prohibited.


  1. Frank November 29, 2009 at 2:05 pm - Reply

    How did you deal with the fiberglass in your clothes and skin? I have an attic and would like to work in there, but with the white insulation it is very messy and gets over everything.

  2. Bob Jackson November 30, 2009 at 8:38 am - Reply

    Working with fiberglass for an hour or two on a small job – a couple of rolls of insulation – doesn’t bother me. I do wear long pants and long sleeves. A dust mask and gloves are recommended, but I’ve not been bothered handling the glass with my bare hands. It probably depends a lot on your skin sensitivity. Take a shower afterwards and run your work clothes through laundry in the normal way. You will see a lot of very short fibers and dust floating in the beam of the flashlight when working in the attic, so a dust mask a good choice.

  3. john January 21, 2010 at 3:24 pm - Reply

    Before you touch any insulation please note that if it was laid before 1995 it may contain asbestos. If so do not touch it get the removers in a dust mask is not enough and 20 to 40 years down the line a slow and painfully death is a possibility. Look up HSE asbestos as a general guide

  4. Alan February 12, 2010 at 4:06 pm - Reply

    I have just converted my 2 car garage into a game room and was going to branch 2 lines into it off my central system. Do I need to put a return air vent into the main house where the air handler gets its return air for the main house?

    • Bob Jackson February 12, 2010 at 5:23 pm - Reply

      Yes, you need a return air vent from the converted garage back to the main house. It’s best to locate the supply air vents near the windows (if possible) and install the return vents on the opposite side of the room to mix the air.

      Also think about replacing the garage entry door with a standard interior door as the original door would have a weather seal to prevent drafts from the unheated garage. The gap between the bottom of an interior door and the floor is a significant contributor to the return air flow pathway.

  5. Matt May 29, 2010 at 3:44 pm - Reply

    Thanks so much for this How To guide. I will be tackling the exact same job this weekend and your guide was a great help!

  6. Brian McFarlin February 8, 2011 at 12:30 pm - Reply


    Your How to Guide will definitely come in handy with the project I’m about to tackle…which is almost identical to yours.

    I’m curious, is there a method to determining the best ceiling location to add the new register? My current register is in the NW corner of the room. The door to the room is in the SW corner…and there is a return air vent just outside of this door. I was thinking of putting the new register in the SE corner of the room (that is, diagonal from the current register).

    • Bob Jackson February 8, 2011 at 7:06 pm - Reply

      I take it you have a room over a garage with an A frame roof and dormer windows. The southeast corner would be an good choice. A better choice is the northeast corner because it’s the farthest away from the door & ceiling air return duct in the southwest corner. The idea is to avoid dead spaces and equalize the air circulation. However, if the attic trusswork or low rooflines make it too difficult to install a new ceiling vent in the NE corner, the SE location should do fine.

      Do choose a ceiling register with adjustable louvers and vanes to adjust both the volume (balance) and direction of air flow.

  7. Brian McFarlin February 13, 2011 at 12:53 pm - Reply

    Thanks for your response Bob. I appreciate your willingness to respond to questions about such an old project.

    When adding the additional register to your daughters room, were you at all concerned about how it would affect the amount of air coming out of other registers in the house? Was there a notable difference in the other rooms?

    • Bob Jackson February 13, 2011 at 4:53 pm - Reply

      Your central AC system shouldn’t be running so close to it’s maximum capacity that adding a single register would starve the airflow from other rooms. If you install a ceiling register with adjustable louvers as I recommended, you can adjust (restrict) the airflow to equalize it with the other ceiling register(s). This might be necessary if the new branch line is taken off the main trunk where the air pressure is higher.

      As for my home, I have two 3-ton compressors and air handlers which independently serve the upstairs and downstairs living spaces, so overall system capacity wasn’t a concern.

  8. Pete May 17, 2011 at 12:10 pm - Reply

    My house suffers from probably what most others do — 2nd floor hotter than the rest of the house in summer and colder in winter. I have a colonial with an integral two car garage with the master suite over the garage. The temperature variance is greatest in that room. I have attempts to adjust this by adjusting the baffles in each distribution line (where I can get to them), and opening/closing the registers each season. Is there a better way without costing a great deal? Would professionally balancing the system help?

    Thanks for your reply

    • Bob Jackson May 17, 2011 at 4:37 pm - Reply

      You’ve done the basics by adjusting the baffles and registers. I assume the room is too hot or cold even if the door is open, therefore return airflow isn’t an issue. Your options are better air supply and/or better insulation.

      Air supply: Short of installing new duct work, an in-line booster fan might help. I have not tried these and your mileage may vary.

      Insulation: Better insulation will reduce the overall heating and cooling needs. Is the space between the garage ceiling and the 2nd room floor insulated? Drill a small inspection hole or two in the garage ceiling to find out. There are companies that can install blown insulation inside existing walls.

  9. Jim June 23, 2011 at 11:15 am - Reply

    I am partiioning a large room with two supply ducts and want to move one suply to the new room. It is a first floor bedroom and was hoping to avoid ripping down the drywall ceiling. Do yo know of any tricks/products avaialble to accomplish this? Thanks, jim

    • Bob Jackson June 23, 2011 at 7:37 pm - Reply

      If you don’t want the mess and trouble of tearing open the ceiling drywall to reroute the ductwork, then build a new run for the duct extension along the corner of the ceiling with 2×4 framing and drywall as so:
      Ceiling Ductwork Extension Design

      The construction detail for the ductwork extension is:
      Celing Ductwork Extension Design Detail

      Key construction elements are:
      1) Remove the ceiling register and vent boot from the vent to be moved.
      2) Build the box to conceal the new ductwork along the top corner of the room from the old vent through the wall partition to the new room. This technique is not uncommon for remodels and blends in nicely when finished and painted.
      3) Run new flexible duct of the same size through the box framing and keep the corner turns wide and gentle for the best airflow.
      4) Tie the new ductwork into the old ductwork in the ceiling and seal with HVAC tape. If you can find a rigid duct connector for the two sections, that’d be best.
      5) Install a new vent boot and register in the new room partition.

      An air return duct will be needed in the new room. You could build a return duct the same way as the supply duct extension, or if noise won’t be a problem, install a pass-through grill in the wall partition.

      Suppose the old vent is a few feet from the wall and building a duct extension 90 degrees out from the wall would look silly. Just cut a hole in the ceiling next to the wall between the floor joists (make sure it’s the same joists where the air duct is located!) and with long arms, reroute/extended the old air duct to the wall and into the new corner box you’re building to the new room. You’ll have a small amount of ceiling drywall repair to cover up the old vent hole.

  10. Jim June 24, 2011 at 12:17 pm - Reply

    Hello Bob,

    Great solution as it involves new construction rather than the mess of tearing up the ceiling.

    Thanks for the advice. Jim

  11. Alan June 25, 2011 at 7:40 pm - Reply

    I live in a 3 story townhouse, with the lower-level being the basement, the main level consisting of the living room, kitchen and 1/2 bathroom and the 2nd level having the master bedroom/bathroom, 2 smaller bedrooms and a full bathroom in the hall. Each level is approximately 800 sq feet (20 ft wide X 40 ft deep. The attic has the trunk line (from the basement) that has (6) 6″ flexible duct lines going to all the rooms upstairs, with 2 going to the master bedroom.

    My problem is that the 2 smaller bedrooms never get comfortable in the winter or summer unless the thermostat is set to an extreme temperature while the master bedroom becomes excessively warm or cold. I went into the attic and tried moving the flexible ducts to make them straighter and also added at damper to one of the ducts (at the trunk)going to the master bathroom. I plan on adding more dampers to the other two ducts going to the master bedroom to see if this improves airflow to the smaller bedrooms.

    I should also add that there are 2 electronic dampers in the basement on the trunk line that supplies the heat/AC to the basement and main level. I was told I have a 2-Zone system (not 100% sure). The thermostat in the hallway upstairs turns the system on/off, along with being able to switch the Heat/AC on/off, and the thermostat on the main floor can do everything except turn the system on/off.

    The air return is at the top of the second level near the ceiling.

    I am looking for possible ideas to correct the situation, including adding second ducts to the smaller bedroom (directly frim the trunk), adding more dampers, etc… I need ideas and/or solutions..

    Thanks in advance..


    • Bob Jackson June 26, 2011 at 8:41 am - Reply

      > The air return is at the top of the second level near the ceiling.
      So there’s just the central air return in the 2nd floor hallway and no air return ducts in the two smaller bedrooms?

      Step 1:
      How strong is the air flow to the two smaller bedrooms on the 2nd level? Is it about the same as coming from the ducts in the master bedroom?

      Step 2:
      Does the air flow to the smaller bedrooms change noticeably when the door is closed? Are the smaller bedrooms uncomfortable all the time or only when the door is closed?

      A) If the airflow to the smaller bedroom(s) with the door open is significantly weaker than the master bedroom, balancing the air flow with dampers could be a solution.

      B) If the airflow drops significantly with the door closed, then a new return vent and duct would be appropriate, but may only be a marginal improvement.

      C) Since you have attic access to the trunk line and six existing 6″ supply duct lines, the ideal solution would be to install a new 6″ duct line from the trunk to each bedroom with a new 6″ return air duct line to each room. Install the new supply vents on the opposite side of the room away from the existing vents and doorway. Place the return duct near the door. Use an adjustable register on the new vents to adjust (balance) the air flow; simpler than dampers inside the ductwork and easier to tweak.

      The advantage of new duct lines is it not only increases the airflow, but diversifies the airflow to equalize the room.

  12. Alan June 26, 2011 at 2:00 pm - Reply

    Thanks for the prompt reply Bob…

    > The air return is at the top of the second level near the ceiling.
    So there’s just the central air return in the 2nd floor hallway and no air return ducts in the two smaller bedrooms? <<

    Yes, there is only one central air return. Nothing any of the bedrooms.

    The existing 6" air duct line in each of the smaller bedrooms is in the center of the ceiling and you're advising me to put a new 6" air duct line (from the trunk)at the opposite end… I don't really want to move the exisiting one(s) unless I have to so i am wondering if I should put the new ducts close to wall that has the windows, which is opposite the wall that has the door??

    As for putting in the new 6″ return air duct lines; can I use flexible duct or do I have to use rigid??

    I haven’t had a chance to check about air flow changes as of now.

    Thanks again…

    • Bob Jackson June 26, 2011 at 4:07 pm - Reply

      Right – don’t move the existing air ducts. It’s best to install the new air ducts above the windows at the opposite wall from the door. Windows are a source for the heat & cold from the outdoors even when closed, so putting the new vents in the ceiling above the window helps to mediate the hot/cold spot here. Since the window is at the far side of the room opposite the door, that’s even better so the air will travel the full length of the room to reach the new return vent above the door.

      > As for putting in the new 6″ return air duct lines; can I use flexible duct or do I have to use rigid??
      Flexible duct is fine for a single return branch line. Flexible duct is internally supported by a wire coil and the negative return air pressure is not much, so the flexible duct won’t collapse. All the individual room returns in my home (built in 2002) are flexible duct lines back to the rigid trunk line.

  13. Alan June 26, 2011 at 4:27 pm - Reply

    Thanks for the information on placement of the new returns/supply ducts. Before I make that move I am going to reinstall the existing lines from the trunk, eliminating the bends coming off the trunk – I was reading that the bend/angles should occur 1 or 2 feet from the trunk. I will use the proper strapping to hang them if necessary and use just the length needed.

    When I can I will take some before and after pictures. I am learning that houses, condos, & townhouses built in masses tend to use inferior parts, or not at all – none of the existing supply ducts have dampers in place, just the one that I put in… They are also lazy when doing things the right way, so I am going to “MAKE IT RIGHT!” –> stolen from Mike Holmes!!

    • Bob Jackson June 26, 2011 at 6:51 pm - Reply

      Before and after photos would be welcome; I’ll post them here with your permission.

      > They are also lazy when doing things the right way, so I am going to “MAKE IT RIGHT!” –> stolen from Mike Holmes!!
      Quality and workmanship are often problems on any home. I once converted a covered porch into a new room on former house in Florida. The blueprint had to be signed off by a certified Civil Engineer for wind loading (hurricane country) to obtain the building permit. The Civil Engineer I hired was mostly working on expensive mansions and condos as a independent inspector for the homeowner. He showed me notebooks of photos with building code violations, shoddy work and botched jobs. He said quality is often a problem no matter what the house’s price range. Enter “goober” in the search bar at the top right corner of for examples of poor workmanship I’ve found and corrected on my home.

  14. Jessica July 7, 2011 at 9:17 am - Reply

    We are going to add another vent to our son’s bedroom because it gets very hot during summer and cold in winter. The room has 2 1/2 outside walls and one large window. The one wall that is entirely an inside wall has a bathroom door and a closet, so that wall is pretty full already. The only vent into the room now is under the large window. Is it correct that we should place the other floor vent on the other outside wall? I want to make sure that we get good air circulation in the room. Due to the design of the room, the other place we are thinking about adding it due to ease of install is in front of one of the sliding closet doors. The intake vent is above the bedroom door which is the wall opposite of the one we are thinking of placing the new vent on. Any resources/information for vent placement?

    • Bob Jackson July 7, 2011 at 5:24 pm - Reply

      > Is it correct that we should place the other floor vent on the other outside wall?
      That would be the best choice. Supply vents should be located by windows (as you already have) or as far away from the entry door as possible to maximize the distance the air travels to the return vent for equalization.

  15. Mike January 19, 2012 at 7:57 am - Reply

    Bob – The home we recently purchased has a “bonus” room upstairs above the great room that was added after the home was built. It’s approx 20 x 9 and has a spiral staircase going up. It appears reasonably well insulated (examined from attic). We have 2 ac units and there are 2 vents from each going to the attic room, but there is no return up there. The only way for air to get out is to be pull down through the spiral stair entrance by the main returns in the great room. The air always seems stale and it’s uncomfortably hot in the summer – even with both ac units feeding the room. Would it make sense to add a return (or returns) up there? Thanks.

    • Bob Jackson January 19, 2012 at 3:53 pm - Reply

      How does air in the room feel when the door is open for an hour or two? An open door is the same as a return vent. If the room is comfortable when the door’s open, then add sufficiently sized return duct. If not, then a larger air supply is needed to keep up with the summer heat.

      See my advice dated December 14, 2011 at 11:00AM in this post for duct sizing calculations.

  16. Mike January 20, 2012 at 9:04 am - Reply

    Bob – Thanks for the reply. There is not a door in the traditional sense. The sprial staircase goes up from the great room but the opening at the top is probably the same square footage as a door. I’ll check out the link you provided. Thanks again.

  17. Gardner March 2, 2012 at 11:17 am - Reply

    Thank you for the great post! I have to do the exact same thing. I have a question about placement though.

    Sorry for the long description-A diagram is worth 1000 words!

    We have a combo kitchen/utility room about 8′ x 24′. There is an exit to the back yard on the north (8′) wall. On the very far southern side of the east (24′) wall there is a door to the dining room. The air return is in the hallway on the other side of the dining room from the kitchen.

    We’re dividing the room with a wall and pocket door so the kitchen (south end) will be about 8′ x 15′ and the utility room (north end) will be about 8′ x 9′. There will be windows on the west (24′) wall in the kitchen at about 4′ and 13′ (on center) from the south wall. And a window in the utility room on the west wall about 6.5′ (on center) from the north wall.

    Currently there is one supply vent on the ceiling about 7′ from the south wall (4′ from the east and west walls). We want to add one more supply vent but I’m not sure of the best location. Initially I thought it best to place it in the utility room, just north of the new pocket door and leave the original vent where it is. However, space is getting tight since I want to install a small secondary attic entrance in the utility room and there is a ceiling light as well. Also seems like the air would just be blowing right out the door to the back yard.

    So, then I started thinking that if I placed the new vent on the northern end of the kitchen (just south of the pocket door) enough air would get thru to the utility room (essentially just laundry and storage) and the kitchen would be better cooled. (I would probably move the original vent further south so the two were equally spread out in the kitchen.) It would also keep the new vent farther from the exit door on the north wall. However, now worried that if we end up keeping the pocket door closed a lot, it would get stuffy and perhaps damp in the utility room. We’re on the gulf coast and prone to mold and mildew.

    Any thoughts or help would be most appreciated. Also is there any issue with the proximity of the vent to a ceiling light fixure?

    Thanks again.

    • Bob Jackson March 3, 2012 at 10:22 am - Reply

      What I would do is:
      1) Install a new supply vent with louvers in the utility room for temperature and humidity control. The air flow volume can be balanced by opening or partially closing the louvers. The new ceiling vent is ideally placed above the window.
      2) Make an air return path from the utility room by ensuring there’s a 1 inch gap between the floor/carpet and the bottom of the pocket door; or install a simple pass-through vent in the new wall partition a foot or so above the floor.
      3) Relocate the existing kitchen supply vent from the south wall to the northwest side of the kitchen above the 2nd kitchen window, inside the new wall partition. This will equalize the air flow by making it travel diagonally across the kitchen towards the return vent in the dining room hallway. HVAC vents are ideally near windows where it’s warm in the summer and cool in the winter to minimize hot/cold spots.

      > Also is there any issue with the proximity of the vent to a ceiling light fixture?
      I’d keep the vent at least 12 inches away from the light fixture, preferably 3 feet or more. You might want a recessed light fixture (gets hot) and if you were to decide a ceiling fan was better at a later date, the vent wouldn’t be directly above the fan blades.

  18. Gardner March 3, 2012 at 4:44 pm - Reply

    Hi Bob,
    Thanks for the quick reply and answers!

    I have a follow-up question to your answer #3

    > 3) Relocate the existing kitchen supply vent from the south wall to the northwest side of the kitchen above the 2nd kitchen window, inside the new wall partition. This will equalize the air flow by making it travel diagonally across the kitchen towards the return vent in the dining room hallway.

    I should have mentioned that directly across from this new window will be our 43000 BTU stove and a 450-500 cfm exhaust hood. We do a lot of high heat cooking, which is when we need the AC the most. If we move the old AC vent to above the original window then it will be right across from the door to the dining room (and the air return vent). So maybe it makes sense to leave the original vent where it is, right in the center of the new kitchen’s floor plan. Is there another option I’m missing?

    Thanks again,

    • Bob Jackson March 4, 2012 at 9:05 am - Reply

      With the extra details you’ve described, yeah – leave the AC supply vent where it is. You can always move it later.

  19. Gardner March 4, 2012 at 3:13 pm - Reply


    Thanks again for the article and the quick answers!


  20. June April 7, 2012 at 9:38 am - Reply

    We are turning part of porch into a room. How do vent this space

    • Bob Jackson April 7, 2012 at 10:53 am - Reply

      You’ll need to look at your ductwork and work up a plan for running new supply and return branch lines to/from the room. The method for running new branch lines will vary greatly depending your ductwork configuration and accessibility. Is the ductwork in the attic or floor crawlspace? Can you get access to the main trunk to tap off the branch lines? What’s the best way to route the branch lines – in the wall, ceiling, floor? It’s difficult to give specific advice without knowing the particulars of your home.

  21. Ashley April 18, 2012 at 10:31 pm - Reply

    Dear Bob,

    Many helpful insights and advice on this comment board!

    My husband and I are renovating the finished basement of our 1940’s brick townhome in Washington, DC to ready it for a tenant. We hired a contractor to convert a 6′ x 12′ “storage room” into a bedroom. The storage room currently exists outside of the original foundation (it is located under our front porch) and connects to the main basement living area through an interior/exterior door.

    The contractor has sealed and insulated the outside walls of the renovated room and we are now determining how best to run a duct from the main living area into the room, through the cinderblock foundation. The ductwork is located between the basement ceiling and main floor, luckily bording the wall between the renovated room and the main living space. However, both the outflow and return ducts are located side by side, so in most rooms of the house the outflow and return vents are placed on the same wall, 2′ – 5′ apart (not ideal).

    Here are our questions:
    1. Do you have any tips for running the ducts through the cinderblock foundation?

    2. Considering that the outflow and return vents would be located on the same wall in the renovated room, is it even worth installing a return vent?

    Many thanks for your insights.

    • Bob Jackson April 19, 2012 at 7:27 am - Reply

      > 1. Do you have any tips for running the ducts through the cinder block foundation?

      The cinder block foundation is load bearing and you must provide a means to transfer the load if a hole is made in the wall to run the ductwork to the new room. The 6ft x 12ft room is very small, so a 4 inch diameter branch duct will do.

      The installation process is:
      1) Purchase an angle iron lintel and cut it so it’s two cinder blocks in length.
      2) Grind out the mortar joint above the cinder block where the new duct will be run. Also grind out the mortar joint 1/2 the length of the cinder blocks to the immediate right and left of the main block.
      Grind out the mortar joint only as deep and thick as the angle iron lintel. A masonry blade on an angle grinder should do nicely. It will generate huge amounts of dust! Have a helper hold the nozzle of a shop vac next to the grinder wheel to catch most of the dust.
      Test fit the iron lintel and tailor the mortar joint until it fits.
      3) Mount the lintel in the mortar joint with an appropriate masonry epoxy or Type S mortar.
      You now have a load transfer system over the cinder block where the new duct will be routed.
      4) Mark a 5 inch circle in the center of the left or right side (hollow core) of the cinder block, avoiding the center rib if possible.
      5) Drill a series of 1/4 inch holes with a masonry bit along the marked circle, leaving a 1/4 inch gap between each drill hole.
      6) Punch out the 5 inch hole with a small mason’s chisel and hammer, working between each drill hole. Clean up the edges of the knock-out so it’s smooth.
      7) Reaching inside the hole you’ve just made in one side of the cinder block, drill a pilot hole out the far side to mark the center.
      8 ) Repeat steps 4 to 6 on the other side of the block, using the pilot hole in step 7 as the center to mark the perimeter of your hole.

      > 2. Considering that the outflow and return vents would be located on the same wall in the
      > renovated room, is it even worth installing a return vent?

      A return vent isn’t necessary if you leave a 1 inch gap between the door bottom and floor or carpet.

  22. LEN April 19, 2012 at 9:49 pm - Reply

    Hi Bob,

    This comment board is fantastic! Thanks for doing this!
    I recently built an extra room in my garage that I will use as an office. The room is 10′ x 13′ and my air handler is located directly above this room. I would like to run a branch into this room. There are two doors to this room. One is an exterior door and the other a standard door leading to the garage. What would be the best way to do this? Would the same supplies for the project you did get this job done? Your input is greatly appreciated!

    • Bob Jackson April 20, 2012 at 4:29 am - Reply

      With the air handler directly above the new room, it’ll be easy to install new supply and return duct branch lines off the main trunks to the ceiling.

      Install your new supply duct near an exterior window or door. The best place for the return at the opposite side of the room from the supply vent, or above the interior door.

      The same ductwork installation materials that I’ve shown here are all you need for the attic branch lines.

  23. Henry May 11, 2012 at 7:56 pm - Reply

    Bob, Fantastic blogs! I am going to do this exact project this weekend! I live in central Florida.
    My son’s bedroom is the only bedroom with a “vaulted” ceiling; about 14 feet. To save some money, we have beentrying to keep the thermostat around 80 degrees. His room is ALWAYS 8-12 degrees more! It is very uncomfortable. The room only has one 5″ supply duct with a 8″x12″ register…near the TOP of the wall!
    the A/C is heated up before he can feel it as it travels down. I want to add a register on the next wall 90 degrees but much lower about 8 feet off the ground. I already cut vent hole above the doorway across the room, close to the ceiling to let out some of the hot air. I will take some before and after shots and temp readings for your readers.
    Thank YOU for all your help!

    • Bob Jackson May 12, 2012 at 8:06 am - Reply

      The AC ductwork for the room seems undersized. Vaulted ceilings are often a problem because it’s next to the roof with no space for extra insulation and incurs lot of heat transfer from solar heating into the room. I look forward to your before & after results.

  24. Cary July 24, 2012 at 11:58 pm - Reply

    Hello Bob,
    I really like the way U explain everything in detail without fanfare.
    The best to U.

  25. Brian January 3, 2013 at 9:42 pm - Reply

    Hi Bob I’ve been reading through your blog here and it is all great info, I am getting ready to add an air register in my garage to try and moderate the temperature in the garage as currently it is extremely hot in the summer and extremely cold in the winter. I planned to run a new line off my attic unit, but my question is do I need to add a return air line and if so how do I go about doing this? My other option is to ‘t’ off an existing duct line that is in my master bedroom. Thanks for any help you are able to give.

    • BobJackson January 3, 2013 at 10:21 pm - Reply

      Have you checked the garage exterior walls and attic space for insulation? Garage walls and attics are normally not insulated by the home builder. Uninsulated walls and attic will make it very difficult to keep the room heated or air conditioned. You can hire a company to inject insulating foam into existing walls. Attics are much easier to insulate with fiberglass batts or blown insulation.

      > but my question is do I need to add a return air line and if so how do I go about doing this?
      A return air duct and branch line would be the best solution to ensure maximum airflow. Install the return duct the same way as the supply duct. Install the supply air vent on the far side of the room by the windows and place the return air vent on the opposite side of the room near the door to the main house.

      An alternate approach for the return air is to leave a 1 inch gap between the bottom of the entry door from the garage to the house. Since the entry door to a garage normally has weatherstripping, you’ll most likely have to replace the exterior type door with a standard interior door when converting the garage to air conditioned living space. You can try this easy solution first and if the garage is still too warm, open the door for several hours to see if it makes a difference. If the garage is only comfortable with the door open, then you’ll need to install a return air line.

      If the garage is comfortable with the door open, instead of replacing the door another option is to install a simple pass-through grill on both sides of the drywall between the studs above the door. Use a vent grill that’s at least 25% larger in area than your supply air register. Be careful not to cut the load bearing studs and locate any electrical wiring. A stud finder with an “AC” (Alternating Current) detection option is helpful for locating wiring.

  26. Joseph Ciea February 25, 2013 at 6:40 pm - Reply

    Hi Bob~

    Your post is helpful. I have a question. My arizona room is 30 feet length and 10 feet width (this room was an conversion from a patio). I am going to build a wall in this room to make a fourth bedroom. The room currently has one vent on the ceiling for the entire room and it is located 2 inches away from the proposed wall. We can’t move the wall closer to the vent due to a window being in the way.

    An AC guy came and he wasn’t helpful or give me any idea to a solution. He stated we can’t move the vent or add a vent because of the 2×4 beam fountain between ceiling and roof. There is no room to do anything. I was wondering if we could use the current vent (6″x12″ vent size) and split the vent into the proposed fourth bedroom and the family room by attaching it to the proposed wall? Can current vent  attach extend by use the flexible ducts & 6-in x 12-in Galvanized DuctUsed for in-line transition? After that, add 2×4 by where flexible duct set and drywall cover the extension in the room?

    If this is not an option- how far apart should the new wall be from the current vent on the ceiling?

    Your feedback will be very helpful- thanks.

    • BobJackson February 25, 2013 at 9:43 pm - Reply

      Without running the ResDuct™ – Residential Air Duct Calculator, the 6in by 12in duct in a 30ft x 10ft room strikes me too small for the room, but you would know better living in the Arizona desert.

      See my comment dated December 14, 2011 at 11:00 am in this project for more details air duct sizing.

      1. Run the air duct calculators (see above links and discussions)
      2. If the calculators indicated the 6in by 12in air duct is sufficient for the 300 sq. ft. room, go ahead and try splitting the air duct.
      Do provide an adequate return air flow pathway in the new room. See my other comments for return air flow options.
      3. Should your experience with the existing comfort level in the room and/or air duct calculators show that the 6in x 12in duct is undersized, consider building an L-shaped interior soffit to tap into the main trunk line in another part of the house and route air to the new room along the ceiling and wall to avoid the 2×4 ceiling joists. See my soffit illustration in the comment June 23, 2011 at 7:37 pm (scroll up). I have an interior soffit in my home.

  27. Scott June 8, 2013 at 12:31 am - Reply

    Hi Bob, thanks for putting your work out there. Great information for us beginners/DiYers! I’m planning to tackle the challenge soon on an exterior storage room that is attached to main house and insulated but only accessed through exterior door under breeze way. The room stays noticeably cooler than the garage just with the insulation. It will be used as a dog house for the summer months for my aging dog.

    If I understand correctly, once the supply is installed, I can run a return the same exact way OR I can leave a crack at the bottom of the door which will allow for the exchange? Is one of these methods preferred for the use I am looking at for the room? Anything else I’m missing?

    Thanks in advance for your advice, it is appreciated.

    • BobJackson June 8, 2013 at 8:22 am - Reply

      A return air vent & duct is the best solution because it allows for an unobstructed return air path that is being pulled by the air handler.

      > leave a crack at the bottom of the door which will allow for the exchange?
      A 1 inch gap may work, but might not allow for sufficient return air volume depending on the room size. Your project description indicates the storage room has only an exterior door; a 1″ door bottom gap would vent air to the outdoors which would be somewhat wasteful.

  28. Scott June 9, 2013 at 12:04 am - Reply

    Thank you. You are correct that the gap would be wasteful but I mentioned it as I read the suggestion in a previous post. My inclination was to have a dog door serve for the exchange but I think i’ll be best with a return vent and duct.

    • BobJackson June 9, 2013 at 7:27 am - Reply

      You must’ve misread my advice for a different situation. I don’t recommend venting HVAC ducts to the outdoors.

      People have asked me about converting garages to climate-controlled living space. The entry door from the garage to the house is an exterior door with weather stripping to seal out drafts. If that door is serve as return air path, the weather stripping should be removed from the door jambs and the (typically metal clad) exterior door replaced with a lightweight interior door having the 1 inch gap at the bottom for air flow. The air flow is a closed loop system.

  29. Cynthia June 23, 2013 at 9:17 am - Reply

    Thanks for this helpful guide. Your descriptions and photos are just what I was looking for.

  30. Chelle July 11, 2013 at 3:20 pm - Reply

    Hi Bob. I ran across your post looking for advice on our situation. We have a split level house, and the basement below our living room in unfinished. My husband is using it as an office and we noticed it has no vents. Since it’s unfinished, we can see 2 ducts running across the ceiling that heat/cool the living room and wondered if we could just pop a hole in one of them to get some air flowing in the basement?

    • BobJackson July 11, 2013 at 6:30 pm - Reply

      I assume you have flexible air ducts, that are round and feel springy as shown in the project.

      Figure out which duct is the supply and return duct. The supply duct conveys chilled air from the air conditioner to the rooms, while the return duct takes air from the room back to the AC unit to complete the cycle.

      The two branch ducts for the living room are sized to provide an air flow to keep that room comfortable. Splitting the duct could make the living room too hot or cold. You wouldn’t “pop a hole” in the branch duct because:
      * it would be ragged and probably noisy with bits of duct insulation blowing off.
      * the airflow would uncontrolled and undirected.

      The best approach is to install two new supply and return branch ducts from the corresponding trunk supply and return ducts. Install vent boots between the floor joists on opposite sides of the area that’s used as an office, at least 10 or 12 feet apart. Louvered vent registers will provide airflow control.

      If you must tap into the living room air ducts, cut the living room duct in two at the appropriate location near the basement office, install round metal duct wyes to split the airflow, connect the new basement office branch ducts, vent boots and louvered registers. Keep the new branch ducts as short as possible for better efficiency.

  31. Dave Tull December 6, 2013 at 12:30 pm - Reply

    Bob, I am adding a couple of flexible ducts to main floor rooms from basement main trunk lines. Does it matter if I connect to the end of a trunk or should I always come off the side of the trunk line? It seems that the end connection would provide more air flow than from the side but perhaps too much? Thanks

    • BobJackson December 6, 2013 at 8:28 pm - Reply

      Oh yeah! A branch duct takeoff in the end of the main trunk duct will provide more airflow… and that would be bad because it can cause air to be drawn in from the upstream supply vents.

      Branch duct takeoffs must never be installed in the end of the trunk duct because of the Venturi effect. The air flow in the trunk duct has speed and momentum and will travel in a straight line to the branch duct takeoff at the end of the trunk. Air flow doesn’t “like” to change directions and will travel in a straight line if possible.

      The high speed airflow in the trunk causes the Venturi effect at the upstream branch duct takeoffs, which means the air pressure will be lower at those upstream locations and suck air into the trunk duct. The result is no fresh supply air to those upstream vents and rooms. This article has a great example: Rooms Not Getting Enough Air? This Duct Design Flaw May Be to Blame.

      The DUCT SYSTEM DESIGN CONSIDERATIONS by Roger M Hensley, CMS of the Refrigeration Service Engineers Society – The HVACR Training Authority™ states on page 3:
      * Keep branch run starting collars 24 in. from the [trunk] end caps.
      * Never locate a takeoff in the [trunk] end cap.

      The example ductwork layouts in Figures 2 through 5 illustrate branch duct takeoffs from the side of the trunk duct, never from the end cap.

  32. Dave Tull December 7, 2013 at 8:01 am - Reply

    Thanks! Very Helpful.

  33. mark May 17, 2014 at 11:16 am - Reply

    Hi Bob
    I went through all discussions. Very instructive.
    I have similliar issue as many before. I am adding new register in new room partitioned out from thee garage space.
    Does it matter how far from the beginning of the trunk you can install new branch?
    The existing branches are 6′ next 8′ and so on from the begining of the trunk. Can I install new one like 3′ from the begining.

    • Bob Jackson May 17, 2014 at 11:34 am - Reply

      > Can I install new one like 3′ from the beginning?
      A branch duct takeoff located 3 feet from the beginning of the trunk exceeds the recommended minimum 18 inches. Use a wye fitting for the branch duct takeoff and louvered vents to balance the air flow. See my comment dated January 4, 2014 at the end of this project for more information which also has a link to the Residential Air Duct Calculator.


      • mark May 17, 2014 at 9:26 pm - Reply

        thanks a lot..great links..very nice of you to provide a helping hand :-)

  34. Frank May 22, 2014 at 9:32 pm - Reply

    Bob, my back bed room is at the end of a run that supplies 4 other rooms before the air gets
    bedroom. I believe this is a 16 inch diameter run. I’m thinking of disconnecting my bedroom from this run, and running a direct run from the plenum straight to my bedroom. Will this increase my air flow to the bedroom since it will be a direct run instead of at the end of run that feeds 4 other rooms first? My bedroom has one supply that is 8 inches in diameter.

    Also, I noticed that all the runs that come off of my plenum come out of the sides, nothing from the top. Can I attach this new run to the top of the vertical plenum, or should i come off of the side like all the rest?

    • Bob Jackson May 23, 2014 at 4:15 pm - Reply

      > my back bed room is at the end of a run that supplies 4 other rooms
      > before the air gets bedroom. I believe this is a 16 inch diameter run.
      Is that 16 inches inside diameter or outside diameter including the insulation jacket? If it’s 16 inches inside diameter it’s a fairly large duct and I would look at the branch duct configuration for design errors. If the other 3 bedrooms are comfortable then you may only need to install louvered vents in all bedrooms to balance the air flow.

      To your question about installing a new home-run branch from the air supply plenum to the far bedroom… I strongly recommend against doing that because:

      * The new branch duct will probably be a smaller diameter than the existing trunk ducts and imbalance the airflow.

      * Since you ask about installing the branch duct on top of the supply plenum I assume the existing trunk ducts occupy most of the side space.

      The purpose of the supply plenum is to mix the air flow from the air handler. Blowing air has momentum and doesn’t like to change direction. A duct take off on top of the plenum will provide a straight pathway for the air flow, limit mixing and greatly imbalance the system. You also want to avoid creating a Ductopus.


      If you need to rearrange the ductwork my recommendations is to install a rigid duct board trunk distribution box (see the gray shaded boxes at the bottom right & left of the diagram) and attach the branch flex ducts to it. I have this duct configuration in my home.

      The rigid duct board distribution boxes solve the problems of:
      * Not enough space on the supply plenum for duct takeoffs.
      * Equalizing the air flow to the branch ducts

      If you install the distribution box more centrally to the bedrooms it will help to equalize the lengths of the branch ducts. Install the 16 inch trunk duct from the air handler either in the end or top of the distribution box and locate the branch duct takeoffs on the sides away from the box ends.

      Use the ResDuct™ – Residential Air Duct Calculator to estimate or validate branch the duct size. See my comment dated Feb. 25, 2013 for more info.

      You can send photos bob (at) if you’d more specific recommendations.


  35. Frank June 3, 2014 at 7:53 am - Reply

    Thanks so much for your reply. I have learned a lot from your website.

  36. Scott R July 3, 2014 at 3:04 am - Reply

    My wife and I just added a room in the back of the garage. We finished the garage as far as insulation and sheet rock are concerned. We had an extra space in the back of the garage that was 1/4 of the way closed off. We decided to close off the wall completely and make it a room to play music. He currently have an HVAC unit/ Heat pump. There isn’t any air flow in the room currently. It is carpeted and has soundproof foam on the front and back wall. The room is 13′ X 15′. What is your suggestion to run a ceiling vent in there to keep it cool in the summer and warm in the winter? The room is also well insulated throughout. Any advice would be much appreciated.

    • Bob Jackson July 3, 2014 at 1:16 pm - Reply

      Two 6 inch flex branch ducts connected to the supply and return trunks should do it. Locate the supply duct and ceiling vent at the far side of the room away from the door. Place the return on the opposite side of the room. The idea is to promote maximum air circulation. Install an adjustable louvered ceiling vent on the supply duct to adjust the air flow. If connecting to a rigid distribution box then you can put the takeoff on the side of the box (but never at ends). If tapping off a larger flex trunk duct then you’ll need to install a sheet metal wye fitting for the branch line.


  37. Mishelle July 15, 2014 at 11:59 am - Reply


    A couple of years ago we converted a portion of our garage into another room aka…Man Cave. there is a vent in the room directly from the heatpump unit(located directly on other side of Man Cave wall) we have problems heating and cooling the room. We were told we needed to add a return to this room, it seems obvious that the return can be installed very easy directly thru the wall and into the unit just as was done for the vent…does this seem like accurate information we have been given? We want to install the return vent ourselves…any tips?

    Thanks in advance for any suggestions/tips!

  38. Ali September 7, 2014 at 9:29 am - Reply

    Hi Bob,

    Wow love this post. I saw a similar question but the person asking never responded with info. We are enclosing a porch into a family room. 16×24. It has10 feet ceiling. We want to add several vents for heating and cooling from our HVAC. When we replaced several years back we asked for a bigger unit to accommodate this. It is run from our floor. How do we go about this. Run several flexible duct off main box? I was reading about air duct sizing? Thanks for any info you can give.

    • Bob Jackson September 7, 2014 at 12:17 pm - Reply

      Hi Ali,
      A 16×24 foot room should be fine with a single supply and return duct located at opposite ends of the room so the air flow will will fully mix. The supply duct would ideally be located at one outside corner and the return duct at the far end inside corner (diagonally opposite corners).

      The duct size can be estimated as so:
      16 ft x 24 ft = 384 sq ft.

      Using 1.04 Cubic Feet per Minute (CFM) per square foot of air flow requirement, you’ll need:
      384 sq ft * 1.04 CFM/sq ft = 399.36 CFM of air flow. Call it 400 CFM for easier math.

      Plugging the following values into the Residential Air Duct Calculator:

      Supply & Return Duct Type: Flexible
      Airflow – CFM: 400

      Click the Calculate button.

      Round Duct Supply & Return Sizes: Exact Diameter 12.36″ @ 480 fpm

      The 12.36 inch duct rounded up to 13 inches as calculated factors in friction losses. Since 13 inch flexible duct is not a standard size your choices are to either 14 inch diameter duct or use 12 inch flexible duct for the supply and return branch lines.

      This estimate assumes the new flex duct branch lines are < 25 feet long, the room is reasonably insulated and no special conditions exist such as a wall of windows with lot's of sunshine. If these assumptions are good, then you get by with 12 inch flex duct. If not then 14 duct would be better and you can adjust the airflow with louvered vents. The main duct sizing consideration is summertime cooling because you can always use a portable electric heater in the winter for extra warmth.============> Run several flexible duct off main box?
      Take care the “main box” is a trunk line or distribution box and not the supply plenum at the air handler. It’s almost always a bad idea to run a branch duct from the air handler supply plenum because you can really mess up the air balance, pressure and flow. The rigid duct board trunk line or distribution box must be large enough to accept a 12 inch (or 14 inch) start collar for the new 12 inch flexible branch duct. Also see the Duct System Design Considerations to familiarize yourself with the basics.

      Hope this helps,

  39. Ravi October 5, 2014 at 3:18 pm - Reply

    Hello Bob,

    Good info on Air Ducts.

    I have a few questions.

    I bought a house which was built in 1964. The Wall register and return Air grille are located at the intersection of the wall and floor. They are also thicker (3/4″ from the wall) and the flooring (carpet and tiles) was cut to accommodate this thickness of the grille. It looks odd and the flooring needs to be cut whenever someone changes the flooring. I bought new wall registers and Air grilles which are thinner and it creates a gap on the flooring.
    So, I plan to move both wall registers and the return air grille 1 foot away from the floor on the wall.
    1. Can I move the Air grille 1 foot away from the floor anywhere on the wall?
    2. Is there any duct work attached to the current return air grille?
    3. If so, how do I add ducts to the new location?

    4. For wall register, I know duct is attached to it. How do I move it 1 foot up?
    5. How do I connect existing duct to the new location? Do I need to cut open the complete area?

    Please confirm.

    Advance thanks for your clarification.


    • Bob Jackson October 6, 2014 at 9:10 pm - Reply

      Hi Ravi,
      Could you email photos to help me better understand the exact arrangement of your vents? I’ll try to answer as best can I now.

      Floor registers commonly have a flange so it can sit on top of the wood, tile or carpet to accommodate different flooring material thicknesses. A gap between the rectangular register body that fits in the floor opening and the duct vent boot is normally not a problem. The vent boot should be sealed where it enters the subfloor to prevent air leakage in the crawlspace or joist bays. Most of the air will flow through the register louvers while a small amount may blow beneath the register flange, but it’s all going into the room where you want it.

      > 1. Can I move the Air grille 1 foot away from the floor anywhere on the wall?
      You want to relocate a supply air vent from the floor to the wall? That can a very difficult job because you’d have to saw through the 2×4 wall sill or sole plate. You also don’t want to saw the sill or sole plate in a load bearing wall.

      > 2. Is there any duct work attached to the current return air grille?
      Probably not in an older home when the building codes allowed the wall stud cavity to be used for return air. You can check easily enough by removing a grill and looking inside with flashlight for a sheet metal wall stack. See Figure 28-41 in that link for what a wall stack looks like.

      > 4. For wall register, I know duct is attached to it. How do I move it 1 foot up?
      That would be messy and challenging because you’d have to remove the grill, determine which direction the ductwork is going (up or down), cut open the drywall, extend or shorten the ductwork then repair a big hole the drywall.

      > 5. How do I connect existing duct to the new location? Do I need to cut open the complete area?
      Most likely as explained above.

      Hopefully your photos will help us figure out a better solution.


  40. emmett freeman February 15, 2015 at 11:31 pm - Reply

    My home is a two story house, with no heat on the second floor. The only heat is coming up the staircase on one side of the house. If I cut a vent between the first and second floors, will heat circulate throughout the second floor.

  41. Nick Cronk August 18, 2015 at 2:11 am - Reply

    I live in a one story house and the vents are floor level because the furnace is in the finished basement. Recently At&t came and drilled two holes in the exterior walls where the return ducts are, possibly going through it. But I don’t know if, when vents come from a lower level, they run horizontally along the first floor walls, or do they pop up in which case At&t didn’t actually puncture my ducts.
    Please, can anyone respond? Thanks.

    • Bob Jackson August 18, 2015 at 7:20 pm - Reply

      Remove the return vent register and look inside with a handheld mirror and flashlight to inspect the duct for puncture holes. If the return vents are located at or above the baseboard there may be a short section of sheet metal duct called a “wall stack” with metal vent boot between the 2×4 wall studs. Some homes don’t have sheet metal wall stacks and simply use the space between the stud bays as the return duct; there will be vent boot and flexible or sheet metal duct in the subfloor if you look inside the wall.

      Overall a small drill hole in a return duct isn’t a significant concern if the hole is within the building envelope (air conditioned/heated living space). On the other hand, if the hole were in the attic drawing in hot air you’d want to patch it with metal foil HVAC tape or mastic.

  42. Jesus Miron May 16, 2016 at 8:50 pm - Reply

    Bob I am turning my 2 car garage to a man cave, do I need a return vent? And how many a/c ducts do I need to install?

  43. Kevin Leek July 16, 2016 at 11:49 am - Reply

    I have 2 rooms that each have a 6 inch duct, that connect back to an 8 inch duct which goes to the plenum. The split occurs around 12 feet from the plenum, with the one duct going 12 feet to the laundry room (7×12) and the other duct going 18 more feet to a bedroom (11×15). The bedroom doesn’t seem to keep up with the hot Oklahoma summer. We took off both registers and each register box has a 45 degree angle to them. Removing the register covers seemed to make the rooms cooler, some debate from family members. Would turning the register covers a different direction solve the problem? Would partially closing the laundry room register send more air to the bedroom? Would replacing the 6 inch duct that goes to the laundry room with a 4 inch duct force more air to the bedroom and be the best solution? The bedroom is on the southwest exterior wall of the house (hottest side), while the laundry room it in the middle of the house structure with no external walls. Thanks, Kevin

    • Bob Jackson July 16, 2016 at 3:42 pm - Reply

      Hi Kevin,
      Excellent description of your situation!

      The rule of thumb for duct sizing is 1 CFM per square foot of floor space according to How Contractors Really Size Air Conditioning Systems:

      “Of the 79 contractors who supplied estimates of their CFM-per-square-foot rule-of-thumb, 42 (53%) use 1.0 CFM/square foot.”

      1 CFM isn’t scientific but a reasonable value to make some estimates.

      The total floor space of your two rooms is 11×15 + 7×12 = 250 sq ft (rounded up). Running this through the Residential Air Duct Calculator for a flexible trunk duct results in a duct 11 inches in diameter.

      Running the calculator for each room using flexible branch duct results:
      * 7×12 = 84 sq ft laundry room: 7-1/4 inch exact size duct
      * 11×15 = 165 sq ft bedroom: 9-1/4 inch exact size duct

      What you should do is:
      * Replace the undersized 8 inch trunk supply & return duct with 12 inch duct (flex duct is only available in even sizes, so round up 11 in. to 12 in.)
      * Replace the bedroom 6 inch duct with supply & return 10 inch duct.
      * Leave the laundry room 6 inch duct as is. It’s a bit undersized but no one spends much time there, it’s in the middle of the house where there’s no sun loading, and the bedroom has priority.
      * Use a 12x10x6 sheet metal stub wye fitting to connect the laundry and bedroom branch ducts to the 12 in trunk duct. This will maintain the greatest airflow velocity. Cut the flex duct insulation jacket so you can pull it over the stub wye and seal it with HVAC metal foil tape. The wye must be completely insulated to prevent condensation. You probably won’t find a 12x10x6 stub wye at the home improvement stores so try a local HVAC supply at the counter sales desk.

      You must replace the bedroom vent boots and adjustable vents with a 10 inch parts to match the 10 inch supply & return ducts. The supply and return ducts must be the same size otherwise it will constrict the airflow. Adjust the supply side louvered vents to balance the airflow between the two rooms. Use fixed vents (no adjustable louvers) on the return side.

      Also see:
      * How to Install Round Sheet Metal Duct
      * How to Add a Room Air Duct with Speedi-Boot
      * How to Install an Air Duct in a Suspended Drywall Ceiling using Speedi-Boot.


      • Kevin July 23, 2016 at 12:39 pm - Reply

        Thanks for the help, especially the speedy-boot link. have a couple questions. What would happen if I put in a new wye, 8x8x6, then put in an 8 inch duct to the bedroom with 10×10 register? Not sure, but I think our return ducts are replaced by 2 14×30 ducts. Again thanks for the words.

        • Bob Jackson July 23, 2016 at 6:31 pm - Reply

          The air duct calculator (assuming 1 CFM per sq ft of floor space) suggests a 10 inch supply & return duct for the too warm bedroom that currently has 6 inch duct. If you replace the 6 inch duct with 8 inch it will be 79% increase in airflow capacity (see the area comparison below). The question is will that be enough to make the room comfortable on the hottest days? Is the bedroom is 5°F or more warmer than the thermostat setting on hot days, or only 2°F or 3°F degrees? If only a couple of degrees then 8 inch duct should be sufficient. On the other hand you can always throttle excess duct capacity by adjusting the vent louvers, inline damper or both.

          Duct area comparison (airflow is proportional to area):
          6 inch dia = 28 sq in
          8 inch dia = 50 sq in (79% larger vs 6 inch)
          10 inch dia = 79 sq in (58% larger vs 8 inch, and 282% larger vs 6 inch)

          Area of circle = π * r^2.

          > Kevin wrote:
          > “other duct going 18 more feet to a bedroom (11×15)”
          We’ve been focused on upgrading the single supply branch line to the bedroom. Have you considered installing a new (2nd) branch line to the bedroom? An 11ft x 15ft room with two 6 inch branch lines will increase the airflow and improve overall circulation in room by eliminating dead areas. Two smaller ducts and vents will be quieter vs a single large duct & vent.

          * Replace and extend the 8 inch trunk line with 10 inch duct. The idea is to drastically shorten that 18 feet long 6 inch branch line to the bedroom.
          * Install a 10x10x6 Straight Thru Wye off the 10 inch trunk to the 6 inch branch in laundry room.
          * Further downstream use a 10x6x6 Wye Branch to serve the existing and new 6 inch branches to the bedroom.
          * Install a new 6 inch branch line from the Wye branch fitting to the bedroom. The new 6 inch branch line should be installed along the exterior wall away from the existing supply and return vents.
          * Upgrade the bedroom return duct to 8 inch to handle the increased supply air.


  44. Kevin July 23, 2016 at 4:56 pm - Reply

    Thanks Bob!

  45. Kevin July 24, 2016 at 1:21 am - Reply

    One last question. I don’t think we have return ducts in our house. I think we have 2 14in x 30in inlets that we have replaceable filters in. Are those our return ducts?

    • Bob Jackson July 24, 2016 at 11:50 am - Reply

      Those are centralized return ducts; the air filter confirms it’s a return. Hold a Kleenex tissue up to the return grill and it’ll be sucked against it.

      If a room with a door (e.g. bedroom) lacks a dedicated return duct, the room can become overly pressurized when the door is closed thereby limiting the supply air. You can often hear the change in air flow as the door closes, going from a quiet flow when the door is open to a higher pitched whoosh with the door closed. Sometimes the lack of air return pathways will push the door closed within the last couple of inches. This tells you the gap under the door is too small to meet the return air flow requirements.

      An alternative to a return air duct is a through-the-wall or attic transfer duct transfer. See Figures 15, 16 and 17 on page 19 of Advanced Strategy Guideline: Air Distribution Basics and Duct Design published by the U.S. Dept. of Energy. The through-wall transfers are designed to minimize noise transfer. The attic transfer duct connects the bedroom to the hallway.

  46. V. Milasinovic August 6, 2016 at 5:01 pm - Reply

    Bob, not sure if someone mentioned this in the sea of comments above (none of which I admittedly read) :-), but another issue you have in this installation the way it was originally done is that your flex duct is laid across the joists in the attic. Flex duct is required to be supported off the joists via 1.5″ cloth strap spaced out no more than 4′ apart and with no more than 1/4″ sag per foot of duct. This prevents damage to the ductwork, and allows for proper air flow through the duct. It also helps with capacity loss due to kinked duct at each register boot connection. You can loose as much as 60% of capacity just from the 90 degree bend in the ductwork that isn’t designed to be installed in such a way.
    All new construction and remodels, contractors can’t get away with laying flex duct across the joists like that, but even as early as few years back a lot of them still did it this way.

    • Bob Jackson August 6, 2016 at 6:26 pm - Reply

      The flexible duct support requirements you stated are only partly correct and wrong on certain points.

      The 2015 International Mechanical Code – Chapter 6 Duct Systems states:

      “603.10 Supports.
      Ducts shall be supported in accordance with SMACNA HVAC Duct Construction Standards—Metal
      and Flexible.
      Flexible and other factory-made ducts shall be supported in accordance
      with the manufacturer’s instructions.”

      Master Flow (the brand of flex duct I used) hasn’t published instructions – at least that I can find. So let’s see what The Air Diffusion Council ( who “represents North America’s leading manufacturers of Flexible Air Ducts and Air Connectors” says in the Flexible Duct Installation Requirements (ADC IR5E).

      > Flex duct is required to be supported off the joists

      That is incorrect. See Page 8, Figure 24:

      “Flexible ducts may rest on ceiling joists or truss
      supports. Maximum spacing between supports shall
      not exceed the maximum spacing per manufacturer’s
      installation instruction.”

      It’s better to have the flex duct laying on top of the ceiling joists and/or insulation. Way less sag, better support and somewhat less exposure to the hot (or cold) attic air.

      > via 1.5″ cloth strap spaced out no more than 4′ apart and with
      > no more than 1/4″ sag per foot of duct
      It’s 1/2″ inch 1/4″ per Page 7, Section 4.6 and Figure 19: “Supporting shall be provided so that the maximum centerline sag is 1/2″ per foot”. To achieve 1/2 inch or less sag between supports requires either:
      * Closely spaced supports on less than the maximum allowed 4 feet intervals
      * And/or the flex duct to be pulled tight which is only feasible on straight runs when well anchored at ends, e.g. with metal duct fittings. Joists on 16 inch centers is much better and allows for bends & turns.

      > All new construction and remodels, contractors can’t get away
      > with laying flex duct across the joists like that
      I’d be very interested to see the Building Code references forbidding that practice because it’s contrary to the 2015 IRC Mechanical Code and Air Diffusion Council’s installation guide.

      Also see: How to Install Round Sheet Metal Duct where I replaced the trunk flex duct with rigid sheet metal duct.


  47. jonathan September 1, 2016 at 12:13 pm - Reply

    hi there, i have a house where the master bath has 1 supply ac and a separate vent for toilet room, should i install a vent for the shower area?

    another full bath has only ac supply. but no vent, should i install a vent? it also has a small window (which i believe was used to vent the moisture?) there is a huge return vent outside in the hall.

    but baths are on the 2nd floor. this house was built in the late 1990s, i live in dallas texas.

    thanks for the article!

    • Bob Jackson September 1, 2016 at 1:05 pm - Reply

      The Building Code requires a ventilation fan in bathrooms without a window. Building Codes state the minimum requirements and that’s what the home builder did:
      * Bathroom with no window: fan in the water closet (toilet) only but not the main bath
      * Bathroom with window: no fan… open the window.

      See the Bathroom Ventilation Guidelines by The Home Ventilating Institute for fan sizing and placement. The short story is a fan should be installed in each enclosed room, e.g. fan for the main bathroom and a 2nd fan in the toilet area.

      See Panasonic WhisperCeiling Vent Fan Old Work Installation Steps for more info.

  48. doug dewey September 19, 2016 at 6:28 pm - Reply

    hi i have a question i am trying to connect a six inch flex duct to my existing 12 inch flex duct how do i attach the 6 inch starting collar to the 12 inch flex duct thanks doug

    • Bob Jackson September 19, 2016 at 7:07 pm - Reply

      Start collars are only used when taking off from a rigid duct.

      To connect flex duct to flex duct you need a sheet metal Tee Wye fitting. For example this 12″ x 12″ x 6″ Tee Wye. The specifications are:
      * 12 inch diameter main inlet
      * 12 inch diameter primary outlet
      * 6 inch diameter branch outlet

      I doubt the home improvement stores will have this fitting in stock. Order it online or check with a local HVAC supply store.

      Connect the Tee Wye to the flex in the usual way: pull the flex inner core over the end of the fitting, seal it with HVAC tape or fiberglass mesh tape & mastic, then pull the insulation jacket on and secure it with a large HVAC zip tie. Home Depot has 48 inch long duct zip ties.

      Take care to install the Tee Wye such that the Wye points downstream and completely cover the sheet metal fitting with insulation to prevent condensation.


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