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

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, spray foam or mastic to prevent air leaks (see above photo). Caulking is easiest option because it can be applied from inside the room in a clean and precise bead. Mastic and spray foam are excellent sealants but should only be applied inside the attic so it doesn’t mar the painted ceiling because both products are sticky and can be messy.

The vent boot sealing wasn’t required 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”). An outline is traced in the trunk rigid air duct and cut out with a utility knife. Here’s 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:

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 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 installed two bags of Owens Corning R-30 insulation in 24inch by 48inch batts. Two bags of 88 square feet each covered the attic. Cost of each bag is about $65. The 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 comfort of the bedroom. The fiberglass batts were laid over the white blown-in insulation.

R-30 Fiberglass Insulation Batts

R-30 Fiberglass Insulation Batts

Take care,
Bob Jackson

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

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

    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 #

    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 #

    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 #

    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 #

      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 #

    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 #

    Bob,

    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 #

      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 #

    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 #

      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 #

    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 #

      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 #

    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 #

      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 #

    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 #

    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..

    -Alan

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

      > 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?

      Recommendations:
      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 #

    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 #

      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 #

    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 #

      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 HandymanHowTo.com for examples of poor workmanship I’ve found and corrected on my home.

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

    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 #

      > 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 #

    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 #

      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 #

    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 #

    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 #

      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 #

    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,
    Gardner

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

      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 #

    Bob,

    Thanks again for the article and the quick answers!

    Gardner

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

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

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

      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 #

    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 #

      > 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 #

    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 #

      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 #

    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 #

      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.
      Thanks!

  24. Cary July 24, 2012 at 11:58 pm #

    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 #

    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 #

      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 #

    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 #

      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.

      Recommendations/Options:
      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 #

    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 #

      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 #

    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 #

      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 #

    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 #

    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 #

      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 #

    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 #

      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 #

    Thanks! Very Helpful.

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