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Community and Q&A

Garage Ceiling Thermal Break

HhsHwZFSCs | Posted in Energy Efficiency and Durability on

The house is located in Atlanta GA and has bonus room over the garage. I currently have only R11 insulatiion in between the 9″ rafters on the garage ceiling/bonus room floor. I plan to remove drywall and blow in cellulose over the R11 batt insulation. I wanted to attach XPS 1/2″ rigid foam to the underside of the floor joist to help with thermal bridging especially in the Summer when the hot cars are parked in the garage. I will the attach 1/2″ drywall over the XPs foam. The garage faces West so it gets heavy afternoon sun especially with the door open.


1) Will I gain any benefit doing this or in containing the Summer heat or is this overfkill?
2) Is there another approach to address thermal bridging in this situation if so what do you suggest.



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  1. davidmeiland | | #1

    The extra 1/2" of foam doesn't provide much added R value. I would remove the drywall and the fiberglass, get a blower door installed somewhere so that you can completely airseal the garage ceiling, then install the cellulose. If you're going to go with foam, I would try for a couple of inches rather than just the 1/2". You might have to do a bit of detailing in order to reinstall your garage door operator so that it's rigid enough.... can't just screw it to drywall and foam.

  2. Expert Member
    CARL SEVILLE | | #2

    Scott - While the added insulation and thermal break will help, your biggest problem is going to be air infiltration, and if you don't address that, you may find that the insulation doesn't make as much of a difference as you expect. If you have knee walls above the garage ceiling with uninsulated attic areas next to them, make sure the insulation in those walls is installed well and put up foam board or some kind of rigid material on the attic side, sealing all the joints well. Also, block and airseal from the bottom of the knee wall to the garage ceiling drywall to eliminate air movement below the bonus room floor. I would do this first and see how much it helps before you strip the ceiling and re-insulate it. If you do reinsulate, the thermal break will help but in a climate like Atlanta it doesn't make as much difference as you think. Check out this detail:

  3. RMGheHhbJx | | #3

    Hi Scott, Carl is exactly right. Knee walls need to be six sided enclosures just like regular walls, capped top and bottom and backed. Also, Air sealing from the bonus room floor to the garage ceiling is required to take control of the bonus room envelope. Also remember that warm air rises and many times the bonus room is the highest room in the house. By caulking and sealing all penetrations, especially in the ceiling you can offset the stack effect compounding your problems.

  4. user-884554 | | #4

    Better check with your local codes official too if you end up removing the drywall from the garage ceiling. He will likely tell you that the ceiling (and walls) of an attached garage must be drywalled using a 5/8" type x material (fire rated). The above info is dead on. The thermal addition won't be nearly as significant as blocking the air pathways. Tough job I know first hand. Doing the exact same project in Indy. Kneewalls are the main culprit in my case. I have two choices: rip the drywall off the finshed side of the room at the kneewall, or remove a portion of ther garage ceiling like you are considering. Either way, what a pain! I will be using a spray applied single component air sealant (EcoSeal) and backing the kneewalls with 2" ductboard. Fortunately I have top and bottom plates. With your garage oriented to the west, so is a good portion of the attic on at least one side of the bonus room. Without a doubt that is a significant contributor to the current conditions in the bonus room.

  5. HhsHwZFSCs | | #5


    I have three knee walls above the garage. One on the left, one on the right and one to the back. The one to the back has an open run of floor joists. I plan to cut XPS pieces and spray foam them in place around these floor joist section openings. All Kneewalls have 2" XPS on their interior walls. There ceilings are insulated with R 19 batt and 1/2" XPS over it. Angled ceilings are difficult to to seal with rigid foam. Back to my question. To install 2" XPS will cost about $30 a 4x8 sheet and I need 12 sheets. Is the benefit of sealing the thermal bridging worth the cost? Not to mention I will have to deal with moving an electrical outlet and lighing fixture boxes down 2 3/4 inches for the lowered ceiling prior to installing drywall. I do plan to use fire rated drywall. Then there is the garage door rail supports to contend with. You and Carl's opinions are appreciated.

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