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Community and Q&A

Sealing garage below bonus room

icandoamazong | Posted in General Questions on

I’m not sure which category this should go in, but here goes nothing. To finish out a bonus room over a gabled garage, a room that has knee walls, what are the requirements for sealing the room from the garage both from an exhaust fume perspective and insulation? Is making sure the sheetrock seams are taped up and all holes for lights/outlets in the garage ceiling are filled with spray foam good enough for the exhaust fumes? For the insulation, would it work to put rigid foam, sealed with spray foam, at the end of each floor rafter under the knew wall? The current knee wall, on the uninsulated side, has thin foil poly structural panels that are stapled into the studs. For this, do I need to tape off the seams and the ends? ¬†Are these panels okay for the job?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Icando...

    It's super important that you air seal the garage ceiling both to keep hazardous gasses out of the living space and for comfort and efficiency. Perhaps the easiest thing to do is air seal below the floor joists with a continuous layer of rigid foam, sealed at the perimeter with caulk or canned spray foam and at the seams with tape. This will not only give you the air sealing you need but also a good start on thermal performance in the floor. You'll also need to seal the wall top plates all around the perimeter. The you can insulate and air seal the kneewalls and roofline in a number of ways.

    Here are some articles to read:
    How to Insulate a Cold Floor
    Insulating a Wood-Framed Floor Assembly
    Insulating Behind Kneewalls
    Airtight Drywall
    How to Hang Airtight Drywall

    1. icandoamazong | | #2

      Thanks for the reply. I live in a mild climate. Because of that, would I need the rigid foam between the floor joist, or would sealing the sealing the garage sheetrock with caulk/canned-foam and tape?

      Also, for the current knee wall, on the uninsulated side, has thin foil poly structural panels that are stapled into the studs. For this, do I need to tape off the seams and the ends? Are these panels okay for the job?

      1. GBA Editor
        Brian Pontolilo | | #3

        Hi Icando...

        When it comes to air sealing the garage ceiling, either approach would work. The benefit of the continuous rigid foam on the underside of the joists is not only the air sealing, but it breaks thermal bridging. Even with the continuous foam, you would still insulate between the joists with another material like fiberglass batts or blown in cellulose. Perhaps in a mild climate you are less concerned with insulation and would prefer to install airtight drywall on the ceiling and just insulate between the joists. That's your call. Refer to the articles above for R-value recommendations for your area.

        I'm not sure what the material is on your kneewalls, but I assume that it is some sort of radiant barrier panel. Radiant barriers are marginally effective used right in the right situation. If it is in good shape, you could use caulk and tape to air seal it, then insulate the stud bays with fiberglass or another insulation. But I doubt it is adding much value, so if it is not in good shape, take it out and use rigid foam as an insulator and air barrier on the attic side of the kneewall.

        1. icandoamazong | | #4

          Got it. Thanks.

          For the tap between the rigid foam, what do you recommend I use? I have seen everything from foil tape to regular clear box tape.

  2. Expert Member
    Peter Engle | | #5

    Brian, The sheet goods applied to the OP's walls are probably Thermoply, or something similar. Depending on the specific flavor, these are often very good air and vapor barriers as well as a thin structural sheathing product.

    Icando, If you give us the specific product name from the labels, we can probably tell you more about it. Most likely, it will work fine to close off the unconditioned side of your walls, but there may be some risk of condensation on the interior face of the panels. Let us know where you live and we can give better advice on condensation potential and proper wall construction.

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