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Continuous foam on INSIDE of garage wall?

Lance Peters | Posted in Energy Efficiency and Durability on

CZ6A, Ottawa Ontario

I’d like to insulate the perimeter of my garage slab with foam to isolate it from the foundation walls. Is there any reason why I can’t continue that foam layer continuously upwards on the inside of the studs? The stackup from outside in might look like this:

1. WRB
2. Sheething
3. 2×6 studs with fibergalss or cellulose
4. 1.5″ foam
5. Gypsum

I guess the air/vapor barrier would be the foam so it would need to be taped, though there won’t bee much moisture in the garage during the colder months. The wall could dry to the outside. The garage won’t be heated, save for whatever heat comes up through the uninsulated slab hopefully keeping it just above freezing.

Anyone see any issues?

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Lance,

    The tricky bit is the transition from the stem walls to the framing above. Typically there is a rim of concrete around the interior of the garage where the foundation is visible between the sloped-slab and the framing above. It serves a useful purpose keeping water off the drywall.

    Making the foam continuous means either aligning the framing above with the interior of the foundation wall, or figuring out how to cover the foam below the framing. In either case you probably don't want to bring drywall right down to the slab.

    Remember that California Corners and other typical framing variants don't work with interior foam. It also makes drywall returns on windows a lot more dicey.

  2. GBA Editor
    Martin Holladay | | #2

    Lance,
    Malcolm's points are all good. But to answer your question: You can install a continuous layer of rigid foam on the interior side of your studs if you want. (Malcolm's correct, of course, that it is unlikely that this layer of foam will be continuous with the rigid foam on the interior side of your concrete stemwalls.)

    For more information on this issue, see "Walls With Interior Rigid Foam."

  3. Lance Peters | | #3

    Malcolm/Martin, thanks for your thoughts.

    I'm leaning towards 2x8 framing for the garage lined up with the inside of the foundation wall. The walls will be nearly 12 feet tall, and my local code compliance guy indicated a 2x6 wall would need to be on 16" centers at that height. If my Engineer will approve 24" centers with 2x8's the lumber cost difference will be negligible while giving me a thicker insulated wall. I can get the siding lined up with correct batten thickness on the outside, and on the inside my foam layer will line up with the slab layer nicely. I'll use something other than gypsum for the lowest 6" or so of wall finish, perhaps that fancy composite deck material.

    Great point about the corners, Malcolm! I'll be sure to plan ahead for drywall, both in the corners and the wall/ceiling joint.

  4. Expert Member
    Malcolm Taylor | | #4

    Lance,

    I hadn't even thought about the wall/ceiling joint - good spot!

    I used Timber-tech decking as baseboards in a commercial kitchen last year. Pretty bullet proof.

  5. Dick Russell | | #5

    I have a similar situation in my garage, with foam insulation inside the foundation walls, top to footing, framed wall above insulated and sheetrocked, and no insulation under the slab. The attic space is insulated to R30, and the overhead door is insulated to "R18" (probably no better than R6-8 "whole door"). To protect the above-slab foam from thermal and impact damage, I had the builder cover it with Durock (cement board) up to perhaps a quarter inch or more above the concrete, then cap the exposed part of the top of the foundation and foam with a 2x6, with a dado cut on the under side to capture the top of the Durock.

    In terms of freeze protection of the garage, the low level of heat coming up through the slab keeps the interior from going below about 37-38 F nearly all of the winter. The coldest I've seen it, as measured about six inches off the slab, is close to 32 after a few days of zero to single-digit temperatures. I've never seen ice on the slab, even close to the door. This is in central NH.

  6. Lance Peters | | #6

    Dick, thanks for the info! My plan is very similar, to not insulate below the slab and use that as a source of heat to keep the garage above freezing. I don't need it warm, just enough to keep the cars thawed out.

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