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Use rigid foam as backing for 2×4 walls?

Charlie Doe | Posted in General Questions on

I have a 64x40x12.5 garage that has a 64x18x8 room above it that I will be using for a woodshop. I live in Vermont so it gets cold here in the winter. I want to insulate the garage and room above it in 2 different zones and put vents in the floor that I can just open up if I want to get heat up stairs. I wish I had the trusses designed so that I had a 2×6 wall upstairs. instead I have 2×4 walls. I wanted 6″ of insulation. Would it be alright to use unfaced rigid foam (maybe 1″ or 2″) as a backing and then use 2×4 faced insulation inside? The rigid foam would give me insulation properties and it would provide a backing so the insulation doesn’t peak off the face and fall down over time. I’m worried about moisture and condensation. I know rigid foam would be pricey (around $1k in materials), but I think the time savings of cutting/installing 2×2 blocks to make the walls wider and then applying strapping would make it worth it. Also any advice on insulating between the floors? Would that section require unfaced insulation?

Attaching a picture that better explains my questions. Thanks!

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  1. Expert Member
    Dana Dorsett | | #1

    In US climate zone 6 (all of Vermont), 2" of foam (any type) on the exterior of a batt-insulated 2x4 wall provides sufficient dew point control, and it does not need facers on the batts. But 1" foam is not enough- you need at minimum of R7.5. If faced batts are used for installation convenience or lower cost, kraft facers are fine, but aluminized facers reduce rather than enhance resilience.

    With a vented attic space there is no need for interior side vapor barriers on any of the ceilings. With 2" of foam on 2" of foam on the exterior side of the 2x4 assembly there is no need for a vapor barrier on the interior side of the 2x4 wall- it only impedes drying.

    On the garage walls you probably don't need vapor barrier either, if it's not fully conditioned space. Siding type makes a difference too. If you feel the urge (or the inspector insists), using 2-mil nylon instead of 4-5 mil polyethylene would be preferred, since it becomes vapor open if the wall ever needs to actually dry. (Both of the major box store chains carry Certainteed MemBrain now, but you may have to order it online and have it delivered to your store in some cases.)

    A 2x4 wall with 2" of continuous foam outperforms a fiber insulated 2x6 wall with no continuous foam, so this is really a superior solution. You would need a minimum of R11.25 foam for dew point control on a 2x6 fiber insulated studwall in your climate.

    The ceiling & floor foam under the room can be faced or unfaced, as long as aluminized facers are avoided.

    Rigid foam is pretty cheap if you use reclaimed or factory seconds goods. These folks trade primarily in factory-seconds roofing foam, and have depots in both Bennington & Barre:

    In MA there are at least a dozen outfits trading in reclaimed foam at 25-35% the cost of virgin stock goods which is probably a good deal cheaper than that factory-seconds vendor. (Used 2" polyiso runs $10-15 per 4x8' sheet from multiple vendors near me, 3" runs $15-20.)

  2. GBA Editor
    Martin Holladay | | #2

    Installing rigid foam on the exterior side of your kneewalls is fine. You might, however, prefer to install insulation along the roof slope. For more information on this issue, see “Two Ways to Insulate Attic Kneewalls.”

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