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Community and Q&A

Thin insulation behind drywall

ezc | Posted in General Questions on

I’m trying to finish out the interior of a former shop space as living quarters by installing 1/4″ drywall over horizontal wood plank walls. While I’m at it, I was wondering if there was some sort of extremely thin insulation I could sandwich behind the drywall to help insulate the (uninsulated) exterior walls. I don’t want to decrease the interior space any more than I have to, and removing exterior clapboards to insulate isn’t in the budget. Thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    By "plank walls," do you mean an old-fashioned house built of solid squared timbers (sort of like log walls)?

    If so, you obviously can't insulate the stud bays (because there aren't any studs). Your plan will work, but it isn't worth doing if the insulation is "extremely thin." I would advise that it's hardly worth doing if you aren't willing to install a continuous rigid foam layer that is at least 1 inch thick. (That's R-4 to R-6, depending on what type of foam you choose.)

    Of course, 2 inches of rigid foam would be even better -- and 4 inches would be best.

    For more information, see Walls With Interior Rigid Foam.

    If this building actually has empty stud bays (and the "planks" are just interior cladding), then the best way to proceed would be to fill the empty stud bays with dense-packed cellulose.

  2. ezc | | #2

    It's platform-framed and the walls are finished on the inside with 6" horiz. tongue & groove pine boards. An inch of rigid foam would be good except for all the retrimming. I'll look into filling the stud bays. Thank you for your advice.

  3. GBA Editor
    Martin Holladay | | #3

    Call up an insulation contractor. It's easy to fill the stud bays with cellulose -- all the contractor needs to do is drill a series of 4-inch-diameter holes in the boards (one hole per stud bay).

    Here is a link to an article with more information: How to Install Cellulose Insulation.

  4. ezc | | #4


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