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Community and Q&A

Insulating a walkout basement

NormanWB | Posted in Energy Efficiency and Durability on

I have a partial basement planned for a new house that also has an extensive crawlspace area. I plan to seal the crawlspace using poly on the floor and foam board on the walls. Backed up to this crawlspace will be a walkout basement area. The walkout wall will be standard framing on a footer, Since this basement is mostly for storage and an occasional guest, I do not plan to insulate the slab,but want to insulate the perimeter. Since we have termites and other critters, I plan to insulate to the inside. For now, only the structural walls will be in place and plumbing roughed in, as I plan to add rooms and fixtures at a later date.

So, since this is conditioned space what insulation should I put in now and what can I put off,for when I finish the area?


CZ 3A Greenville, SC
~4 hours to totality!

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  1. GBA Editor
    Martin Holladay | | #1

    In most locations, wall insulation is required by code, and your local authorities probably won't issue an occupancy permit until the insulation work is complete.

    If you live in a rural area beyond the reach of codes, I suppose you can move in any time you want, whether or not the insulation work is complete.

    The 2012 International Residential Code requires framed walls in Climate Zone 3 to have at least R-20 insulation between the studs. Assuming that you have 2x6 walls, this can be achieved by any insulation material with an R-value of R-3.63 or more per inch -- including high-density fiberglass batts, mineral wool batts, blown-in fiberglass, or dense-packed cellulose.

    With this type of insulation job, careful installation matters more than the type of insulation you choose. Install the insulation carefully, without any voids.

    Remember that, no matter what insulation you install, you need a good interior air barrier. Taped drywall will work, as long as you remember to install airtight electrical boxes (or detail your electrical boxes to prevent air leakage).

    If your walkout basement includes any concrete walls, follow the advice given in this article: How to Insulate a Basement Wall.

  2. NormanWB | | #2

    Thanks for the input, Martin.

    It will have concrete walls on at least two sides, perhaps three. Since I do not plan insulation under the slab, how should I insulate the slab edge?

  3. GBA Editor
    Martin Holladay | | #3

    Attached is an illustration showing a basement with interior wall insulation, but no slab insulation.

    Of course, a continuous layer of horizontal rigid foam under the slab (along with vertical insulation at the slab perimeter) would be better, if you decide you can afford it.


  4. joge2468 | | #4

    I’ve been searching for insulation advice for a walkout basement, and this is the only thing I’ve found on GBA. Martin, am I to understand the below grade masonry walls are to be treated with foam only, and the framed walls like any 2x6 wall? What about where the wall is half masonry, half framing? What about where a shower is to be installed? What is the detail there, where one intends to use kerdi or similar? Thank you in advance for any guidance you can provide.

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