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New home build in Iowa (Climate Zone 5)

user-6759891 | Posted in Building Code Questions on

Is it possible for a new home build in Iowa, climate zone 5, to not have under slab insulation? What’s the min code requirement for a poured concrete basement with walkout basement?

Is it normal to also have external foundation just sprayed with waterproof membrane and tiles?


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is it possible for a new home build in Iowa, climate zone 5, to not have under slab insulation?"

    A. It's possible. When you say "under slab insulation," I assume that you are talking about a continuous layer of horizontal insulation. For information on code requirements, see this article: Insulating a slab on grade.

    Here is the summary:

    1. In most building codes, there are no requirements for horizontal insulation under slabs (unless the slab includes hydronic heating pipes), but there are usually requirements for vertical insulation at the perimeter of the slab (unless your area has been designated as an area with heavy termite infestations). Code requirements for perimeter insulation can be met partially with horizontal insulation.

    2. Local codes may differ from the summary above. Contact your local building department for more information.

    3. Some parts of Iowa may not have any building code enforcement whatsoever.

    4. GBA recommends that all slabs in your climate zone include continuous horizontal sub-slab insulation. If you have a walkout basement, it's particularly important to include vertical insulation at the slab perimeter, extending well below grade, on the walkout side.

    -- Martin Holladay

  2. Dana1 | | #2

    The foundation wall needs to be insulated, even if you just water proof & tile the exterior side of the above-grade portion. You may need to use something special for the waterproofing to be able to reliably affix the tile though.

    The IRC calls out R15 continuous insulation on foundation walls or R20 if broken by studs. The latter is a mold risk. In zone 5 you can get the same thermal performance at low moisture risk with 1" of foil faced rigid polyisocyanurate foam (seams taped, edges sealed) trapped to the concrete wall with a 2x4/R13 studwall, using unfaced or kraft faced batts, or blown fiber. Keep the cut edge of the rigid foam off the slab (1/4" is fine). If you're not using foam under the slab, put an inch of EPS (not polyiso) under the bottom plate of the studwall and under the bottom edge of the polyiso on the wall as a thermal & capillary break.

    Skipping the slab foam is a bad idea, even if it's not required by foam. The deep subsoil temperatures are much lower than your summertime outdoor dew point averages, so anything resting on the cool slab in summer risks high moisture content, making rugs etc susceptible to mold. With even an inch of under-slab EPS you can avoid the "musty basement smell" related to mold growth on items resting on a cold slab.

  3. GBA Editor
    Martin Holladay | | #3

    Here is a link to an article with more information on insulating basement walls: How to Insulate a Basement Wall.

    -- Martin Holladay

  4. user-6759891 | | #4

    Thanks for the helpful Martin and Dana. So you don't see an issue with using the foil-faced polyiso on the interior if the exterior has a sprayed on waterproofing and/or XPS insulation? I thought that may potentially cause a double vapor barrier situation, so I was thinking of about 1.5" of EPS that would get me a min R7.5.

  5. user-6759891 | | #5

    And a 2x6 stud wall with batts.

  6. user-6759891 | | #6

    And if the foundation is already poured and there's no under slab insulation, then 1" of high density closed cell EPS under sub floor should ok?

  7. GBA Editor
    Martin Holladay | | #7

    If you plan to install insulation in your proposed 2x6 wall, you have to provide enough rigid insulation to keep the interior surface of the rigid insulation above the dew point during the winter. A conservative approach would be to follow the guidelines for above-grade walls in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    According to the table in that article, an insulated 2x6 wall in Climate Zone 5 needs to be paired with rigid insulation that has a minimum R-value of R-7.5. So if you want to use foil-faced polyiso, the polyiso should be about 1.5 inch thick. If you want to use EPS, the EPS should be about 2 inches thick.

    Either polyiso or EPS will work in this application. There is no need to worry about a "double vapor barrier."

    One inch of rigid foam under the subfloor is OK, but 2 inches would be even better.

    -- Martin Holladay

  8. user-6759891 | | #8

    Thanks Martin... much appreciated.

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