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Community and Q&A

Definition of “floor”?

GBA Editor | Posted in Building Code Questions on

In the IECC 2006 code, that I need to adhere to in a planned addition, it’s not clear to me whether the “floor” (requiring R-19 in my climate 4 in Maryland) refers in my case to the concrete basement floor. Do I need R19 under the slab of my conditioned basement, which would be 6ft below grade?

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  1. GBA Editor
    Martin Holladay | | #1

    According to section N1102.2.7 of the IRC, "Slab-on-grade floors with a floor surface less than 12 inches below grade shall be insulated in accordance with Table N1102.1." (This is the table that specifies that floors in your climate zone should be insulated to R-19.)

    Unfortunately, this section of the code is poorly written. It appears to be silent concerning how builders should insulate:
    1. Slab floors (like yours) that are more than 12 inches below grade; or
    2. Slab floors that are at grade; or
    3. Slab floors that are above grade.

    However, the wording of this section implies that insulation requirements for floors more than 12 inches below grade differ from those in the table. I know that builders routinely omit insulation beneath basement slabs, although in my opinion these slabs should be insulated (if only to limit condensation during the summer).

    Talk to your inspector. In my opinion, you won't regret installing 2 inches (or more) of XPS below your slab. That will allow your floor to be carpeted. Without the sub-slab insulation, carpet can't be installed because of moisture and mold arising from summer condensation.

  2. Riversong | | #2

    The R-19 requirement is for a wood-framed floor over unconditioned space. 2006 IECC Table 402.1.1 requires R-10 slab insulation for 2', which includes the slab edge and either vertical extension downward or horizontal extension sub-slab.

  3. Richard | | #3

    Thank you, Martin and Robert. So for my basement floor, I should be looking at the slab requirements:

    Climate Zone Slab R-value, Depth (4)
    4 except marine R-10, 2 ft

    IECC 2006 Table 402.1.1 Insulation and Fenestration Requirements by Component (1)

    4. R-5 shall be added to the required slab edge R-values for heated slabs.

    So for the bottom line answer, the code doesn't require that I insulate under my basement concrete slab, since it lies below 2 ft.

    In terms of undertanding it, I have seen in various county explanations that the "2ft" requirement refers to the frost line. Essentially all parts of the country are required to have R-10 for whatever part of the house is in contact with sub-grade soil that is above the frost line. The frost line is either 0 ft, 2 ft or 4 ft, by climate zone. If the slab is below the frostline, then it doesn't need to be insulated. As to the rationale for this, I have read the threads here on GBA regarding the passivhaus method that raise questions as to how warm exactly that soil is lying under the house below the frost line. I guess the thought is, especially for my mixed climate, that the cooling the house receives in summer might possibly balance out the slight heat loss through the heating season (with the low delta T). Martin - I see your recommendation for sub-slab R-10 doesn't refer to energy conservation but to the comfort of being able to have carpet as an option. Do you know if the BSC recommendation of "5-10-20-40-60" with "R-10" under the slab is based on energy conservation?

    What I am not sure I understand (in terms of the code rationale) is that if the basement wall extends below the frost line, then I do need to insulate it to R-10 (in climates 4-8). The basement wall doesn't have a frostline (or "depth") criterion. I imagine this is because of the thermal bridge, i.e., the heat of the wall will go right out the upper portion of the wall. And I guess the rationale is that the concrete slab is not coupled to the walls to suffer heat loss to the same extent. Does this rationale I am imagining seem accurate?

  4. GBA Editor
    Martin Holladay | | #4

    Basically, you've got it.

    My only comment concerns your assumption that code requirements always have a rational basis. Some do, some don't. The ones that lack a rational basis are historical accidents, or political compromises, or measures based on obsolete assumptions since disproved by recent research.

  5. Riversong | | #5


    The sub-grade and sub-slab insulation requirements are based on air temperature (climate zone) and not frost depth, which varies in the lower 48 from zero to 60".

    The table 402.1.1 R-value requirements are based on the boundaries of your conditioned space. Basement wall requirements (R-10 continuous or R-13 between studs for climate zones 4-8) are for basement walls that are at least 50% below grade. Any wall less than 50% below grade must comply with wall standards.

    Slab insulation requirements are only for slabs less than 2' (zones 4 & 5) or 4' (zones 6, 7 & 8) below grade.

    However, these are MINIMUM requirements for energy efficiency. It always pays to go beyond the minimum, and consider comfort and condensation as well as increased energy efficiency.

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