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Basement slab perimeter insulation

CTSNicholas | Posted in General Questions on

I was told that the updated code for Zone 5 required R-10 Foam for 2′ around the perimeter of my basement. Local code has not updated to the modern one that states this, but I would like to do this anyway. My question is…does this mean 2′ of foam going vertical against the footing, or 2′ of foam horizontal butted up against the footing that goes around the inside perimeter of the basement? I am trying to understand what the benefit of just 2′ does?


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  1. GBA Editor
    Martin Holladay | | #1

    You have stumbled upon a code question that confuses many builders. Perhaps no section of the IRC is as poorly written as the section explaining requirements for slab insulation.

    To learn more about the issue, see Insulating a Slab on Grade.

    In that article, I wrote:

    “Insulation requirements for slab-on-grade floors can be found in section R402.2.9 of the 2012 IECC and section N1102.2.9 of the 2012 IRC. Both codes state, ‘Slab-edge insulation is not required in jurisdictions designated by the building official as having a very heavy termite infestation.’ Otherwise, both codes require that ‘slab-on-grade floors with a floor surface less than 12 in. below grade’ need vertical insulation at the slab perimeter. These codes do not require any insulation for slabs that are more than 12 in. below grade. The codes are silent about whether above-grade slabs need to be insulated.

    “This omission is curious, since most slab-on-grade homes have above-grade slabs. Most inspectors, however, require above-grade slabs to be insulated in the same way as below-grade slabs. In climate zones 1, 2, and 3, slabs are not required to be insulated unless they include hydronic tubing, in which case they are required to have R-5 vertical insulation extending downward from the top of the slab to the footing.

    “In climate zones 4 and 5, the code requires certain slabs (below-grade slabs that are less than 12 in. below grade) to have R-10 vertical insulation at the slab perimeter, extending downward from the top of the slab to a depth of at least 2 ft. If the slab has hydronic tubing, the minimum R-value of this insulation increases to R-15. The same R-value requirements apply in climate zones 6, 7, and 8, but the insulation must extend to a depth of at least 4 ft.

    “There are no requirements for horizontal insulation under slabs, but the requirements for perimeter insulation can be met partially with horizontal insulation. This compliance option is explained this way: ‘Insulation located below grade shall be extended the distance provided in Table N1102.1.1 [namely, 2 ft. in climates zones 4 and 5, and 4 ft. in climate zones 6, 7, and 8] by any combination of vertical insulation, insulation extending under the slab, or insulation extending out from the building.’ ”

  2. CTSNicholas | | #2


    So it seems I have mistaken this wording. I will have a basement, with timber framing on the main level. This seems to be designated for ranch houses with no crawlspace or at least with the concrete slab floors only 2-3' deep where my basement is going to be 8' below grade in some areas, and some parts with no grade since it is a walkout.

    Current code here requires no insulation, but if it were to be updated to 2012, would there still be any need for whatever this R-10 for 2' means? It sounds like it is for footings perhaps, but definitely not 2" of foam placed horizontal along the gravel 2' along the inside of my basement foundation like I thought before.

  3. GBA Editor
    Martin Holladay | | #3

    Sorry I misinterpreted your question. Here is the scoop on basements: How to Insulate a Basement Wall.

  4. GBA Editor
    Martin Holladay | | #4

    If you have a basement slab that is more than 12 inches below grade, there are no code requirements for slab insulation, as I said in my first response.

  5. CTSNicholas | | #5

    Yeah, I am not sure where I got the R-10 2' perimeter information originally. Plenty of info out there to get mixed up. With a walkout basement it would prove to be more work to detail the rigid foam on the exposed areas versus a standard basement with soil all around the foam. At this moment, I am leaning towards the inside insulation method. I don't think there will be any sub-slab insulation however (zone 5B). Just the radon vent system, gravel, 6 or 10mil poly, rebar mesh, and then concrete.

  6. GBA Editor
    Martin Holladay | | #6

    It's unclear from your comment whether your basement is a walk-out basement, or whether you are just making a general comment about walk-out basements.

    If you've got a walk-out basement, you definitely want vertical exterior insulation near the door -- if only to keep the footings from frost heave problems.

    If I were you, I would re-think your plan to omit horizontal insulation under the slab. Just because the code doesn't require it, doesn't mean it's not a good idea. If you install horizontal insulation under your slab, the slab will stay warmer during the summer, so there will be less chance of condensation on mold, and your basement will smell better. You'll also be able to install carpeting later if you want to. Carpeting will be impossible without sub-slab insulation.

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