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Insulation ratio Zone 4A

rhl_ | Posted in General Questions on

I have a very thick (12”?) foundation wall above grade. I’m going to add rigid foam board insulation on the inside and outside of the wall.

if I recall I need at least 16% on the outside correct? So if I’m doing 2” of polyiso (at R-13.5) on the interior then I need ~R-16 total for at least R-2.5 on the exterior? I was thinking of going for at least R-10 on the exterior with 2” of R-5 board.. 

Am I following the rules correctly ? 

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  1. user-2310254 | | #1

    Hi Ryan,

    Foundation or basement walls can be insulated on the exterior or the interior. See Martin's article: Zone 4A requires R-10 on one side or the other.

    I am no expert, but I think you are confusing the sheathing and foundation guidelines. Or do you have frame wall sections in your basement (a daylight basement)? Maybe you can clarify for us.

  2. rhl_ | | #2

    This wall isn’t below grade, so I would have guessed you’d have treated it like an ordinary wall, and wanted higher insulation than just R-10.

    On the interior I want to avoid framing the wall and just use insulation with furring strips, then drywall.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      The only reason there is a ratio for interior and exterior insulation for walls is to keep the sheathing dry. You can use any combination you want on concrete walls.

      How did you end up with a 12" thick foundation wall?

      1. rhl_ | | #6

        Well, the house is like 100 years old.

    2. Expert Member
      Dana Dorsett | | #7

      In zone 4A, R8 continuous insulation on the exterior of a mass wall would meet IRC code minimum performance. As long as no more than half the insulation is on the interior it the R8 can be split between interior & exterior. (1" EPS + 1" EPS makes it.) So a continuous R10 on the exterior would be code-compliant with or without additional insulation on the interior.

      If more than half the total R is on the interior the total has to add up to at least R13.

      >"I was thinking of going for at least R-10 on the exterior with 2” of R-5 board."

      The only 2" board labeled R10 would be XPS, which is only warranteed to R9 (read the fine print), and would eventually drop to R8.4 (the same as EPS of similar density) as the climate damaging HFC blowing agents slowly leak out over several decades. The predominant HFC in the mix for all US made XPS is HFC134a, with a 100 year global warming potential of 1400x CO2. EPS is blown with comparatively benign pentane, a hydrocarbon with a 100 year GWP of only 7x CO2, most of which escapes the foam in the factory where it is recaptured rather than vented to the atmosphere, and has a stable R-value over time. The difference in environmental impact is not subtle:

      Polyiso is also blown primarily with pentane along with a mix of other hydrocarbons, and has roughly the same impact as EPS.

      If there isn't a capillary break between the foundation sill and the top of the wall using unfaced EPS would be preferable to foil facers, to allow the wall some drying capacity for any moisture wicking up from the footing. Using 2" layers of reclaimed roofing foam (either fiber-faced polyiso or unfaced EPS) would be fine, and far greener than any virgin stock foam.

  3. user-2310254 | | #3

    Hi Ryan,

    The 16% minimum would apply if you were in Marine Zone 4C. Since you are in 4A, any amount of foam will be sufficient to keep the sheathing safe. Here is a blurb from one of Martin's articles (

    "If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don’t have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem."

    1. rhl_ | | #5

      Oh I see. That’s for walls. It’s different for roofs. Great.

      And I guess code minimum is going to be something like R-13 or so.

      1. Expert Member
        Dana Dorsett | | #8

        See the MASS WALL R-VALUE column of TABLE N1102.1.2,

        R13 continuous insulation on the interior side would be needed to meet code, but it only takes R8 continuous insulation if it's on the exterior (or if at least half of the R8 is on the exterior.)

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