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Application of 2/3-1/3 rule when rigid foam is vapor barrier

I submitted a question a couple of weeks ago: "Rigid Foam Between Walls" in response to which I received numerous very helpful answers. Thanks again.

I'm in New Brunswick Canada. We're getting close to to making the final decision regarding the wall system. It has basically come down to either ICFs (6" of concrete with 3.25" of foam outside and inside) or a double wall system involving, from exterior to interior: vinyl siding, housewrap, 2x6 exterior wall 24" oc with blown in blanket fiberglass, 2" rigid foam, poly vapor barrier if required by inspector, 2x4 interior wall 16" oc with fiberglass batt, sheetrock.

I understand that there are other double wall set ups that might provide more R value for my money, but this is the one my builder is comfortable with. I also understand the pros and cons of ICFs and I definitely don't want to get into that debate here.

My question is specifically about the 2/3 - 1/3 rule and it's application in relation to my double wall assembly. Ignoring for now the issue of the poly vapor barrier (because I'll only include it if the inspector insists), if the 2" of rigid foam insulation is a vapor barrier, does that 2 inches of foam still count toward the total insulation on the exterior side of my vapor barrier? OR will I have, in effect, a 2" thick vapor barrier and therefore, only the blown-in fiberglass in the exterior wall will count as insulation on the exterior side? In that latter scenario I'd only have an R24 (exterior) to R12 (interior) ratio. That's exactly 2/3 to 1/3 but I'd be a lot happier with the wall if I knew the R value of the foam would also function as part of the insulation on the exterior side. Thanks.

Asked by David Williams
Posted Feb 14, 2018 1:27 PM ET


2 Answers

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The so-called 2/3 1/3 rule isn't scientific. It was a rule of thumb invented in the 1970s with no basis in science. In fact, the amount of exterior insulation you need to keep the first condensing surface warm enough to avoid condensation problems depends in part on your climate, so there will never be a universal rule.

For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.

If you consider the interior surface of the rigid foam to be your first condensing surface, then you want the R-value on the exterior side of this surface to represent at least 36% of the total R-value of your wall (assuming you are located in Climate Zone 6).

You have an R-41 wall, and the insulation in the 2x6 wall plus the 2 inches of rigid foam has a combined R-value of R-28. Since R-28 is 68% of R-41, you're fine.

I'm curious: Will this wall have any exterior sheathing?

Answered by Martin Holladay
Posted Feb 14, 2018 2:04 PM ET


Thanks Martin. Much appreciated. Yes. 1/2" plywood. I just forgot to put it in the list. Thanks for the link.


Answered by David Williams
Posted Feb 14, 2018 2:48 PM ET

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