Is rigid foam as sheathing cheapest way to meet code?
I know: it depends.
I found GBA’s detail of 3/4 rigid foam on the framing with 1/2″ ply-osb sheathing on top. this gives an easy nailing surface for shingles. So if I can use diagonal bracing, (steel or 1 x4) and xps foam and cellulose this will give me R= 22.25. (Good for Zone 5) and also 18.5+ 3.75. My question is now I don’t have to worry about the condensation point because my sheathing is outside my insulation?
If I put the foam on top of the cdx, then I have to increase it to 1 1/2″ to prevent condensation on my cdx, right? Plus for shingles I have to provide either more ply or horiz strapping with weep holes for my shingles, because I assume 1 1/2″ foam is too much to nail shingles through?
Thanks for any help you can give me on this.
Keith
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Replies
Keith,
I'm not familiar with the GBA detail you are talking about. Do you have a link?
Here's one of three.
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Keith,
OK. That's one of Steve Baczek's details. In my opinion, it will work in Climate Zone 4 or anywhere warmer -- assuming, of course, than an engineer and the local code official approve your plan for wall bracing.
I don't think it will work in Climate Zone 5 or colder, because the rigid foam isn't thick enough. (For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)
Even if the OSB sheathing is on the exterior side of the rigid foam, you don't want condensation running down the interior face of the rigid foam in January -- and that might happen if the foam is too thin.
Thanks. I was wondering about that with the condensation.
What I took from the" Calculating..." article is I should just jump to 7.5 R-value for rigid so I can avoid the code req. for inside vapor retarder being more than latex paint. I will ask my shingle question under a different heading.
So for this job, I will look at 2x6 stds and flash foam and cellulose to get the required R-value:20