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I'd like to insulate an exterior 2x4 wall to R-21. It looks like I'll need to use Rigid Foam of some sort. I'm not finding any products at my local store which are 3.5 inches thick. At the moment, I'm finding:

  • Rmax Thermasheath-3 Polyisocyanurate at 2 inches which provides R-13.1
  • Owens-corning Foamular XPS at 1.5 inches which provides R-7.5

(that adds up to 20.6, which after considering the exterior wood and siding and the interior sheer wall, I should technically be at > R-21)

Is this my best bet? Simply put these two foams one on top of the other? Is there a better system or single product I can buy in 3.5 inch thickness? Should one or the other be on the outside?

Thanks for any advice, I'm new to insulation work.

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I had a similar wall that I insulated exactly (well, almost) the way you're describing. Here's a cross section of the wall. The left side is towards an unheated workshop.

enter image description here

I don't think I used 1.5 inch XPS because it would have caused the sheet rock to bulge slightly.

Another option is what they call an R13+5 construction. With this technique, which can be used for new construction or when you're tearing down to the studs, fills the 3-1/2" wall cavity with something like Rockwool (R13), then fasten foam board insulation with an R-value of 5 to the outside of the wall, before your house wrap and siding. The chart excerpt below is from a local building code information brochure.

enter image description here

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You can do this with spray foam, look carefully as they won't all meet your requirements which would be minimum R-6/inch. I found at least one, Tiger Foam which meets R-6/inch though.

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