How do I get an R-20 wood wall?
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Insulation method to get R-20 in a 2×4 framed wood stud wall?
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Closed cell spray foam will get you there.
Closed cell spray might get you to R20 center cavity, but the severe thermal bridging of 3-3.5" studs brings the whole-wall performance down to R11-12, even at a full 3.5" depth, and including the gypsum & siding. By contrast a full 3.5" of cellulose or open cell foam is still in the R9.5-10 range, a difference that could be made up with half-inch rigid foam (any type) on the exterior at much lower cost.
To hit ~R20 whole-wall with 2x4 framing put 2" of rigid polyiso on the outside of the sheathing and cellulose or open cell foam in the wall cavities.