I'm building a shed kit in Minnesota and need to insulate the exterior walls to meet the code requirement of an R-value of 21. The problem is that my 2x4 framing only provides 3.5 inches of cavity space, which makes it tricky to achieve that R-value. I know closed-cell foam would do the job, but it's pretty pricey. I'm looking for any creative or cost-effective alternatives that could help me out. Thanks!
3 Answers
You might want to think about making your walls deeper by attaching additional material to the existing studs. Rip 2 inches off the sides of a 2x6 and fasten it to your 2x4 studs, then you can use 5.5-inch insulation. Adding exterior foam to the studs before siding could also do the trick!
Consider layering some foam board on the outside of the walls. If you're using 2 inches or more, it could really help with your insulation goals.
Sheets of Polyiso foam work great for this—they're around R6 per inch. Your 3.5" cavity should give you at least R21! Plus, it tends to be cheaper than closed-cell foam. Or you could use R15 rockwool in the cavities, added with 1 or 2-inch Polyiso sheets outside for some extra insulation and vapor barrier benefits. This approach also helps minimize thermal bridging through the studs.