This week, I’m sharing a few observations, some technical, some not, from a deep energy retrofit of my own home in Burlington, VT. Part 4 of the series.
No, I’m not talking about the standoff in Reservoir Dogs.
Putting high R-value insulation in a cavity but allowing thermal bridging across the studs or rafters means you’re not getting that nominal r-value. Wood is about R-1 per inch. I’ll call SPF R-6 per inch. What to do, what to do.
Now, I could have simply put all the insulation on the outside of the wall and roof. But on the existing structure, the roof had just been redone when I bought the house and I didn’t want to pay for it again. In the addition, I wanted to align interior and exterior surface planes, and I wanted to maximize both R-value and the slope of a shed roof that was limited by second floor windows. And the exterior insulation in deeper thickness is a lot more work for one guy tackling a project himself. (Also see previous comment about the ready availability of 6″ screws, but the special-order nature of 10″ screws).
I decided on cavity insulation. I decided that would be closed-cell SPF. And I wanted something like an R-50 wall assembly and an R-60 roof. I wanted to use the cavities since the space was at a premium. While to a large extent with the walls, I addressed thermal bridging with 3.5″ of exterior insulation, I wanted to do more. Hence the standoff.
Essentially I used scraps of 1.5″ XPS to hold 1.5″ furring strips off the inside face of the studs and rafters. This provide a cavity that would 3″ deeper, and by isolating the stud from the furring strip. After filling the now-deeper cavity with SPF, I got a thermal break from either the XPS or the SPF. And the mudroom is the quietest room in the house.
Tomorrow? Maybe I’ll talk about minimizing any risks from pinhole leaks.