Musing from a Deep Energy Retrofit: The Spray Foam Standoff

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Thermal Break-XPS Standoff

From left to right, 2×2 nailer, 1.5″ XPS spacer, 2×4 stud, exterior sheathing.

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.

Thermal Break in rafter cavities with XPS standoff

Same idea with the rafters…

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).

Cavities full of SPF

I spec-ed 6″ of SPF in the built-up cavity…but most of the walls got a bit extra, and had to be shaved back a bit in some areas. Finished wall R-value I’m calling R-55. (Or R-60 on a really cold day.)

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.

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About the Author:

Mike Rogers is the President of OmStout Consulting. A nationally recognized expert in residential energy-efficiency, he works with contractors and programs to scale sustainable market approaches to improving homes. More on Google+


  1. Sydney  February 26, 2015

    Mike, that sure is a lot of foam. How many lifts did it take to fill the cavity with closed cell SPF? Have you done a parallel path calculation? -Sydney

    • Mike Rogers  March 3, 2015

      I love you, Sydney! Not only because you take the time to read my blog posts, but because you ask nerdy questions. (And because you appreciate just how pretty–how pretty? so pretty!–the Grand Canyon is, but that’s another story.)

      The walls were three lifts (two and a very light 3). The Ceilings were 4-5 lifts.

      A parallel path calc should be taken with a large block of salt in this wall. I think the math is much more complicated than parallel paths would suggest, but differential equations don’t sound as good as another Heady Topper. But here’s what it looks like, with some gross simplification, and omitting some air-film surfaces. I call the opaque wall an R-45. Now, ask me about the marginally efficient windows…

      Parallel Path R-Value Calculation


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