High density rigid foam strips as rainscreen battens
marcit
| Posted in Green Building Techniques on
Would it be possible to use 3” strips of high density 1” foam insulation board (class A fire rated) as rainscreen battens covering studs to create a thermal break for the studs as long as the siding was nailed through to the studs? I am considering Raycore 2×6 16” OC panels covered by Ox structural sheathing with 1×3” strips of insulation over the studs. I know I could get 25-100 psi compressive strength s rigid insulation that is class A rated.
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That's an interesting idea. There was a recent discussion of doing that on a more conventional wall and there was a strong consensus that foam strips wouldn't be robust enough. Maybe if you got up to 100 psi foam.
But I'm not convinced that Raycore makes much sense to use in the first place, for the reasons discussed in this old thread here.
https://www.greenbuildingadvisor.com/question/raycore-is-it-the-product-for-me
Thanks for the response, Charlie. If you have a link to the old thread I would love to look at it. Code allows construction with structural panels and siding to be nailed over up to 1" of foam without furring strips so I thought that as long as the siding was rigid enough to be nailed over furring strips (not needing backing support between them) that one should be able to just make the insulation intermittent to act as the furring strips.
As regards the RayCore panels, I have gone back and forth on this. The cost of labor has skyrocketed, and these are easy to build with as long as you design carefully, create straight walls that are easy to finish, provide an easier way to get an airtight home without insulation gaps that does not rely on lots of fiddly details that workers rarely get right, require fewer specialty trades and in general simplify the whole process. Greener alternatives to foam will come (there are cellulose-based foams on the way that are very promising).
There are greener foams availble right now. Polyiso, EPS, Graphite EPS (aka GPS or neopor) and even some spray foams all avoid the HFC blowing agent that is the chief culprit here. This is a serious climate impact, not a vague preference for more natural materials because they make you feel good.
I was curious to see how well this would work, so I ran a quick Therm model of the assembly.
Took a 2x6 wall with MW batts, rain screen plus siding. This works out to R18.8. If you go with 5.5" wide 1" thick foam strips instead of wood strapping, it bumps the assembly R value up to 19.6 (1" of continuous EPS is R21.9, my model is not quite right as it should be R17 batt only and R20.7 with 1" EPS).
So it is slightly better than wood strapping. Going to 2" of EPS is a small bump up to R20.25, so not much gained. 3" wide strip is too narrow to do anything.
I think this might work better for walls with a lot of thermal bridging like steel studs. The strip needs to be pretty wide to have much of an effect though. The thicker the foam the wider it needs to be.