I’ve talked about the wall assembly before when we were deciding whether to build a double wall or go with spray foam (we decided to go with spray foam). I even had an elaborate post written about it that I never published since it didn’t really go anywhere. Then I talked about it some more when we were considering redoing the exterior first (nope). Finally, I talked about it when we demoed a wall on the first floor to get a look at the sheathing and again when when we found out we have old asbestos-laden siding. So I’m apparently obsessed about it, and so as not to dwell on it any longer I’m making a final post about it, never to drone on the topic again (at least until we reside the exterior in a few years).

While the earlier posts were full of questions, this post is full of the decisions we’ve reached and why we made them. There are a lot of factors to consider, like the insulating value, the ability of the wall to dry so it doesn’t trap moisture and grow mold, the thickness (both on the outside where we’re close to neighbors and the inside where we don’t want to lose square footage), and of course the cost.

The animation below shows the spray foam and drywall added on the inside, and then the house wrap, rigid foam, furring strips, and finally the siding.

Let’s walk through what this means. On the inside we’ll fill the wall cavity (3 3/4″ thick) with closed cell spray foam (applying directly to the existing wood plank sheathing). The foam insulation has an R-value of about 7 per inch, giving us about R-26. Unfortunately, the studs are only about R-5 and comprise 10% of the wall, so our actual rating is about R-24. Then we add drywall and paint.

On the outside, once we’ve removed the existing vinyl and cement siding, we install house wrap over the plank sheathing (performing proper flashing on window edges) then add 2″ of polyiso rigid foam. The rigid foam adds R-12 and functions as a thermal break. Then we add furring strips (steel, not wood) which act as a capillary gap behind the siding so it doesn’t overheat in the summer and doesn’t wick moisture, and finally the new cement fiber siding (which doesn’t contain asbestos). Adding the minimal R values of the sheathing and drywall brings our total wall insulation to about R-36, well above the minimum R-30 recommended for our region.

Our challenge will be finding good prices on all of these materials. Rough calculations put our costs for this assembly at $6.92 per square foot, which I’m hoping we can bring down considerably. The good news is that I’m done thinking about walls for a while. I can focus on HVAC, which is a whole other can of worms.


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