Double Wall, Spray Foam, or Both

Double Wall Section

We’ve met with several architects and we’re waiting for a couple of proposals. One of the common themes in talking with them is the suggestion that we should insulate with spray foam. We’ve been talking about building a double wall with rockwool insulation, and the architects are almost unanimous in saying that spray foam insulation in the walls is the way to go.

Now they are the experts, but I come back to the reasons we planned to do this in the first place: the insulation value of the double wall with rock wool is 25% better and costs less than half as much. There are other factors though: spray foam is an air barrier, and it would seal up gaps in our leaky old house. Not having to build the double wall would save a lot of work plus we wouldn’t be making an already narrow house even more so. On the other hand, we’re planning to replace the exterior sheathing after we remodel the inside, so using spray foam would mean adding some new layer to the inside before applying, so we don’t break the foam seal when we remove the exterior boards. When we do the exterior we’ll be using a house wrap and taping all seams, which should seal the house nicely, without the expense of spray foam.

I’ve been referencing BuildingScience.com, which has some R-Value Recommendations for walls based on climate zone. In Chicago, which is Zone 5, they recommend an R-30 wall. Closed-cell spray foam yields an R-value of  6 per inch. Filling the 3 3/4″ thick outside walls would give us R-22.5. In addition, we’re planning a 1″ layer of insulation on the outside of the sheathing, which will give us another R-4. Adding the minimal contributions of drywall and sheathing and we’re up to about R-28. Compare that to the double wall, as in the diagram. Each 3 1/2″ thick batt of rockwool provides R-14. Adding the exterior insulation and the rest of the wall brings that total to R-33 while also reducing thermal bridging. Of course, there’s another option that’s kind of the worst of both worlds: do both. We could spray foam the outer wall and then still build the inner wall, insulating it with rockwool. The insulation value would be a stunning R-42, but carries the highest cost by far as well as the most work.

Deciding the right course can often be tricky, but in this case it can be done with straight economics. Using houses on the market in our area today I can estimate the value of the square footage I would lose with the double wall. Then I subtract the extra cost of spray foam and the increased cost of heating and cooling. The numbers come out in favor of spray foam. So despite my inclination toward the double wall, I’m open minded enough to see the better decision and change my mind.