What we learned from exterior insulation.

Siding was going up and it was supposed to be a smooth day. My eyes were burning, my skin was itchy, and I seemed to have found a loophole in physics. I was whaling on a nail trying to fasten a starter strip onto my plywood rain screen. Each blow did absolutely nothing except shake dust from my 4” layer of mineral wool insulation, which was absorbing all of the force from my hammer. After 5 or 6 blows, the nail fell comically to the ground. This was the proverbial icing on my RockWool cake.

This was the first project I’ve done where I used such a robust layer of RockWool. At almost every stage of the project, it presented a new challenge and a handful of associated costs. I had to step my foundation out. I had to buck out my doors and windows. I had to continue insulating above the roof line and up the parapet walls. My parapet capping ended up being around 14 inches! Each buck-out needed to be ripped, beveled, glued, flashed, and capped. The bends were complicated, liquid-flash is expensive, and it’s really difficult to accurately account for compression after your rain screen is fastened.

I love RockWool, don’t get me wrong. It’s all I’ll use sub-slab, and I love the ComfortBatts for my wall and ceiling applications. This was my first time using ComfortBoard for exterior insulation, and to be honest, many of the difficulties I encountered were 100% avoidable. In my own defense, next time I’ll know what to look for. If I land a project where I need exterior insulation, I’ll know how to execute the assembly seamlessly. My foundation walls will be blocked, I’ll use Thermal Bucks and the windows, and I’ll use smooth-shank screws if my nails stop working :)

For my next gut-rehab, new construction, or deep energy retrofit, I’m going with a double stud wall with dense pack cellulose and an intelligent vapor barrier that dries to the interior.