This is the fourth and last post delving into the anatomy of high-performance wall assemblies, beginning with the Karuna House in Oregon. This week’s post explains our approach at the Madrona Passive House.
The wall assembly at Madrona Passive House shows how Hammer & Hand’s approach to high-performance envelope construction is evolving. With each project we are working to simplify assembly and hew as closely as possible to standard construction techniques. This is good for our installers in the field and for project budgets. The wall at Madrona Passive House takes advantage of the compressive strength of mineral wool to suspend the home’s exterior insulation with the rain screen battens’ fasteners.
The wall assembly’s air barrier and WRB consists of Zip System sheathing (sheets of OSB with a factory-applied weather-resistant coating) with fluid-applied sealant at the seams.
A 3.5-inch exterior monolithic layer of Roxul mineral wool insulation (A) adds R-14 of insulative value to the assembly, while 5.5 inches of high-density fiberglass insulation (B) brings another R-23. The wall’s total insulative value (including insulation, sheet goods, air films, etc.) is R-39 (center of cavity) or R-34 (whole wall).
Madrona Passive House’s 1Ã—6 knotty cedar siding (A) serves as the primary barrier to water intrusion. As in the other three wall assemblies described in this series, the rainscreen cavity behind the siding (D) allows water to drain away. The Roxul mineral wool (B) is hydrophobic, so it provides another barrier to water, while the Zip sheathing WRB (C) serves as the final barrier.
Because the OSB of the Zip sheathing is a vapor retarder, the assembly is semi-permeable to the inside (B), slowing vapor diffusion from the home’s interior into the assembly. The mineral wool warms the Zip sheathing, preventing moisture accumulation there. And because the mineral wool is very vapor-open, vapor diffusion to the outside is fairly unimpeded. The assembly’s ventilated rainscreen (A) adds drying capacity, and therefore durability, to the wall.
With Madrona Passive House’s wall we have simplified the assembly down to six material layers; by comparison, Karuna House and Pumpkin Ridge Passive House both have seven layers, while the Glasswood Retrofit has ten. That’s progress!
Zack Semke is the manager of business development at Hammer & Hand, a company specializing in high-performance building with offices in Washington and Oregon.