The Building Envelope: A Solid Approach

The building envelope, the collection of assemblies that separate the inside from the outside, is arguably the most important part of any home. It’s the primary barrier against the elements and the key to comfort, durability, and efficiency. For the Spring Street Passive House, we landed on a set of assemblies that balance performance, carbon, and occupant health.

Our primary wall assembly is a 12-inch-thick double-stud wall. This creates a large cavity that we can completely fill with dense-pack cellulose, an insulation made from recycled newspaper with very low embodied carbon. This is wrapped on the exterior with a continuous layer of rigid mineral wool board insulation, eliminating thermal bridging through the studs. The result is a highly-insulated, vapor-open wall system that can dry out if it ever gets wet.

This "belt and suspenders" approach is about more than just hitting an R-value target. It's about building in a way that is robust and forgiving. We intentionally prioritized wood- and fiber-based materials, minimizing the use of plastics and foams. This not only lowers the home's embodied carbon footprint but also contributes to a healthier indoor environment by avoiding materials that can off-gas harmful chemicals. This envelope is our home’s armor, but it’s an armor designed to stay nice and dry over time.