Detailing Walls With Rigid Foam
Navigating the challenges of exterior insulation isn’t the nightmare you might think it is.
Synopsis: In this article, builder Steve DeMetrick shares construction and design details for efficient and trouble-free installation of exterior foam sheathing. His method employs Zip System sheathing for structure and air-sealing, 2-in. foil-faced foam for exterior insulation, and a felt-paper weather-resistive barrier behind a rain screen.
Wall construction has changed dramatically since I started in the trades 20 years ago. The 2×4 walls insulated with R-13 fiberglass batts that everyone built back then don’t come close to complying with today’s energy code in climate zone 5, where I live. And even with today’s stricter codes, building just to code is like settling for a D in school. A house that only meets the minimum standard is the worst that can legally be built.
On this house, the combination of 2-in. foil-faced polyisocyanurate exterior foam and 6 in. of fiber insulation create a high-performance wall that exceeds the IRC requirements. But even meeting the minimum wall R-values required by the new energy code can be hard to achieve with cavity insulation alone. In climate zone 5, walls are required to be at least R-20, and standard batts yield R-19. In cold climates, exterior insulation in addition to the cavity insulation is becoming a de facto code requirement. But there are pitfalls, including moisture condensation, detailing challenges around windows and other penetrations, and the lack of a solid base for attaching the siding. Here’s how I navigate them.
As a consultant on this build, I worked with carpenter Andrew Gallant of Gallant Builders on the wall details. Our combination of foil-faced foam and Zip System sheathing has created walls that are essentially impervious to moisture on the outside, meaning that they can only dry inward. To allow this, the wall cavities will be insulated later with unfaced fiberglass or mineral-wool batts, and the…