A Better Wall with Exterior Foam
Why a longtime SIP builder turned to rigid polyiso for insulation
I’ve been a home designer, builder, and timber framer for more than 25 years, and I recently calculated that my projects have consumed almost 2 million sq. ft. of SIPs (structural insulated panels). I still use SIPs almost exclusively for roofs, where they serve as insulation, short-span structure, and premade overhangs. When my firm used SIPs for sidewalls, however, there was a great deal more work involved in cutting openings and routing mechanicals, so almost 20 years ago, we started developing an alternative. Our goal was to find an open-cavity option that made installing mechanicals easier and was still well insulated.
The solution, which I refer to as the matrix wall, consists of conventionally framed 2×6 walls with either wet-sprayed or netted dense-pack cellulose in the cavities and polyisocyanurate foam on the exterior. Instead of using the foam as the weather-resistive barrier, I install housewrap and a rain screen of 1x strapping. The assembly yields an R-30 wall with negligible thermal bridging and a managed dew point.
Of course, thermal performance means little if the walls allow water and air infiltration, and establishing durable water and air barriers lies in the installation details: foaming sheathing seams, taping housewrap, and flashing windows.
We also think that our solution is better than the currently popular “flash and fill” option that uses an inch or so of closed-cell foam sprayed against the inside of the stud bays. Placing the insulation inside the wall cavity doesn’t address thermal bridging, and this technique also has been shown to develop gaps when the framing dries and shrinks.
Here is a look at one project where we did most of this work on site.
Wall assembly is part of a larger system.
Because it’s more efficient to build as much of the wall assembly as we can in our…