If you install flanged windows on the exterior side of a sheathed 2×4 or 2×6 wall, and you then install exterior rigid foam, you end up with an “innie” window. Innie windows work well. Before you can install furring strips and siding over the exterior rigid foam, though, you need to come up with exterior trim details (sometimes called “exterior jamb extensions”).
Anyone installing innie windows needs to install weather-resistant (and waterproof) details to trim the exterior jambs, the exterior head, and the wide exterior sill. This isn’t impossible, but it’s fussy work that is rarely detailed well. Some builders use solid PVC trim (cellular PVC like Azek); some builders use copper flashing. Almost all builders who come up with site-built details like this end up using peel-and-stick flashing for the tricky spots, covered with some type of durable material to hide the peel-and-stick.
A new approach for exterior jamb extensions
During a recent visit to the Cold Climate Housing Research Center (CCHRC) in Fairbanks, Alaska, I saw a wall mockup that displayed an intriguing new way to trim innie windows. Ryan Tinsley, a research scientist at the CCHRC, explained that the trim system was developed (and is now sold) by Northerm Windows of Whitehorse, Yukon. (Capital Glass in Anchorage, Alaska, is a U.S. distributor of Northerm Windows.)
One architect who specifies the detail is John Berg of Stantec, an architectural firm in Whitehorse, Yukon. I called Berg on the phone and asked about the use of deep-set brickmold. “We struggled for at least twenty years with these window details, as more and more insulation was installed on the exterior of the structure and our walls got thicker,” Berg told me.
This window mockup at the Cold Climate Housing Research Center shows cutaway versions of vinyl components sold by…