UPDATED on April 3, 2016
Builders in northern states and Canada often specify exterior wall foam for new construction as well as for residing jobs on existing houses. Installing rigid foam on exterior walls reduces thermal bridging through studs and (as long as the foam is thick enough) greatly reduces the chances of condensation in wall cavities. Current trends favor thicker and thicker foam; many cold-climate builders now routinely install 4 or 6 inches of EPS, XPS, or polyiso on exterior walls.
Builders installing thick exterior wall foam can install windows two ways: with the window flanges in the same plane as the back of the siding — so called “outie” windows — or with the window flanges in the same plane as the OSB wall sheathing — so-called “innie” windows. Either way will work.
Let’s consider a wall with 2×4 studs, OSB sheathing, 4 inches of exterior foam, 3/4-in.-thick vertical strapping, and fiber-cement lap siding. How should the windows be installed?
A builder who prefers “outie” windows would do it this way:
On the other hand, a builder who prefers “innie” windows would do it this way:
The innie vs. outie debate has been going on for decades. Back in 1984, here’s what builder John Hughes of Edmonton, Alberta, had to say about the debate: “It’s possible to locate windows and doors anywhere on these broad sills, but most people like to keep windows flush with the new exterior wall. This creates a wide sill inside the house, looks good on the outside, and eliminates the necessity of installing a broad, weather-resistant exterior sill.” (The quote is from Hughes’ article, “Retrofit Superinsulation,” published in the April/May 1984 issue of Fine Homebuilding.)
Both innie and outie windows have strong advocates. When Energy Design Update reported on the innie-versus-outie controversy in July 2002, building scientist Joe Lstiburek…