Double-stud walls use common materials and familiar assemblies to create a low-tech, energy-efficient wall with lots of room for thick insulation. This framing method virtually eliminates thermal bridging through the studs and greatly reduces sound transmission through walls.

The basic strategy is simple: The exterior walls are built from two parallel stud walls with a gap between the rows for extra insulation. Many builders use two parallel 2x4 walls with a 5-inch gap between them to create a 12-inch-thick wall. Of course, the wall can be thicker or thinner as circumstances dictate.

The most commonly used insulation for this method of construction is dense-packed cellulose, although other types of insulation (including blown-in fiberglass, mineral wool batts, or open-cell spray polyurethane foam) can certainly be used.

For more information, see GBA Encyclopedia: Double-Stud Walls.

Protect the head of the window from rain by tucking the window in from the face of the wall. Overhangs don’t help much on the coast — water blows sideways, up and down — and on taller buildings, an 18-inch overhang doesn’t help the ground-floor windows at all. On a building with 12-inch thick well-insulated walls, we’ll slide the windows back 3 1/2 inches so that every window has an individual built-in overhang right above the window head. That means we install most windows as “in-betweenies” instead of “innies” or “outies.”

Treat the window flange as an installation aid only, not as a water or air seal. Expect that water will be able to enter behind the flange during extreme conditions, and always provide a pathway out. Follow the manufacturer’s instructions; your warranty depends on it.

Since water will probably get behind the flange at some point, make sure it can’t pool on bare wood…