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.

A good window installation converts a hole in the wall into an integrated part of all three important barriers that make up a wall assembly: the air, thermal, and water barriers.

Because all windows leak at some point, rough openings need to be designed to handle water entry. The rough sill should be flashed with either a pre-formed manufactured sill pan or a site-built pan.

This detail shows a site-built sill pan installed in a double-stud wall. The water-resistant barrier (WRB) can be asphalt felt or plastic housewrap.

For more on windows:

GBA Encyclopedia topic page

Video Series: Flashing a Window in a Foam-Sheathed Wall

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…