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:
Video Series: Flashing a Window in a Foam-Sheathed Wall
All of the GBA window details reflect two basic principles: Flash and drain both the rough opening and the window unit; and weatherlap all components as much as possible. The inclusion of both rigid exterior insulation and a vented air space in just about all of the details means that there are many ways to achieve flashed and properly weatherlapped rough openings…
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