Double Stud Wall / Window Jamb with Furring

Double-stud walls create plenty of room for thick insulation

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 bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. 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.

Great windows are only great if installed well

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 wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation.. 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

All of the 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 and window units. Check out more than one sill or head detail to see different ways of skinning the same cat.


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