Double-stud walls are a great way to create a thick, well-insulated wall with very little thermal bridging as part of an energy-efficient home. They don’t require designers and builders to learn about a new product, and building the house doesn’t require techniques that they are not already familiar with. Builders typically construct the exterior load bearing wall first, then the floors and roof. The double, interior wall is built afterwards, but is still just an additional stick-framed wall. The tricky part comes when it’s time to frame and finish the windows and doors which will be installed in uncommonly thick walls.
The location of the studs in a double-stud walls is not important, they can be lined up, or staggered. Of course, you’ll likely want them to be on 16 in. or 24 in. centers to easily layout sheet goods like sheathing and drywall. At the rough openings for doors and windows, the framing of the outer and inner wall must align perfectly. It is common, as shown in these details, for the rough opening to be laid out and framed with extra space so the carpenters can line the opening with plywood or OSB, sometimes referred to as a “buck,” that connects the two walls. The plywood has many purposes. It holds the insulation in the walls, brings the air barrier into the opening, and give a fastening surface for the drywall returns or extension jambs that will be used to finish the opening.
Doors and windows can be placed flush with the exterior face of the wall, like in a traditionally-framed house (called “outies” in a double-stud wall), they can be placed in the interior wall (called “innies”) or they can even be placed somewhere in the middle (called “in-betweenies”). There are advantages and disadvantages to…
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.
In-swinging exterior doors installed in thick double-stud walls usually require a frame-within-a-frame approach. The 12-inch-thick double-stud wall needs an oversized rough opening. Within that rough opening, a second (smaller) rough opening is framed with 2x4s; this smaller rough opening is located toward the exterior of the larger rough opening. This framing method allows the door to open without impinging on the deep interior jambs.
In general, exterior above-grade walls should have continuous air, water, and thermal barriers.
Installing rigid insulation as the exterior sheathing can accomplish all of these tasks, but you need to account for shear resistance (provide wall bracing) in other ways, and you must provide special details if you expect the foam sheathing to perform as a water-resistive barrier (WRB).
A double-stud wall insulated with dense-packed cellulose is an excellent thermal barrier; however, double-stud walls still need at least one additional air barrier to limit air leakage through the cellulose.
Check out the advanced framing section of Construction Detail Library for alternative techniques…