Double-wall construction is one option for high R-value walls. They are relatively inexpensive to construct and use readily available materials that construction crews may be more familiar with than other high-R-value options such as structural insulated panels (SIPS) and insulated concrete form (ICF) walls. Double-wall construction consists of two stud-framed walls set up next to each other to form an extra thick wall cavity that can be filled with insulation. Because the interior and exterior framing are separated by insulation, thermal bridging is also reduced or eliminated. Building Science Corporation, a Building America research partner, investigated several forms of double-wall construction including double-stud walls, truss walls, and offset frame walls in a study on high R-value walls (Straube and Smegal 2009). Some of the forms they investigated are described below.
Double walls would be designed and specified by the architect and implemented by the framers. Site supervisors should ensure that framers and trades responsible for air sealing, insulating walls, and installing windows are knowledgeable or trained in techniques required for double-wall construction and that skill level expectations are included in the contracts for these trades.
How to Construct a Double-Stud Wall with Cellulose Insulation
One form of double-wall construction consists of an exterior 2x6 or 2x4 stud-framed structural wall and a second 2x4 nonstructural wall built to the inside with a gap in between of several inches. If the studs in each wall are installed at the same spacing (e.g., 24-inch on-center) they can be staggered, although research has shown only minor improvement (<R-1) when staggering the studs (CARB 2009). Plywood boxes must be installed around the rough-in spaces for installing windows, which are typically installed flush with the exterior wall. The cladding attachment is the same as normal stud-framed construction practice.
The example shown here uses a 2x4 exterior structural wall built at 16-inches on-center and a second 2x3 wall built at 24-inches on-center that is nonstructural but is used to support drywall and electrical services. The two stud walls plus the gap in between provide a 9.5-inch cavity for cellulose insulation, which would have a clear-wall R-value (for that section of the wall without interruptions) of R-34 or a whole-wall R-value of R-30 (Straube and Smegal 2009). In this example, a Class I vapor barrier of 6-mil polyethylene is installed on the exterior side of the interior wall to control air leakage and vapor diffusion (see Straube and Smegal 2009 for more on moisture control). A vapor barrier may be appropriate in very cold climates, but is not necessary in warmer climates. If one is installed, it should be located as shown here on the exterior side of the interior wall and care should be taken that insulation on both sides of the vapor barrier is fully aligned with the barrier the entire length of the wall.
Figure 1 - This double-stud wall, consisting of a 2x4, 16-inch on-center exterior wall and a 2x3 24-inch on-center interior wall, provides 9.5 inches of wall cavity space filled with cellulose insulation for a whole wall R value of R-30
How to Construct a Double Stud Wall with Spray Foam
A combination of insulation types can be used. In this example, high-density spray foam provides increased air sealing and moisture protection, decreasing the risk of wintertime condensation on the interior side of the exterior wall while less expensive cellulose provides additional R-value. This example again uses a 2x4 exterior structural wall built at 16-inches on-center and an interior 2x3 nonstructural wall built at 24-inches on-center providing a 9.5-inch cavity. The exterior wall is covered with rigid fiberglass foam sheathing, and then the inside surface of the exterior wall is covered with 2 inches of high-density spray foam. For climate zones 6 and higher, a Class I vapor barrier of 6-mil polyethylene is installed on the back of the interior wall. (Class I vapor barriers are not recommended in warmer climates [See Straube and Smegal 2009 for more on moisture issues]). The cavities remaining in the interior wall and between the polyethylene and the spray foam are filled with a total of 7.5 inches of cellulose, for a calculated whole-wall R-value of R-32.4 and a clear-wall R-value of R-36.2 (Straube and Smegal 2009).
Figure 2 - This double-stud wall - consisting of a 2x4, 16-inch on-center exterior wall and a 2x3 24-inch on-center interior wall - has 2 inches of high-density spray foam applied to the interior surface of the exterior wall for air sealing and moisture control, plus 7.5 inches of cellulose insulation providing for a whole-wall R value of R-32.4
How to Construct a Truss Wall
The truss wall uses two sets of studs like the double-stud wall, but in this case the interior wall is the load-bearing wall. The exterior wall is attached at each stud and hangs cantilevered outside of the foundation wall, which frees up floor space compared to a traditional double-stud wall. The interior and exterior wall studs are aligned and connected with plywood gusset plates toward the top, middle, and bottom of each pair of studs, and a plywood cavity closure at the top and bottom of the stud cavities. These gussets and closures provide stability so that the walls can be further apart, allowing more room for insulation. The bottom edge of the exterior wall drops below the sill plate, providing space that can be filled with insulation along the exterior side of the rim joist, thus minimizing the thermal bridging that can otherwise occur through the rim joist.
In this example a 2x4, 16-inch on-center interior load-bearing wall is connected to a 2x3, 16-inch on-center exterior nonbearing wall spaced to provide 11 inches of cavity width that is filled with cellulose insulation for a calculated whole-wall R value of R-36.5 or clear-wall value of R-40.5 (Straube and Smegal 2009). This wall, which is designed for climate zones 6 or higher, has a Class I vapor barrier of 6-mil polyethylene installed between the interior wall and the drywall. (For warmer climate zones, no vapor barrier should be installed.) Because of the location of the plastic in this wall assembly immediately behind the drywall, it will be perforated by electrical wiring and plumbing. If the wall is not air sealed at the drywall layer, warm moist air could get into the wall cavity through these perforations and condense on cooler outer framing. (See Straube and Smegal 2009 for more on moisture issues).
Figure 3 - This truss wall - consisting of a 2x4, 16-inch on-center interior load-bearing wall connected to a 2x3, 16-inch on-center exterior nonbearing wall with plywood gussets - is spaced to provide 11 inches of cavity width that is filled with cellulose insulation for a calculated whole-wall R-value of R-36.5 or clear-wall value of R-40.5