Construction: Framing for Increased Thermal Efficiency

The 2 x 4 (38 x 89 mm) has been the standard-size wall stud since light framing was invented. In recent  years, however, pressures for heating fuel conservation have led to energy code requirements for more thermal insulation than can be inserted in the cavities of a wall framed with  members only 3½ inches (89 mm) deep. One solution is to frame walls with 2 x  6 (38 x 140 mm) studs, usually at a spacing of 24 inches (610 mm), creating an insulation cavity 5½ inches (140 mm) deep. Alternatively,  2 x 4-framed walls may be covered either inside or out with insulating plastic foam sheathing, thus reaching an insulation value about the  same as that of a conventionally insulated 2   6-framed wall. For even greater insulation performance, 2   6 studs and insulating sheathing may be used in tandem. Alternatively, in very cold climates, even deeper wall  assemblies that can achieve greater insulation values may be constructed of two separate layers of wall studs  or of vertical truss studs made up of pairs of ordinary studs joined at intervals by plywood plates. Some of these constructions methods are illustrated in Figures 7.17 through 7.21.

Figure 7.17
Insulation levels in walls of light frame buildings can be increased from the R13 to R-15 (RSI-90 to RSI-104) of a
2 x 4 stud wall to R19 (RSI-132) or more by using either 2 x 6 framing and thicker batt insulation (left) or 2 x 4
framing with plastic foam sheathing in combination with batt insulation (right). The foam sheathing insulates the
wood framing members as well as the cavities between them but can complicate the process of installing some types
of siding.
Figure 7.18
Window and door headers in 2 x 6 framing require special
detailing. Two alternative header details are shown here in
section view: (a) The header members are installed flush
with the interior and exterior surfaces of the studs, with an
insulated space between. This detail is thermally efficient
but may not provide sufficient nailing for interior finish
materials around the window. (b) A 3 x 2 spacer provides
full nailing around the opening.
Figure 7.19
Two alternative corner post details for 2 x 6 stud walls, shown in plan view: (a) Each wall frame ends with a full 2 x 6 stud, and a 2 x 4 nailer is added to accept fasteners from the interior wall finish. (b) For maximum thermal efficiency, one wall frame ends with a 2 x 4 stud flush with the interior surface, which eliminates any thermal bridging through studs.
Figure 7.20
With the two framing methods shown here, walls can be insulated to any desired
level of thermal resistance.
Figure 7.21
A raised-heel roof truss provides plenty
of space for attic insulation at the eave.

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