Installation and applications

The issue of thermal bridging in steel-framed walls has been attrib­uted to the reduction of whole-wall R-values in these applications. In a study performed by National Association of Home Builders (NAHB) Research Center, R-values measured for steel-framed wall assemblies (including gypsum wallboard and plywood sheathing) were lower than values typical for similar wood-framed walls. For example, a 6" steel stud wall with R-19 batt insulation measured only R-10.1, demonstrating a 47 percent loss in insulation value at the center of the cavity. The wood-framed walls only lost 10 percent of the nominal insulating value.8

Tests show that foam sheathing helps block the rapid heat loss that would otherwise cut in half the value of cavity insulation. In fact, the application of foam sheathing increased the insulating val­ue of the steel stud wall by more than its rated R-value. For exam­ple, the test wall has an R-value of 10.1. Adding Iм of XPS with an R-value of R-5.0 increased the wall’s R-value to R-16.2. The effi­ciency of the Iм XPS board was 122 percent. By blocking the ther­mal bridge, foam sheathing restores some of the insulating value of cavity insulation that had been bypassed by the steel studs.8

In comparison, foil-faced polyisocyanurate insulation had a notably lower thermal efficiency. Another test wall increased from R-10.1 to R-17.1, whereas the nominal value of the polyisocyanu­rate board itself was listed at R-6.8. The foil facer may be the rea­son the board’s efficiency rating was a slightly lower 102 percent efficiency rating.8 (Fig. 11.8).

Placing foam tape on the face of the studs also was studied. The tape increased the insulating value by only R-0.5. It started out at 5/16M and was compressed directly under the screw to only У8". There appears to be little to gain by increasing the thickness of the wall cavity. The best-performing wall measured in this study was 35/8M thick with an R-ll batt and 2" of XPS insulation. It surpassed a wood-framed R-19 wall by a value of R-0.5.8

З’/У Steel Studs 16-18 да. 16" o. c.

2" Air Space