Metal framing’s light weight, dimensional stability, and resistance to fire and decay are all advantages over wood. For illustrations of these benefits of metal framing, look no further than the contents of this issue of Walls & Ceilings. Aside from the aforementioned benefits of steel over wood, there remains another issue with metal framing that is currently being addressed and corrected: metal’s thermal performance.
According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers, heat loss from a wall is affected by framing as well as insulation. ASHRAE reports that steel conducts heat more than 300 times faster than wood and, although much more information is needed, current data reveals that steel framing does have a negative impact on the energy performance of residential buildings.
ASHRAE recently prepared a set of correction factors based on several calibrated hot-box tests for 2x4 walls. Solid, 16- to 18-gauge, C-channel steel studs were used and only straight wall sections were tested. The “framing factor” or the percent of the wall occupied by framing was 12 percent for walls with studs spaced 16 inches on center and 9 percent for 24 inches o.c. spacing. Correction factors for other wall configurations were extrapolated from those results. For comparison, consider two walls with 2x8 studs and framing spaced 24 inches o.c. With wood studs, the wall would have an insulating value of R-23.8. ASHRAE’s test results show a wall with steel studs at R-12.3.
Several factors that affect thermal performance were not considered in ASHRAE’s tests. The effects for other parts of a building, such as roof and ceiling assemblies, walls with corners, and the wall-to-roof and wall-to-floor connections are not known. Screws and other fasteners may also promote heat loss. Factors not included that may reduce heat loss include thermal breaks in the studs, punch outs in the web, special stud facings and wider stud spacing. Remaining factors that could increase heat loss include thicker gauge steel and framing factors higher than 12 percent of the wall area.
The National Association of Home Builders Research Center also acknowledges the lower insulating value of metal. In its study, NAHB found that R-values measured for steel-framed wall assemblies (including drywall and plywood sheathing) were lower than values typical for similar wood-framed walls.
For example, the 6-inch wall with an R-19 batt measured only R-10.1, 53 percent of the nominal value of the insulation. It isn’t just metal that’s cool: NAHB reports that wood framing reduces the overall insulating value of walls, too. The Bonneville Power Administration’s default value for a similar wall is R-18.2, more than 90 percent of the insulation’s rated value.
The recommendation for remedy according to ASHRAE and NAHB: foam sheathing. With low R-value sheathing, such as plywood, the rapid heat flow through steel studs reduces the insulating value of cavity insulation between 53 and 72 percent. NAHB’s test results showed that foam sheathing increased the insulating value of the wall more than its rated R-value.
This is good news for contractors still perhaps unsure of steel. Next month, W&C will explore various insulation topics like this one.