Steel-framed walls have substantially lower insulating value than similar wood-framed walls according to a research report prepared by the National Association of Home Builders Research Center. A related set of thermal design guidelines show how to make up for most of the thermal shortfall with insulated sheathing.

In the study, 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. That's 53 percent of the nominal value of the insulation. 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, over 90 percent of the insulation's rated value.

A partial list of the test results appears in the table. These new tests agree with a previous test used to develop thermal correction factors for the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) Standard 90.1g. (See December 1994.) However, ASHRAE tested only walls with plywood sheathing and then calculated the impact of adding foam sheathing. The NAHB Research Center tested 23 different wall samples, including many with foam.

These new tests show that foam sheathing addresses two problems with steel-framed walls. First, it helps block rapid heat loss that would otherwise cut in half the value of cavity insulation. Second, it reduces the potential for dust streaking on the inside surface of walls in northern climates.

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.

Test results showed that foam sheathing increased the insulating value of the wall more than its rated R-value. For example, wall D1 has an R-value of 10.1. Adding one inch of extruded polystyrene (XPS) with an R-5.0 would be expected to boost the wall's insulation value to R-15.1. However, results from wall D2 show heat loss at R-16.2, an increase of R-6.1.

Of course, the foam's R-value doesn't really change when it's placed on a steel-framed wall. By blocking the thermal bridge, foam sheathing restores some of the insulating value of cavity insulation that had been bypassed by the steel studs.

Researchers at Oak Ridge National Laboratories, who authored a portion of the report not related to the tests, have a name for this phenomenon. They call it "thermal efficiency of insulation." The efficiency of that one-inch XPS board was 122 percent (6.1 ÷ 5=122).

Interestingly, foil-faced polyisocyanurate insulation had a notably lower thermal efficiency. Wall D3 measured R-17.1, while the nominal value of the board itself was listed at 6.8. That gives it an efficiency of 102 percent. Experts believe the foil facing may be the cause.

Hat Channels

In some buildings, horizontal metal furring, called "hat channels," are installed to help support sheet materials. Because the channels hold sheathing away from the studs, some experts have expected walls with hat channels to have a higher R-value. Walls A7 and A8 with the channels showed slightly higher R-values compared to walls A1 and A2. However, the difference was small- about the same as the R-value of the air gap created by the channels. Comparing walls D1 and D4 showed a larger difference, but it was probably caused by increasing stud spacing from 24 in. to 48 in. on center.

Foam Tape

It has been suggested that placing foam tape on the face of the studs would create a thermal break. However, tape increased the insulating value by only R-0.5. It started out at 5/16 inch and was compressed directly under the screw to only 1/8 inch.
Cold Spots and Streaking

On the wall with only plywood sheathing (A1), the drywall temperature remained constant over the insulated cavity. However, it began dropping rapidly about 2 inches from the edge of the stud. Directly over the stud the drywall temperature was 10°F lower than the temperature over the cavity. The wall with 2 inches of XPS sheathing showed only a 2.5°F temperature drop from cavity to stud.

These cold spots sometimes promote condensation on the inside wall surface. Dust attracted to the water causes streaks, sometimes called ghosting. The temperature of the interior wall surface plays a major role in this process.

The report cited previous research indicating that slight ghosting can be expected when the wall temperature over the stud is 3.3°F lower than the temperature over the cavity, and that severe streaking occurs when the temperature is 8°F lower. In this test, walls with plywood sheathing showed 8°F to 10°F lower temperatures over the studs, making them likely to develop streaks. The report states that no ghosting problems would be expected in any of the wall with insulated sheathing. However, walls with one inch of XPS showed a temperature drop of about 4°F to 5°F. If a 3.3°F temperature difference is the magic number, then these walls face some risk for streaking.

Sheathing vs. Cavity Insulation

Foam sheathing can help counteract the significant thermal penalty of steel framing. For example, the insulating value of a 6-inch steel-framed wall with an R-19 batt and one inch of polyisocyanurate foam (D3) is seven percent lower than a 2x6 wood-framed wall with just an R-19 batt. The best performing wall measured in this study was 3-5/8-in. thick with an R-11 batt and two inches of XPS insulation (A6). That one would surpass a wood-framed R-19 wall by R-0.5.

There appears to be little to gain by increasing the thickness of the wall cavity. Instead, the optimum steel-framed wall would be one with 3-5/8-in. studs and thick foam sheathing.

Roofs and Floors?

This study shows that foam sheathing can correct the most serious thermal flaws with steel-framed walls, reducing heat loss almost to the same level as standard wood-framed construction. One large question remains unanswered: What are the thermal effects of steel roof and floor framing? It's not so simple to attach foam insulation to these components. The answer will have to wait for future research.