Energy Source Builder
Steel Framing Offers Thermal Challenges
Workers build a steel framed house using an "engineered system."
In the housing industry -- notoriously slow for adopting new technology -- the rapid move to embrace steel framing seems remarkable. For a complete rundown on the advantages and disadvantages of steel framing, just leaf through recent issues of any construction industry magazine. One disadvantage is clear: steel conducts heat much faster than wood. When you try your first steel-framed house be prepared to present your code official with heat loss calculations to show that the house meets the energy code. The same will be true for utility home certification programs.
Here's an example of how serious thermal problems can be. A group of energy professionals recently visited a home built with 2x8 steel studs spaced 24 in. on-center. The walls were filled with R-30 fiberglass batts. Using infrared thermography, they measured the temperature of the inside surfaces. With the outside temperature at 42°F, the inside wall surface between studs was 61°, while the temperature at the studs was 51°. They measured 43° in one corner, just one degree warmer than outside. What will happen when it gets cold outside?
Heat Loss
Steel conducts heat over 300 times faster than wood. When steel studs are placed 16 in. on-center the insulating value of the wall is only 40 percent of the insulation material's nominal R-value. For example, putting an R-19 batt between steel studs gives you a wall that's only about R-7.6. Rigid insulation on the outside of a wall can cover the "thermal bridge" by adding it's stated R-value over the whole wall. So, the same wall with one inch of polyisocyanurate board (R-7.2) would be R-14.8.
Even the fasteners become a heat loss issue. The impact of fasteners was revealed in a recent study by Dow, makers of Styrofoam® rigid insulation. They reported that screws attached to steel studs reduced the insulating value of the foam sheathing by 39 percent.
Thermal bridging across a wall is bad enough, but imagine what happens in attics. Metal roof trusses extend like radiator fins from the interior gypsum to the exposed attic. There's no easy fix for this problem.
While rapid heat loss dramatically increases the energy use of such a building, occupants will be more alarmed by condensation problems. Cool interior surfaces allow water vapor to condense. These moist surfaces tend to attract dust and grow mildew. Since the coldest surface is usually over studs, the result could be called the "striped wall syndrome."
Immediate Solutions
- Use steel studs for interior walls. This starts the learning process with a small, manageable step that doesn't affect heat loss at all.
- Buy full width insulation. Batt insulation cut to fit between wood framing is 15 or 23 in. wide. Steel studs are often "C" shaped, so batt insulation should be the full thickness (16 or 24 in.). Unfortunately, most high density batts, such as the R-21 wall batt, aren't available in full widths. Blown-in insulation gets around the problem.
- Build 4 in. thick walls, and hang as much rigid insulation on them as you can. With solid steel framing extra cavity thickness is a waste of money.
- Consider foam core panels over a steel frame. By putting all the insulation outside the frame, you'll detour around the thermal bridge.
- Avoid the "stick for stick" approach -- where you replace each wooden member with a steel one. It may be simpler, but it will probably create an energy hog. If you're serious about steel, investigate various engineered framing systems. With these, structural supports can be four to eight feet apart, which reduces the number of thermal bridges through the insulation. One such system is made by Advanced Framing Systems of Atlanta, Georgia. Studs are 4 ft. on-center. Sheathing and siding attach to horizontal "hat sections" that reduce the wall area subject to direct conduction.
Eye to the Future
In the long run, steel framing will probably be one of several sensible alternatives to "stick-built" homes. An early step in this march to the future is research and development. For example, the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) hopes to begin a study early next year to measure the heat loss of many types of building components, including steel framing. The study should also take a close look at the impact of fasteners on heat loss.
To find a place in residential construction, the steel industry will have to sell products without the thermal liabilities. At least one product already exists. The Delta Steel Stud was developed 12 years ago in Canada. Unlike many studs, which are made of solid steel, the web of this one is more air than steel. Its manufacturer claims that the "lip reinforced, web perforations" create a stud just as strong as a common solid-web steel stud, but with thermal characteristics similar to a typical Douglas-fir 2x6.
The thermal problems with steel can be overcome. In the long run, structural systems will evolve to take advantage of steel's greater strength and to reduce heat loss.
This article appeared in Energy Source Builder #30 December 1993 |
