This article was originally published in the July/August 1994 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online July/August 1994
Studs of Steel
Rising lumber prices and an erratic market have driven some homebuilders to look for alternatives to wood framing, and steel framing is one option that is grabbing their attention. According to a survey of 561 builders conducted at the National Association of Homebuilders' (NAHB) annual conference in January 1994, 64% consider lumber price and availability to be the most significant issue facing them. Of those surveyed, 11% are already using steel framing, and 34% say they plan to use it. Less than 1,000 steel-framed houses went up in 1992, but that number jumped to almost 15,000 in 1993, and the American Iron and Steel Institute estimates that another 75,000 will be built in 1994. However, unless builders change their techniques to ensure the thermal integrity of the alternative frame, steel-frame homeowners could be in for some cold nights.
The problem is that metal studs conduct heat many times faster than wood, serving as a sort of heat-loss superhighway. To prevent excessive heat loss, the studs must be properly insulated and sealed into the walls. The most commonly used and recommended solution is an exterior layer of rigid insulation sheathing to break that thermal bridge. But not all builders who use steel framing are addressing the alternative stud's shortcomings.
According to California Energy Commission calculations, metal-framed wall assemblies with R-30 insulation have an effective R-value of less than 12. Similarly, the National Research Council of Canada reported measuring the R-value of a steel-framed wall section with R-12 batt insulation at only R-6.8 overall. NAHB is developing a simplified procedure for calculating metal-frame-assembly U--values for the American Iron and Steel Institute. (For the unfamiliar, a U-Value is a measure of the number of Btus that will actually flow through a square foot of a given material in an hour, for each degree of temperature difference between either side of the material.) There's some confusion about which method to use, said NAHB's Ed Barber, explaining that different engineers come up with wildly varying values for similar assemblies.
The California Energy Commission in 1993 issued a procedural guide for calculating the overall U-value of metal-framed wall and roof assemblies for builders to use in demonstrating the compliance of their buildings with the state's building-efficiency standards. More recently, the CEC introduced an IBM-compatible software program for the calculation, called EZFRAME, to further simplify the process. The formula sums the resistance of each component of the wall or roof and calculates an average envelope R-value. Because of metal's high conductivity, its R-value is very low, which reduces the overall average accordingly. California requires builders who elect to use metal framing to follow these calculation procedures.
Several techniques can help minimize the heat loss associated with steel-framed construction. For instance, installing rigid-foam insulation on the exterior of steel-framed walls will increase the whole wall's R-value. Another approach is to use steel studs only on interior walls. Buying full-width insulation (16 or 24 inches) to accommodate the shape of the steel studs, or using blown-in insulation, will also help ensure the wall cavity is filled. Since steel studs can reduce the R-value of cavity insulation by more than 50%, framing 4-inch walls and then installing rigid foam is more economical than striving for 6-inch walls. Foam-core panels are another option, since these products are insulated on the outside of the frame, minimizing heat loss from the steel. Rather than the stick for stick approach to building (replacing each wood stud with a steel one), an engineered framing system with structural supports placed as much as 8 ft apart will reduce thermal bridging through the insulation.
With the help of the American Iron and Steel Institute, NAHB is conducting a series of introductory workshops on the topic of steel-frame construction. The association's Tim Waite, who leads the workshops, said that most of the residential steel-frame construction to date has taken place in areas with thermally forgiving climates, Southern California, the Southwest, and Florida. His workshop teaching partner is Bruce Ward, an Oregon-based steel-frame homebuilder who has been putting up Good Cents steel-frame homes since 1978. Energy consumption in Ward's steel-framed homes is comparable to, and in some cases lower than that of similar wood-frame buildings.
Ward's Super Good Cents house in Salem, Oregon, used advanced framing techniques to minimize studs, and thus conductivity, along with exterior R-5 rigid insulation, for an overall calculated R-value of 19.08. New guidelines, which are based on a wood-framed model, now require R--21 insulation in the wall, plus R-5 exterior insulation. We would have a hard time qualifying a steel-frame house for Super Good Cents now, said Salem Electric conservation supervisor Terry Kelly.
-- Abba Anderson
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