Ross Deter, California Energy Commission's Deputy Director for Energy Efficiency,
Soheil Loghmanpour, Associate Mechanical Engineer, Efficiency Technology Office
Metal Home Digest
Leading The Way For Builders ... Designers...Developers
May-June 1996 (Volume 4, Number 3)
Metal Home Digest (ISSN 10858474) is published six times a year (January, March, May, July, September and November) by Modern Trade Communications, Inc., 7450 N. Skokie Blvd., Skokie, IL 60077. Phone (847) 674-2200
Have you decided to switch to steel framing? Considering the high and unpredictable price of lumber in today's market, steel seems a logical alternative to wood construction.
In addition to its stable price, steel framing has other benefits over wood framing, including increased strength-to-weight ratio and better fire and termite resistance. On the other hand, a major disadvantage is that steel is a very good conductor of heat into and out of the building. A poorly designed steel framing system will increase heating and cooling costs considerably.
The California Energy Commission has established energy efficiency building standards that require that steel-framed systems meet a minimum thermal efficiency requirement. This requires construction technologies different from traditional wood frame construction.
Historically, replacing wood with steel stick-for-stick has been a common practice in steel-framed construction. Many designers erroneously assume steel and wood studs are thermally equivalent. Noting that the steel studs are thin compared to wood studs, some argue that the difference in thickness is more than enough to offset the conductive characteristic of steel. Despite their thin web, however, steel studs are much more conductive than wood studs.
Heat flow through a steel-framed assembly involves conduction from the warm side to the cold side by way of stud flanges that are as wide as the thickness of a comparatively-sized wood stud. The heat is conducted between the flanges via a thin web as long as the depth of a wood stud. Calculations show that the web of an 18-gauge steel stud is about 31 times thinner than a "two-by" wood stud; however, steel conducts heat 310 times more efficiently than wood. As a net result, a "two-by" steel stud will conduct 10 times more heat than a "two-by" wood stud.
To make up for this efficiency loss, a designer could use larger studs and add more insulation, but thermal bridging will substantially diminish the benefit gained by additional insulation. The resulting thicker walls also cut down the amount of usable living space inside the building.
High conductivity is not the only undesirable property of steel construction. Highly conductive steel framing attached to typical assembly layers such as gypsum board, plywood and stucco creates two dimensional heat flow, further increasing heat flow through the assembly.
The solution to thermal bridging is to interrupt the path along which heat flows. This can be accomplished in several different ways including: (1) punching large holes in the web without affecting the structural integrity of the stud; (2) spacing the studs as far apart as possible without affecting the integrity of the structure; and (3) installing exterior insulation (rigid insulation installed on the outer side of the construction assembly).
The maximum U-value allowed for a residential wall system in California's residential energy efficiency standard is 0.088 Btu/h-ft2-° F. That is the equivalent of a standard wood-framed assembly using two-by-four wood studs spaced 16 inches on center with R-13 cavity insulation. It has 1/2-inch gypsum board on the interior and building paper and 7/8-inch stucco on the exterior. Replacing the two-by-four wood studs with 18-gauge two-by- four steel studs with no knock-outs increases the overall U-value to 0.187 Btu/h-ft2-° F, over two times the heat transfer of the wood frame wall. However, installing R-6 exterior rigid insulation reduces the overall U-value to 0.088 Btu/h-ft2-° F, matching the performance of the wood wall.
To compute U-values of light-mass, wood-framed assemblies to show compliance with its energy efficiency standards, the California Energy Commission has adopted the parallel path calculation method developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). (See note.) This method, which assumes that heat flow through an assembly is one-dimensional, produces acceptable results for wood-framed systems. It cannot be used, however, for calculating U-values of steel-framed assemblies because heat conduction through metal systems is two dimensional.
To account for two-dimensional heat flow, ASHRAE developed the zone method of calculation. The Energy Commission adopted this technique for calculating U-values of light-mass steel-framed assemblies to show compliance with the energy efficiency standards.
ASHRAE's zone method is complex and lengthy. To reduce the possibility of error when the figures are calculated by hand, the Energy Commission has developed two alternatives based on this method: (1) The Nonresidential ENV-3 Form, which can be used for both residential and nonresidential buildings, and; (2) the EZFRAME software program.
Both the ENV-3 Form and EZFRAME can be used to design energy efficient steel-framed assemblies and to show compliance with the energy efficiency standards. The ENV-3 Form uses pre-calculated metal framing factors developed and tabulated for limited framing sizes and insulation levels, while the flexible, easy-to-use EZFRAME program allows computer users to model many different types and sizes of wood and steel framed systems.
Steel framing can be designed for energy efficiency. By using steel studs with large knock-outs in the web, spacing the studs on 24-inch center and using exterior rigid insulation, thermal efficiency of a steel-framed system can meet and exceed the requirements of the California's building energy efficiency standards. Exterior rigid insulation provides an excellent barrier to the conduction of heat into and out of a building.
California's building energy efficiency standards do not have requirements for the thermal efficiency of wall, floor and roof intersections, or wall framing for doors and windows. Because framing systems are sometimes over-designed for higher structural strength than is required, designers are encouraged to carefully optimize the structural and heat flow characteristics of their framing designs. Where possible, they should minimize the use of framing members to increase energy efficiency.
If you have questions regarding the EZFRAME computer program, need help with a particular assembly or want copies of the ENV-3 Form, please contact the Energy Hotline at (800) 772-3300 or (916) 654-5106, Monday through Friday, 8 a.m. to noon and 1 p.m. to 4 p.m.