Steel Studs Require Exterior Insulation to Avoid Thermal Bridging
From a distance, an unsheathed, light-gauge steel home frame just looks like a shiny version of a stick-built frame. The studs, headers, and rafters are placed in much the same configuration. The parts of a steel frame are cold-formed channels of galvanized steel of various widths and thicknesses. Because components are fastened with screws instead of nails, and because of steel's inherent strength and consistency compared to wood, this type of framing can be a durable choice in areas that see high winds, severe termite problems, or regular seismic activity. Because of its dismal thermal performance, however, steel framing is usually a poor choice for a green home.
See below for:
METAL CONDUCTS HEAT
The gauge depends upon the load
Non-load-bearing partitions can be framed with 25-gauge steel studs with 1 1/4-inch flanges. Structural walls require heavier 20-gauge to 14-gauge studs with 1 5/8-inch flanges.
Steel frames call for unique details
Create a good thermal break. This generally means putting a layer of continuous rigid board insulation on the outside of the studs. Without this layer, heat and cold will have a thermal shortcut from indoors to out (and vice versa). Heating and cooling the home will cost more, and the risk of condensation in wall cavities is greater, which can be problematic for both durability and health reasons.
Be careful with the details. An outer layer of foam insulation needs to be factored into wall thicknesses, window installation, and other aspects of the envelope design. Of particular importance is how the insulation will integrate with drainage planes and air barriers.
Educate yourself and your crew. Unless your framers also work on commercial buildings, there may be a learning curve with a steel-framed home. Steel studs have unique assembly methods, and they support loads differently than wood framing. Construction details and span charts for light-gauge steel framing can be found in "Standard for Cold-Formed Steel Framing: Prescriptive Method for One- and Two-Family Dwellings," available from the Steel Framing Alliance (www.steelframing.org).
Tools for steel framing
The best tool for cutting steel studs is a 14-inch chop saw like the Milwaukee 6180-20, which has a 14-inch carbide or abrasive metal-cutting blade, although aviation snips are great for quick adjustments.
The best fastening tool is a torque–adjustable screwdriver, commonly called a Tek screwdriver or Tek gun. Unlike a drywall screw gun, which sets fasteners based on depth, Tek guns stop turning when they reach a predetermined torque setting, so they’re less likely to strip or break fasteners. These are available in both corded and cordless models. DeWalt’s corded model is linked below, although most power-tool makers offer similar tools.
Metal framers find a 4- or 6-inch pair of C-clamp locking pliers indispensible for holding metal studs while they’re being fastened. They also prevent the stud from being deformed when the fastener first makes contact with the stud face. The clamp can be hung from an extra hammer loop when it's not needed.
Vise-Grip Locking C-Clamp
Check the span tables in the code
The 2006 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. includes fastening schedules, span tables, and construction details for steel framing in sections 505 (floors), 603 (walls) and 804 (roofs and ceilings). The sections are organized in the same way as the code sections on wood framing, with the most basic standards like material quality and maximum building sizes given first. More specific information like span tables can be found later in the respective section.
ABOUT LIGHT-GAUGE STEEL FRAMING
Steel is easily recycled
Wood is much more common than metal when it comes to framing houses, but light-gauge steel studs outperform their wooden counterparts in a number of categories, such as recycled content, rot resistance, and reduced job-site waste. Steel framing has two major drawbacks, however: high cost and much lower energy performance.
MORE ABOUT LIGHT-GAUGE STEEL FRAMING
While there is much to be said in favor of steel framing, the key drawback is its poor thermal performance. Steel conducts heat much more effectively than wood, and even though the web is far thinner in cross section than 2x material, heat loss is significant. The California Energy Commission claims that a steel stud conducts 10 times as much heat as a wood 2x.
This thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. drastically lowers the performance of insulation in wall cavities. A study by the Oak Ridge National Laboratory (ORNL) found that thermal bridging in a conventionally framed wood wall lowers the performance of cavity insulation by 10%; in a steel-frame wall, performance drops by up to 55%. Simply making wall cavities deeper to compensate for this thermal short circuiting is ineffective.
The ORNL research, however, looked at a number of possible solutions and found that steel-stud walls can perform as well as or better than similar wood-frame walls. The most common fix is to wrap the exterior of the building with rigid foam insulation, which provides a thermal break for the steel framing. A steel-framed wall wrapped with foam performs better than a steel-framed wall without foam — but a wood-framed wall wrapped with foam performs better still.
Other options cited in the ORNL report included using spacers to isolate the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. from the studs, and using foam-covered steel studs; both techniques interrupt heat conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow. through the steel frame to the outdoors.
Perhaps the best fit for steel framing is in mild climates such as Hawaii, where steel's thermal performance is much less of an issue. In locations like this, the need to ship materials from great distances, along with moisture and insect pressures, give durable, lightweight steel framing an inherent advantage over wood.
Environmental Building News
Steel or Wood Framing: Which Way Should We Go?
Cold-Formed Steel Framing
Steel vs. Wood Framing, Long-Term Thermal Performance Comparison, Valparaiso Demonstration Homes
Primer on working with light-gauge steel framing, from
"Steel vs. Wood Framing, Long-Term Thermal Performance Comparison, Beaufort, S.C., Homes"
- Roe Osborn/Fine Homebuilding #97