Let's Get More Familiar (Part Four)
arguments for and against the use of LGMF. Ultimately, this discussion will include the following aspects of the framing:
• Thermal performance
• Insect/pest resistance
• Seismic (earthquake) resistance
• Fire resistance
• Mold resistance
• Environmental benefits
Let's begin with the first-and most controversial-aspect on the list: thermal performance.
Achilles' heel?By its very nature, steel is a naturally conductive material, whereas wood has low conductivity and actually has insulating properties. In fact, generally steel is 400 times more conductive of heat as compared to wood. A 11⁄2 inch thick wood stud is plus or minus 10 times less conductive of heat than a 20 gauge metal stud. Thus, when this metal framing is used at the perimeter of a building envelope, between the interior and exterior spaces, thermal bridging will occur. Heat flow increases slightly around a wood stud, but converge on either side of a metal stud at the flanges.
Consider a wall with wood and/or metal framing spaced at 24 inches on center and R-13 cavity-fill insulation. The wood-framed wall has two times the R-value as the same wall framed with metal studs. Energy codes/standards, such as American Society of Heating, Refrigeration and Air-Conditioning Engineers 90.2 and the model energy code, are typically developed around the "U" value (thermal resistance) of an assembly. This is the ability of a wall or other building element to retain warm or cold air within the building envelope.
Use of the parallel path test method, which assumes heat flows straight through a wall and follows the path of least resistance (for the purpose of determining heat flow through a building envelope), is acceptable for wood framing but not for LGMF. Near a steel stud, heat moves sideways through a wall and then travels through it. Recognizing that there was indeed a significant reduction in thermal efficiency when LGMF is used at building perimeter walls, ASHRAE issued corrected values for LGMF exterior wall assemblies.
In the mid-'90s, the National Association of Home Builders research center found that heat flow calculations used by engineers were flawed in that they failed to correctly estimate the poor performance of the framing when incorporated into an exterior wall assembly. NAHB found that mock-up tests and sophisticated computer modeling were the only reliable means of determining heat flow through such assemblies. LGMF used in residential construction for floors, walls and roofs typically uses 24 inches o.c. spacing (due to steel's high strength-to-weight ratio) rather than the standard 16 inch o.c., used in traditional wood framing.
An NAHB study found that this increased spacing (fewer framing members) helped mitigate some of the thermal penalties encountered with LGMF. Care should be taken to avoid "clustering" of steel studs in exterior walls. This can create cold-spots in the wall and following proper insulation requirements for LGMF is a must.
Utopia callingIn effect, thermal bridging undermines the tightness of the building envelope and causes increased heating/cooling loads, thus requiring larger HVAC equipment. In the Northeast, typically 80 percent of the energy cost of a commercial building is for cooling rather than for heating. "Utopian" walls (those without corners) band joists, rough openings, etc., were often used as test models. This created misleading data. Use of less than full-width insulation to fill the open web on one side of the cavity between metal studs results in circumvention of the insulation through the gap at the web of the C-stud open to the cavity. With wood framing, full-width insulation is not used due to the solid wood stud's thickness occupying a portion of the cavity itself.
A study by the Journal of Thermal Insulation, July 1994, found that the R-values achieved in a simple ranch-style home with LGMF exterior walls was 22 percent lower than the Utopian wall's R-values. Also in '94, an NAHB test showed that the use of insulating sheathing (i.e., foam board), in conjunction with the framing, increased the thermal resistance of the wall assembly by about R-1.
Essentially, there are five solutions to help offset but not eliminate the effects of thermal bridging:
• Modify steel studs
• Use of insulating sheathing
• Add "strapping"
• Framing configuration
• Air tightness
Modify steel studsSome manufacturers (such as Tri-Chord) offer "thermal studs" that include perforations or gaps that remove a substantial portion of the web of the stud, thus reducing the path for heat transfer. Another variation includes "nubs" on the stud flanges to provide a self-furring effect for the stud to minimize the contact area between the metal stud itself and the substrate (sheathing typical) material.
Use of insulating sheathingAs mentioned, use of insulating sheathing on the exterior side of LGMF increases the R-value of the wall by about R-1. Translated, tests have demonstrated that this represents up to a 20 percent increase in the overall R-value of the wall assembly. Though not a silver bullet, use of insulating sheathing is the easiest, most cost-effective means by which to increase the R-value of a LGMF exterior wall and offset the effects of thermal bridging.
Add strappingAnother method used to short-circuit the thermal bridge at the flanges of metal studs is to apply a "break" in the form of felt building paper along the entire surface of the stud flange. This serves as a "disconnect" since the felt paper is not conductive.
Framing configurationThough LGMF can be used as a "piece-for-piece" replacement of wood framing, it need not be used that way. Hybrid framing configurations whereby 2x6 or 2x8 wood studs are used at the exterior walls and/or roof rafters (greater depth of member increases insulation thickness/R-value). Trimmers/headers at the floor along the perimeter of the building may also be wood framed (interior bearing/non-bearing walls, floor joists, etc., can be LGMF).
Air tightnessUse of good detailing, particularly around door and window openings (where most air infiltration occurs), is another way of keeping the outside air out and the inside air in. LGMF is particularly good at creating a tight building envelope due to its uniformity and dimensional stability. Designs that minimize door/window openings also contribute to thermal efficiency. Energy codes typically take this into consideration for determining insulation requirements and offer energy credits for doing so.
Insulating LGMFThe American Iron & Steel Institute published the "Thermal Design Guide for Exterior Walls (RG-9405)," in 1993. Appendix C provides guidelines for the amounts of insulation required to properly insulate homes constructed with LGMF. Three methods for determining insulation levels are included:
• Thermal degree days
• Thermal zone map
• Chart method
Most important when insulating LGMF is the complete filling of the cavity between framing members. Spray-applied insulation is acceptable as long as it fills the cavity completely. As mentioned, full-width, friction-fit batts (16 or 24 inches wide) must always be used with LGMF. For kraft-paper faced insulation without flanges (a.k.a. "lips"), the insulation should be taped or glued to the studs to hold it in place. Areas requiring thermal insulation (to avoid cold-spots) in LGMF structures include the following:
- Exterior walls
- Jambs and headers in EW
- Built-up members in EW
- Corner/multiple studs in EW
- Behind outlet boxes in EW
- Full-width between ceiling joists below unheated attics, garages or where there are heated rooms above the ceiling/living space
- Below stairways and within knee walls inside of unheated attics
- In cathedral ceilings
- Around the rim joist/track at building perimeter
- Between joists that are over a crawl space or above an unheated living space
In part five, we'll continue our discussion of the many aspects of LGMF with a look at the resistive qualities of LGMF to insects, seismic movement and fire.