Building With Steel Framing
Is there a place for this thermally conductive material in high-performance buildings?
Sal Lombardo is planning a new home in the New York-New Jersey area (Climate Zone 5) and is looking at a long list of high-performance construction options: double-stud walls, structural insulated panels, insulating concrete forms, Larsen trusses, and walls built with light-gauge steel framing.
Wait a minute. Steel framing, as in the stuff that leaks heat through the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. like a proverbial sieve? Maybe, Lombardo says, it deserves another look.
"Steel seems like a really good option," Lombardo writes in a Q&A post at GreenBuildingAdvisor. "Almost unheard of in my area, despite being upwards of 60% recycled, it lasts forever (relatively speaking), is super strong, straight, creates minimal waste, is not affected by termites, pests, or mold, and is equal or close to wood in cost (depending on who you ask).
"I know it has very high thermal conductivity. However, there are configurations that can abate this significantly," he adds, such as a double layer of 2-in. polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. foam on the exterior.
“Why isn't it more popular?” Lombardo asks. “Am I missing something?”
Those questions are the topic for this month's Q&A Spotlight.
You'll get a good, but not great, wall
According to GBAGreenBuildingAdvisor.com senior editor Martin Holladay, the main problem with steel framing is 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. . Because the effect is so pronounced, all of the wall insulation should go on the outside. Insulation placed in stud cavities won't accomplish much.
“You're right that it's possible to install two layers of 2-inch polyiso, giving you R-26,” Holladay writes, “That's OK, but it's not great.”
If Lombardo could add even more insulation on the outside of the wall — 6 in. rather than the 4 in. he has proposed — he'll end up with a “pretty decent wall,” Holladay says. “Just remember to keep all of your insulation on the exterior side of your wall framing.”
Holladay wrote that cavity insulation in steel framing is “basically worthless” — an assessment that he later admitted was a slight exaggeration — and pointed to comments by Joseph Lstiburek of Building Science Corporation: “Put an R-19 batt in a steel stud wall and you are lucky to get R-5 to R-6 in the real world. That’s equal thermal resistance wise to about 1 inch of rigid insulation installed on the outside of the steel studs.” (For more information from Lstiburek, see “A Bridge Too Far.”)
Holladay also pointed to GBA's own Encyclopedia, which cites a California Energy Commission claim that a steel stud conducts 10 times as much heat as dimensional lumber. The Oak Ridge National Laboratory has found that thermal bridging in a wood-framed wall lowers the effectiveness of cavity insulation by 10%, but performance drops a whopping 55% in a wall framed with steel.
Aren't we overstating the problem?
Lombardo isn't alone in wondering why steel framing doesn't get more serious attention.
"I am also curious as to why steel hasn't caught up a little more attention in residential framing in Canada and the USA," writes Jin Kazama. "I tend to like concrete because of its long-term life. Steel and aluminum are also materials I favor because of the same factor."
Steel framing is very inexpensive when considered from a weight/strength standpoint, he adds. Kazama points in particular to the buildings designed by Blue Sky Building Systems as an intriguing use of steel framing.
Referring to research at Oak Ridge National Laboratory, James Morgan writes that the effect of using steel framing is more complicated than the California Energy Commission would suggest. "I could find no suggestion that the tests support omitting cavity insulation," he says, "and the tests show a pretty good R-20 wall with just 2 in. of continuous outside insulation. If you're headed for R-30 and beyond, though, I can see why piling foam board insulation on the outside of the wall would ultimately lead to a strategy of leaving the cavity empty — why bother with filling a perfectly good service cavity with fiber to get a measly R-9 or so when you can get the same results by adding another inch or so of polyiso on the outside? But this strategy would apply to a wood-framed wall equally well, once you transition from seeing external insulation as thermal bridge and condensation protection to seeing it as the complete insulation package."
Morgan says that an Oak Ridge National Lab research paper mentions advanced technologies using steel studs that “suggest the potential for a thermally efficient structural alternative to wood framing.”
Insulating a steel wall has its challenges
If a steel-framed wall is to have cavity insulation, there is still the question of what kind and how it should be installed. Dense-packed cellulose is often placed behind a "scrim," Morgan says, that's simple to attach to wood framing but not so easy with steel studs.
And if Lombardo is considering the use of fiberglass batts, he'll find problems there as well. "Fiberglass batts are not the best choice," he writes. "If you do go in that direction, a standard 14.5-in. batt designed for wood studs will not fit correctly. The batt has to be the full width of the stud bay and must fit into the web of the steel stud which is not so easy to achieve consistently."
Lombardo has read about a technique for wrapping steel framing in “Stud Snuggler” foam to reduce thermal bridging, which might be adapted to a DIY-friendly approach. And Johns Manville's Spider Custom Insulation, which is blown-in fiberglass, also has possibilities.
Moreover, says Dorsett, batts in widths that do fit steel framing area available in both fiberglass and rock wool.
But in the end, he says, there's still the problem of thermal bridging. “There really isn't a good way around the high thermal conductivity of steel though, even at 24-in. o.c. framing the whole-wall R values will always come in at about half the center-cavity R, whereas with wood studs and R3.2-R4/inch cavity fill the whole wall values come in at something on the order of 75% of the center-cavity value even at 16-in. o.c. spacing,” Dorsett says.
In case of fire, you're on your own
In addition to thermal bridging, there's another problem with steel framing, says Jon Leeth, and that's what happens in the case of a serious structure fire. “The primary reason I opted not to frame my house with steel was by advice from my insulator (cellulose) who is also a fireman,” Leeth says. “His firefighting perspective was that a burning structure will give fairly reliable clues to structural stability when framed with wood. Metal structures get to that magical temperature where the metal turns very quickly from solid to liquid.
“He said as soon as they determine a structure is steel-framed, all firefighting efforts are immediately limited to the exterior of the building. Absolute evacuation and no re-entry aside from human rescue effort. They would not fight the fire on the inside if the building was believed to be evacuated.”
Those concerns are echoed by Malcolm Taylor, president of his local fire department. “Houses framed with steel studs still contain all the other combustible structural components and contents that fuel fires,” Taylor writes. “The unpredictable collapse of light steel stud walls is a well known phenomenon. Whether that's enough of a worry to influence your choice of materials when building a home is another question.”
Our expert's opinion
Here's how GBA technical director Peter Yost looks at the question:
When I was at the NAHBNational Association of Home Builders, which awards a Model Green Home Certification. Research Center not long after Hurricane Andrew, we had a big light-gauge steel-framing project down in Homestead, Florida. They had a very experienced steel framer as the lead contractor and I was amazed at how fast they were at fastening with tapped screws — not nails — and wielding their screw guns as well or better than I swung a hammer. And their cut list accuracy meant very little framing waste, with what they generated very easy to recycle.
So those who think you can’t make steel framing efficient economically I say this: There is a learning curve with any job site change, and switching to steel is simply one of them.
Another real advantage of light-gauge steel framing is the ability to gauge to the load; it is a much better use of materials to be able to move from 12 to 25 gauge as the load/application allows. And as a formed material, every framing member is true, every time.
But from a hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out. standpoint, I can only add — to the excellent points already made regarding energy performance — these two:
- Condensation. Even small thermal bridging sets up for significant risk of interstitial condensation, exacerbated by the next point.
- Buffering capacity. This is the amount of water that a material or assembly can “hold” or tolerate without deterioration (for more, see this research paper). Joe Lstiburek is famous for many things, but his “Joe math” attracts a lot of us builder types. Here is one of my favorites: For a 2,000-sq.-ft. home framed with steel, the hygric buffer capacity is about 5 gallons of water; wood-framed, it’s about 50 gallons; and for masonry walls it’s approximately 5,000 gallons.
So, when condensation happens in a steel-framed assembly, it’s the really low hygric buffer capacity that ramps up the significance of that condensation. The wall may have very little tolerance for that moisture.
My conclusion: if you are considering light-gauge steel framing, use the BSC “perfect wall” approach. Keep all of your R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. to the exterior, and let that steel be part of the interior conditions (or nearly so) of the building. And the more severe the climate, the more stringent my recommendation is.
- Charles Miller/Fine Homebuilding magazine
Jan 22, 2013 12:42 AM ET
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