Cold facts about steel framing: how to fight heat loss when framing with steel.
Then he got a copy of American Iron and Steel Institute's new thermal design guidelines for steel framing. So he added a layer of R-8 foam sheathing on the outside to provide a thermal break. Since the foam was essential, the 2 x 8 wall system was overkill, costing Warren extra money in framing and insulation.
Does Warren sound confused? Yes. Does AISI have all the answers about steel framing and energy performance? Not yet.
While AISI's exterior wall insulation guidelines are important and will help you improve steel's energy performance, they're only the first step. That's because the guidelines are based on tests of sections of walls built with studs - but without built-up jacks, headers, corners, and partition leads (components that add greatly to heat loss). And the tests didn't deal with conductive heat loss from stud walls through their connections to concrete foundations and steel roof trusses.
So the guidelines don't go far enough yet, but they do provide a first-line defense for cutting heat loss from steel framing. Here's a look at the solutions the guidelines offer and the questions that still need answers.
THE PROBLEM: THERMAL BRIDGING
It's no secret: Steel studs conduct close to 10 times more heat than wood. This creates major thermal bridges any place steel framing is covered only with wood sheathing and siding on the exterior. AISI wanted to tackle steel's energy problems, so it hired NAHB's Research Center to supervise tests on various wall systems.
The results, published in AISI's "Thermal Design Guide for Exterior Walls," provide recommended R-values for several steel stud wall systems. Here's what they found:
* Steel exterior walls do lose heating or cooling energy 30 percent to 40 percent faster than wood walls when insulating sheathing is not used.
* To overcome the thermal bridge problem, don't use thicker wall cavities; in fact, there's a diminishing return on each R-value of cavity insulation added.
* A good way to save money and energy is to use wider framing spacing; a wall with 24-inch o.c. steel framing performs about 10 percent better than a wall framed 16-inch o.c.
ONE SOLUTION: FOAM SHEATHING
The first line of defense: Use foam sheathing over steel walls. Installing R-5 to R-10 foam sheathing over steel-framed walls filled with R-11 batts should bring the wall system up to the R-13 to R-18 range (see table, page 178). That will meet code in most climates, including those where cooling loads overshadow heating.
For most builders, switching to steel is enough change to get used to. But if you care about your houses' energy performance, you'll have to get used to using foam sheathing on the exterior, too. Here are some things to consider.
* FRAMING LAYOUT. Most steel framers use 24-inch o.c. framing, but some stay with 16 inches o.c. because of warranty requirements for siding.
* BRACING. If you use foam sheathing, you lose the structural bracing of oriented-strand board or plywood, so you've got to install steel bracing in the frame. Most framers install X-bracing ([ILLUSTRATION FOR PHOTO OMITTED], page 177) using 16 gauge, 3-inch flat strapping material. And some framers use rectangular sheets of 20 gauge steel screwed to studs at the corners.
* SHEATHING. While AISI's guidelines are based on extruded polystyrene, a popular alternative is polyisocyanurate foam because it provides a higher R-value (R-8 per inch).
* HEADERS AND CORNERS. Built-up steel framing loses lots of heat. Fill headers and corners with fiberglass before assembling them, then try to keep them dry during construction ([ILLUSTRATION FOR PHOTO OMITTED], page 177 bottom).
* ATTACHING SIDING. If you use foam sheathing, how do you attach siding? Some builders still sheathe with plywood or OSB, then nail the foam over it. Then they shoot siding into the structural sheathing, not the studs. But don't overdrive the siding, especially vinyl. Others use only foam sheathing and nail siding through the foam to studs. This may be a good time to use one of the new stiffer vinyl siding products.
* STUCCO & EIF FINISHES. Steel producers have caught an earful from builders who've paid extra to have stucco netting applied to steel framing with screws and washers. (According to Del Webb's Kim Bannister, this was one reason for the company's switch back to wood for its exterior walls.) But new [TABULAR DATA OMITTED] pneumatic fastening systems, like Erico's nailers, should bring these costs down.
* CAVITY INSULATION. Be sure to specify full 16- or 24-inch-wide batts.
* MORE FOAM. Look for other places where foam will help. Be sure to cover the bottom of cantilevers and steel floor joists when they are over unheated garages and vented crawl spaces.
California energy consultant Jerry Senderov of ConSol says the cost for energy features to improve steel framing's performance varies with climate, design, and enforcement of energy codes.
Senderov, who works with more than 150 production builders, says those who already use foam sheathing will see smaller cost increases. Using 16- or 24-inch full-width batts adds almost $100 to the cost of a 1,600-square-foot house. Switching from half-inch to one-inch foam could add $100 to $150. If you want to stick with OSB sheathing and apply foam over it, the cost goes up another $600 to $800, says Ohio builder Michael Whitticar.
Searching for ways to make steel framing's energy performance match wood's, researchers and builders are exploring the remaining questions.
For example, Andre Desjarlais of Oak Ridge National Laboratory has been studying steel framing in cooperation with the NAHB Research Center. Desjarlais looked at the effect of extra heat loss through common details like built-up steel corners, headers, and wall intersections.
The result? If you don't include heat loss from these connections and other architectural details - like overhangs and cantilevers - you're ignoring half the potential heat loss from walls in a steel framed house.
And in Oregon, builder Alex Boutacoff has experimented with different ways to create thermal breaks in his steel framed houses. He's used exterior foam sheathing, interior foam, firring strips beneath the ceiling, foam sheathing beneath the ceiling, offset wall studs, and foam beneath floor joists in the crawl space. Recently he's compared infrared inspections of side-by-side houses: one framed with steel, the other with wood.
"We made some pretty good efforts to insulate the steel," says Boutacoff, "but you could see the studs and cold spots in the thermograph. We saw the same cold spots in the wood home, but they weren't as blue as in the steel house." (Blue is bad news because it indicates higher heat loss.)
Further infrared inspection by Bonneville Power Administration engineers reveals more: R-6 foam located inside the studs worked more effectively than foam on the exterior.
The engineers also found that where steel walls connect directly to steel roof trusses, exterior foam on the studs may only redirect heat flow up through studs and out through the uninsulated truss tails. Heat also escapes downward through the foundation. And if you use steel from the foundation wall up, you probably won't slow major heat loss down through frame walls and out the foundation wall, unless the exterior foam insulation extends below grade.
You've also got to watch the joist flanges exposed to cold air in attics or crawl spaces. They'll lose lots of heat, unless you install foam sheathing over them.
AISI and the NAHB Research Center plan to field test more steel houses this winter. Until more comprehensive testing provides answers, you can use the approach some steel framers use to reduce heat loss through steel-to-steel connections: Use wood trusses and wood sills instead of steel. And install foam sheathing onto steel framing, anywhere it's exposed to cold temperatures.
FOR MORE INFORMATION
To see a complete text of the AISI guidelines, and to read "Steel Crazy After All These Years" (BUILDER, December 1994), see BUILDER Online at http://www.builderonline.com
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|Date:||Feb 1, 1996|
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