A raised-heel truss is much like it sounds: the heel of a conventional plated roof truss, where the bottom chord intersects with (and bears on) the perimeter wall plate and is fitted with a vertical member that literally raises the top chord of the truss.
The primary advantage of raising the heel is to allow full-depth, uncompressed wall-to-wall insulation of the attic floor. Combined with a properly designed attic ventilation system and some well-placed air sealing, a roof framed with raised-heel (aka “high-heel” or “energy”) trusses reduces the temperature and pressure differences between the conditioned living space below and the unconditioned attic area, thereby reducing thermal transfer (or heat loss) between those spaces.
That dynamic, in turn, lessens the amount of energy needed to heat or cool the living space and optimizes the efficiency of the HVAC system, perhaps allowing it to be downsized in capacity—and cost.
An Affordable Alternative Unlike some components within a systematic approach to improving a home’s energy efficiency and overall performance, raised-heel trusses aren’t going to break anyone’s bank.
“Plated trusses are a cost-efficient way to frame a roof, and raising the heels is a simple design change for a fabricator,” says Brink Kuchenbrod, owner of Fokus Builders in Missoula, Mont. “I can’t think of a good reason not to do it.”
The value of an energy truss system is enhanced by the fact that it allows builders to use fiberglass batts, the most affordable insulation option for the attic floor.
“With conventional trusses, a builder would have to use a spray-applied foam insulation (on the roof rafters) to achieve the same values and meet code,” says Dave Ford, a designer at Maple Valley Truss in Prudenville, Mich. That’s a higher-cost option for builders.
Framing Details The 2009 International Residential Code, and to a greater extent its recently released 2012 version, require extensive blocking—perhaps even a shear wall detail—between trusses with heels higher than 15-1/4 inches to guard against truss uplift and seismic forces. Previously, builders and framers could extend ventilation baffles to enclose the heel and create the attic insulation cavity. But recent testing at the National Association of Home Builders (NAHB) Research Center in Maryland indicates that a section of 7/16-inch OSB extending from the top plate to the underside of the top chords (thus enclosing the cavity and connecting multiple trusses laterally) may meet the performance threshold of the code.
A full or extended OSB panel covering the heel cavities and extending far down the sidewalls appears to further enhance lateral stability, and may even eliminate the need to install metal connectors at the heel-plate junction to guard against uplift. “A tall OSB panel that connects to the wall structure might serve as a continuous load path,” says Drew DeRenzis, a structural engineer with the Research Center. “If it saves builders money and time not having to apply metal connectors, they’re all for it.”
New Research While the NAHB Research Center and others confirm the center’s findings and work to incorporate such details into the code, builders will likely have to block or at least apply hurricane ties or truss bearing enhancers to better secure the trusses to the wall frame.
In Missoula, for instance, Kuchenbrod and other builders are required to install metal connectors on every other truss and simply toe-nail the others with a couple of 16d sinkers.
Kuchenbrod does both, using the nails to temporarily secure the trusses as he places them, then coming back later with hurricane ties on every truss-wall connection. “It’s another one of a hundred things that work together to make the house more energy efficient,” he says.
There’s also no excuse for a poor insulation job. With the heel height raised a foot or more, batts should not be compressed at all, thus retaining their optimum thermal resistance (or R-) value. And an off-the-shelf, spray-applied foam or similar sealant applied at critical joints before the insulation is installed (such as where the truss chords meet the ceiling below) can block unwanted airflow to enhance the attic’s insulating value and moisture control.
Ventilate Right Consistent ventilation is a critical element of a raised-heel roof system to help regulate temperature and pressure differences between the attic and the living spaces and thus achieve full energy-saving and moisture-mitigating value.
In addition to properly spaced and sized soffit vents (or a continuous eave vent) and a companion ridge vent running the length of the roof’s peak, the system requires perforated baffles from the heel to at least 16 inches up the slope, and perhaps all the way to the ridge.
Attached between and typically fastened to the top chord of the trusses, the baffles contain and direct most of the air along the underside of the roof deck while allowing some of the outside air into the attic space to further regulate temperature and reduce the chance of moisture vapor condensation.
Added Value A raised-heel roof truss frame can eliminate ice dams. That’s because the design does not allow snow to freeze into ice; instead, it melts harmlessly into the home’s drainage system. Also, by reducing the temperature and pressure differences between conditioned and unconditioned spaces, a builder may be able to put his central HVAC ductwork in the attic without suffering too much heat loss through those runs—even more so if the ducts are properly sealed, insulated, and designed for the shortest runs possible.