Truss Designers Should Consider Raised Heels
Originally published by: ProSales Magazine — July 30, 2013
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With energy codes getting stricter, and more homes being built to Energy Star specs, truss designers now have to consider attic insulation requirements in their roof packages. A cost-effective way to get more insulation into the attic is with raised heel roof trusses.
Raised heel trusses make a home more energy efficient by correcting an inherent problem with conventional attic framing: The rafters don't leave enough heel height at the eaves for full depth-insulation. Heel height is the depth of the attic space at the eaves—the vertical distance from the top plate to the underside of the roof sheathing—measured at the outside wall. With a conventional 2x4 truss, this can be as little as four to six inches.
That height may be sufficient if the attic is insulated using a closed cell foam with an insulating value of R-6 per inch, and if the home is in one of the warmest parts of the country where attic insulation requirements are as low as R-30. But the typical attic is insulated using batts or blown fiberglass, with R-values of around 3.5 per inch. And in colder regions, attics may need a total insulation value as high as R-60.
When using batts, a lot of installers simply stuff them into the heel area. This compresses the insulation and degrades its overall insulating value, but a worse problem is that the difficulty of reaching this cramped area begs for sloppy insulation. The result is gaps in the insulation and a lot of cold spots along the outside walls of the home. In northern climates this can be more than an energy drain: It can also increase the chance of ice damming in winter.
Clearly, higher heels are needed.
There are a couple of different ways to get this extra height. One is to simply make the truss longer, cantilevering its bottom chord beyond the edge of the wall. This creates more depth at the outside wall. If the home's design calls for wide soffits, the advantage will be that the soffit framing is built into the truss. The disadvantage is that blocking will need to be installed in the space between each pair of trusses, in order to hold the insulation in place.
The other alternative is the raised heel truss, which is just what it sounds like. In a standard plated truss, metal heel plates directly join the top and bottom chords, while in a raised heel truss a vertical member at the heel (in elevation it looks like a small knee wall) raises the top chord above the bottom chord to whatever height the insulation requirements call for. This lets the builder install full-depth insulation all the way to the outside wall without compressing the batts. It also leaves plenty of room for the installer to work with, so there's no excuse for a sloppy installation.
Because the vertical heel member is flush with the outside of the wall framing, sheathing is typically aligned with the top of the heel (where the vertical member meets the top chord) and extended down the wall. This creates a continuous load path to resist uplift forces, and may eliminate the need for metal connectors where the truss sits on the wall.
In some jurisdictions, eaves blocking—2x blocking with a depth that matches the heel height—must be installed between trusses at the outside edge of the wall top plate to resist wind and seismic forces. The 2009 and 2012 versions of the International Residential Code require such blocking with heel heights exceeding 15 ¼ inches. Note that this requirement applies to raised heel and cantilevered trusses.
The raised heel will, of course, make the exterior wall look taller. Soffit framing added to the wall after the sheathing is in place, along with a fascia board, can help minimize the visual effect.
Before insulating, it's good practice to apply spray foam sealant to the joint between the truss chord and the ceiling, to block air leakage from the house to the attic. This will plug a major cause of heat loss, and will stop moist air from getting into the attic where it can condense on the underside of cold roof sheathing.
As with all attics, a good structural ventilation system will stop the attic from overheating in summer and prevent moisture condensation in winter. A combination of soffit and ridge vents, along with baffles on the underside of the roof sheathing that extend from the heel to at least 16 inches up the slope, should do the trick.