Green Building Library
Insulation

Builder Takes the Pinch Out of Attic Insulation

Are you caught in a pinch over attic insulation?

In a roof with a four-in-twelve pitch as much as 25 percent of the ceiling area will have less than the stated insulation value. Why? Because conventional trusses or rafters choke ceiling insulation down to nothing around the roof’s perimeter.

Cantilever Trusses

To take the pinch out of attic insulation, Brad Ogle, president of MAO Construction in Eugene, Oregon, specifies “cantilevered” trusses for both flat and cathedral ceilings. While the top chord of most conventional trusses rests directly on the top plate, the top chord of a cantilevered truss is at least 17 inches above the bottom chord at the point where it rests on the exterior wall.

Three types of cantilevered trusses are available: “triangular” trusses and “parallel chord” trusses for cathedral ceilings (see Figure 1). All three allow R-38 insulation to reach the edge of the ceiling at the top plate. As a result, the ceiling’s overall insulating value is increased. In a fact, a cantilevered truss with R-38 is nearly equivalent to R-60 in a conventionally-framed, 4-in-12 roof.

More Insulation

The cantilevered truss approach qualifies as “advanced framing” for ceilings and helps a house qualify for the Super Good Cents program. “We use heat pumps,” said Ogle, “and then go with the advanced framing technique for walls and ceilings. It’s an inexpensive way of meeting the energy standard.”

Easy Installation

Ogle believes that cantilevered trusses have many other benefits besides energy savings. He particularly likes the ease of installation. According to Ogle, building with scissors or parallel chord truss is much easier than building a cathedral ceiling out of 2x14 lumber, which is heavy and often warped. He believes that the savings in labor are worth the slightly higher material cost.

For Ogle, a cantilevered truss is also easier to install than its energy-efficient cousin, the “raised-heel” truss, which leaves an open eave and requires 12 extra inches of siding. Placing the raised-heel truss is difficult, Ogle believes, because the nine-foot sheathing is attached to the wall before raising it and ultimately interferes with setting the truss. Because standard eight-foot sheathing is used with cantilevered trusses, the trusses can be moved into just the right position without damaging the sheathing.

Good Looks

Finally, Ogle likes the “look” created by cantilevered trusses. “Cantilevered trusses allow an enclosed soffit, and that’s a feature that home buyers find attractive,” said Ogle.

He estimates that the cost of using cantilevered trusses is only slightly greater than the cost of conventional trusses– about $150 more for a 1,300 square foot house. “Each truss runs three to four dollars more, and the remainder of the cost is in enclosing the soffits.”

For Ogle, the bottom line is good looks, quality construction and a reasonable price. And one more thing: an R-38 ceiling that’s really R-38.
Reprinted with permission: Eugene Water and Electric Board

This article appeared in Energy Source Builder Vol. I, No. 1, January 1989,
©Copyright 2009 Iris Communications, Inc.