New Code Requirements for Unvented Attics

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The concept of building attics as conditioned, unvented spaces has grown in acceptance in recent years, and this construction method has been advocated by the U.S. Department of Energy’s Building America program. A principal benefit of conditioned attics is that ductwork located in the attic is not exposed to extreme conditions.

With ducts inside the conditioned space, energy loss from leaks and heat conduction from the ducts is much less severe. The 2006 International Energy Conservation Code® specifies a default efficiency of 80% for forced-air systems, if ducts are located in an unconditioned attic. Where the distribution system is run entirely in conditioned space, such as a conditioned attic, the default distribution system efficiency increases to 88%, resulting in energy savings.

A new set of requirements allowing unvented, conditioned attic assemblies is contained in Section R806.4 of the 2006 International Residential Code® (IRC). The 2006 IRC requires that the surface under the roof deck, where moisture would condense, be maintained above 45°F on average during the coldest month. The 2007 Supplement of the IRC simplifies this by specifying the insulation levels that will maintain this temperature.

The IRC requires air-impermeable insulation to prevent moisture reaching the underside of the roof deck in all but climate zones 2Band 3B. If air-permeable insulation is used in the exempt zones, rigid board insulation may be needed above the structural roof sheathing to keep the underside of the roof deck above 45°F for the coldest month.

The IRC prohibits the use of a vapor retarder between the conditioned attic and the top-story ceiling of the house so that any moisture that does collect in the attic can escape into the house. A final requirement is that if wood shingles or shakes are used, they must be separated by a ¼-inch air space from the roofing felt over the structural sheathing.

For more information, see the Code Note, Conditioned Attics, at http://resourcecenter.pnl.gov/cocoon/mo ... rceCenter/

article/1520.

For the full requirements of the 2007 Supplement for the IRC, See Section R806.4 of the code at www.iccsafe.org/cs/codes/2007-08cycle/2 ... IRC07S.pdf

Source: U.S. Department of Energy’s Setting the Standard newsletter.