New Requirements: Unvented Attic Assemblies

[MountainDon]

This, from an email update on energy codes received today.

New Residential Code Requirements for
Unvented Attic Assemblies

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
effic iency 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 maintain ed 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 2B
and 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/morf/ResourceCenter/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/2007Supplement/IRC07S.pdf

The DOE November Newsletter may be found here.

Energy Codes software, ResChek, may be found here.