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Unvented attics

Research has shown conditioned, unvented attics can be an effective alternative to ventilating attics for steep-slope roof assemblies.

The concept of conditioned, unvented attics provides for moving ceiling air and thermal barriers to the roof plane, as shown below, where better building airtightness can be achieved.


Figure: Configuration of conditioned, unvented attic

2009 and 2012 IRC section R806 "Roof Ventilation" permits unvented attic assemblies when attic space is completely contained within a building thermal envelope and attic construction meets the code's specific requirements intended to prevent moisture condensation in the attic assembly.

In most instances, using unvented, conditioned attic assemblies results in increased roof surface temperatures as compared with temperatures of conventionally vented attic assemblies. However, it is important to keep the small temperature increases attributed to unvented, conditioned attic assemblies in perspective. For example, the differences between roof surface temperatures in the southern and northern regions of the U.S. are significantly greater than the surface temperature differences between unvented, conditioned attic assemblies and conventionally vented attic assemblies. Similarly, the roof surface temperature difference between a roof system's southern, sun-exposed slope and northern, shaded slope is about the same or greater than the surface temperature difference between unvented, conditioned attic assemblies and conventionally vented attic assemblies.

Steep-slope roof system performance problems caused by condensation and heat history unmitigated by ventilation are cited by some asphalt shingle manufacturers as grounds for their limiting or entirely disclaiming liability under product warranties for asphalt shingles installed over unvented attics. Homeowners considering asphalt shingle installation over unvented attics should consult asphalt shingle manufacturers to verify warranty coverage.

NRCA considers an unvented attic assembly to be a viable alternative to attic ventilation throughout the U.S. In hot and humid climates, where the direction of vapor drive is predominantly from buildings' exteriors to their interiors and high outside humidity is commonplace, NRCA suggests designers consider implementing the unvented attic assembly concept into their designs for one- and two-family residences.