Conditioned (Unvented) Attic

In a normal attic, the builder insulates the attic floor (the ceiling of the rooms below) and vents the attic to outside air. A conditioned attic does the opposite: the builder seals the attic airtight and insulates at the roofline instead, on the underside of the roof deck, bringing the space inside the home's thermal envelope. The attic is then unvented, which is why it is also called an unvented attic. It stays much closer to the temperature of the living space.

The reason to do this is ductwork. In a standard vented attic, the air handler and ducts sit in air that can pass 140 degrees in a Phoenix July. The U.S. Department of Energy notes that running HVAC in vented attics raises heating and cooling energy use by about 10% from duct losses alone. Move the attic inside the envelope and that loss largely goes away, because the ducts now run through cooled space. The PNNL Building America Solution Center describes sealing the attic "completely within the building thermal envelope." Builders usually do this with spray foam or rigid foam at the roof deck, per 2018 IRC Section R806.5.

For example, a custom home in Rio Verde with the HVAC system in the attic can be built with closed-cell spray foam under the roof deck. The attic becomes a semi-conditioned space near room temperature, so the cooled air in the ducts no longer fights a superheated attic. Cooling bills drop and the equipment lasts longer in the milder environment.

A conditioned attic is not just "more insulation." It changes where the thermal boundary sits, moving it from the ceiling up to the roof. Because the attic is sealed, the design must control moisture for the climate, which is why Arizona builders detail these assemblies carefully rather than simply spraying foam and walking away.