Vented Conditioned Attic

In warmer climates unvented attics can prevent the condensation that occurs on ductwork in some vented attic spaces.

Vented conditioned attic.

The irc has permitted unvented conditioned attics since 2006 with certain requirements. A fundamental requirement of an unvented attic assembly is the use of air impermeable insulation on the underside of the unvented roof to prevent air infiltration and exclude airborne moisture from the attic. Either of these systems comes with a premium cost typically two to three times the price of a blown fiberglass or cellulose vented attic assembly. Lstiburek s enthusiasm was rooted in his desire to bring attic ductwork inside a home s thermal envelope.

Unvented design unvented attic design falls into two categories. Systems where condensing surface temperatures are not controlled figure 4 and systems where condensing surface temperatures are. Code requirements vary across the country. Among the early promoters of conditioned attics was joe lstiburek a principal of the building science corporation.

One static vent style is the turbine vent which uses wind to power its enclosed fan all it takes is a light breeze to rotate the blades and suck heat out of the attic view example on the home. By using an air impermeable insulation like spray foam on the roof deck will stop any air movement. Unvented conditioned attics were quite rare before the 1980s. In a roof attic assembly the builder has two choices.

Unvented attics are also said to perform better in areas where high wind events. The thermal boundary can be at the ceiling plane leaving the attic space above uninsulated and vented or the thermal boundary can be located at the roof line creating a sealed insulated conditioned or semi conditioned attic space. This facilitates the design of unvented attics in warm and mixed climates without an additional vapor retarder but requires an additional vapor retarder element in colder regions. The attic space becomes indirectly conditioned as a result of the air leakage heat transfer and vapor diffusion through the ceiling.

The loss of energy efficiency means some designers choose to move the ductwork into the conditioned areas of the home and leave the attics unvented. An unvented attic system relies on an air seal and no vents to make the attic a conditioned space.