Vented Conditioned Attic

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

Vented conditioned attic.

Systems where condensing surface temperatures are not controlled figure 4 and systems where condensing surface temperatures are. 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. 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 irc has permitted unvented conditioned attics since 2006 with certain requirements. Unvented attics are also said to perform better in areas where high wind events. By using an air impermeable insulation like spray foam on the roof deck will stop any air movement. An unvented attic system relies on an air seal and no vents to make the attic a conditioned space.

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. 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. Unvented conditioned attics were quite rare before the 1980s. Among the early promoters of conditioned attics was joe lstiburek a principal of the building science corporation.

In a roof attic assembly the builder has two choices. The roof deck in an unvented attic must be exceptionally airtight. The acceptance of unvented conditioned attics continues to grow among builders and building officials. 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. Code requirements vary across the country.