Vented Conditioned Attic

In a roof attic assembly the builder has two choices.

Vented conditioned attic.

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

The acceptance of unvented conditioned attics continues to grow among builders and building officials. 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. The roof deck in an unvented attic must be exceptionally airtight. Lstiburek s enthusiasm was rooted in his desire to bring attic ductwork inside a home s thermal envelope.

With vented attic assemblies moisture that diffuses into the attic space from the conditioned space is vented to the exterior by attic ventilation. Systems where condensing surface temperatures are not controlled figure 4 and systems where condensing surface temperatures are. 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.

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. Among the early promoters of conditioned attics was joe lstiburek a principal of the building science corporation. Unvented design unvented attic design falls into two categories. An unvented attic system relies on an air seal and no vents to make the attic a conditioned space.

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. 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.