The choice to design a vented or unvented attic within a building has often been a source of confusion for both roofing contractors and homeowners. “To vent or not to vent” a feature article in the January issue of Professional Roofing, the National Roofing Contractors Association’s (NRCA’s) trade magazine, reports that choosing to vent an attic is not dictated by building code.
“Whether a roof is vented or unvented typically happens with the initial design of the house or building,” explains the article’s author, Joan Crowe, AIA, an NRCA director of technical services.
When a building has a vented attic, it has intake vents to allow outside air to enter an attic space and exhaust vents that allow air to exit, with insulation usually installed on the top of the attic floor. A vapor retarder typically is installed in the ceiling plane.
An unvented attic has insulation and an air retarder at the roof level below the roof deck within a building’s thermal envelope.
According to the article, for an attic to be properly vented, the design must include intake vents near the roof’s eaves, and exhaust vents at or near the ridge to allow air to flow in and out of the attic.
Unvented attics, which take advantage of the “whole house” approach to thermal and moisture control, often serve as the location for mechanical equipment. Roofing contractors are advised that converting a vented attic into an unvented attic could result in unsafe conditions within a building.
“With an unvented attic conversion, you are creating airtight construction and that may affect the indoor air quality of a house,” Crowe says.
Although NRCA maintains its recommendation to properly ventilate an attic space, the association considers unvented attics to be a viable alternative if designed and constructed properly.