Moisture-Safe Unvented Wood Roof Systems

Wood-framed pitched roofs have traditionally constructed with fibrous insulation laid on the ceiling plane, and the large volume above this insulation well ventilated with exterior air. However, there is a growing trend toward insulating the underside of the sloped roof deck so that the volume between the ceiling plane and the sloped roof plane can be conditioned and contain HVAC systems, duct distribution, storage and even living space. In this insulation arrangement, ventilation below the deck is more difficult, expensive, and/or impractical to achieve, and so unvented solutions have attracted significant interest (i.e., unvented cathedralized attics).

The primary concern with the use of unvented roofs is the potential for moisture build up at the underside of the roof sheathing during cold weather. Rain leaks are just as dangerous. Research has shown that the good field experience with ventilated attics is due to the removal of moisture that passes through the ceiling plane by diffusion and accidental air leaks.

This paper describes a hygrothermal modeling study, including all of the US climate zones, a range of interior humidity levels and numerous arrangements and types of insulation. The results showed that so long as airtightness is provided, and wintertime humidity is controlled, numerous unvented solutions using either or both spray foam (open and closed cell) and fibrous insulation (cellulose and mineral fiber) can be successful. Climate, the solar properties and exposure of the roofing, the air and vapor permeance of the insulation (s) and interior humidity are the most important factors to be considered in the design of moisture-safe unvented roof systems.