Unvented Hot Roof Design

This article describes various cold climate building solutions for un vented cathedral ceilings and similar under roof spaces offering advice on how to avoid condensation leaks attic mold structural damage when roof venting is not possible.

Unvented hot roof design.

Unvented roofs unvented roofs are increasingly common due to design priorities. As a result unvented roofs have become popular in homes with feature dense roofs think multiple skylights dormers and other aesthetic flourishes. This type of roof assembly is sometimes called a hot roof. They are designed without ventilation openings and the attic is conditioned like the rest of the living space.

These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck. Unvented roof assemblies are becoming an increasingly common construction alternative to traditional vented roofs. Four options for building an unvented roof assembly. An understanding of heat air and moisture as well as the causes of condensation is critical in detailing unvented roof assemblies.

Any type of rigid foam can be used above roof sheathing. Practicality is also enhanced. Unvented roof assemblies should be insulated either with air impermeable insulation rigid foam or spray polyurethane foam or with a combination of foam and air permeable insulation. This definition allows simple unvented roof design just don t install vents in a previously recognizable climate zone ashrae s hot humid zone.

Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction. The disadvantagesof unvented roofs have to a certain degree been overstated in hot climates the extra heat gain from unvented roofs rarely exceeds a couple of degrees celsius. An unvented roof assembly is constructed without any ventilation and is fully sealed on all sides and filled with insulation. Sc historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof.

The drawing shows exterior foam over an unvented conditioned attic but the details are similar for cathedral ceilings. A powerful argument can be made for unvented roof construction in hot humid climates in order to control interior moisture. Unvented roofs operate by the principle that venting is not necessary to control moisture accumulation. If you don t want to use sips or nailbase there are four basic approaches.