Unvented Low Slope Roof

Because the attic space is now inside the thermal envelope blowing a little heat up there is not like throwing twenty dollar bills out the window.

Unvented low slope roof.

However architects of these compact roof assemblies must consider vapor permeance and arrangement of the materials to produce acceptable hygrothermal performance and low risk of condensation in the resulting conditioned attic space. Unvented low slope roofs are never designed to dry to the exterior because low slope roofing membranes are vapor barriers. The insulation can go below the roof deck above the roof deck or both. If the upper edge of a low slope shed roof intersects a vertical wall the most logical approach in my opinion is to detail the roof as an unvented roof.

In unvented flat or low slope roofs with parapets perform a full flood test to ensure leak free installation of roofing membrane. Low slope and steep slope unvented roof systems i e. Whatever method is used the amounts of insulation must achieve r values mandated by local codes. Another approach is to build a cupola or ventilated doghouse in the middle of the low slope roof an approach championed by joe lstiburek.

Like a wall that is severely out of plumb an unvented roof has insulation packed into the framing cavities aligned with the sheathing. Drying of unvented roofing assemblies is by definition only provided to the interior.