Unvented Low Slope Roof

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.

Unvented low slope roof.

While this fact adds some risk to these assemblies compared for example to a traditional vented unconditioned attic with asphalt shingles this type of roof is very common. Like a wall that is severely out of plumb an unvented roof has insulation packed into the framing cavities aligned with the sheathing. Low slope and steep slope unvented roof systems i e. Five ways to insulate an unvented low slope roof of the various methods to insulate a shallow roof applying insulation to the exterior is easier than working in a space with limited access.

Unvented low slope roofs are never designed to dry to the exterior because low slope roofing membranes are vapor barriers. 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. In unvented flat or low slope roofs with parapets perform a full flood test to ensure leak free installation of roofing membrane. Another approach is to build a cupola or ventilated doghouse in the middle of the low slope roof an approach championed by joe lstiburek.

Drying of unvented roofing assemblies is by definition only provided to the interior. Insulation and air barrier are applied directly to the underside of the roof deck are common in applications where designers are seeking cost effective and energy efficient roof assemblies.