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.

Unvented low slope roof.

Like a wall that is severely out of plumb an unvented roof has insulation packed into the framing cavities aligned with the sheathing. 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. 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. 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.

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 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. Another approach is to build a cupola or ventilated doghouse in the middle of the low slope roof an approach championed by joe lstiburek.

In unvented flat or low slope roofs with parapets perform a full flood test to ensure leak free installation of roofing membrane. Whatever method is used the amounts of insulation must achieve r values mandated by local codes.