Unvented Shed Roof Barrier

Figure 2c above left.

Unvented shed roof barrier. There are traditionally four main reasons for venting a roof. Enhancing roofing material life span by reducing sheathing temperature. This phenomenon occurs regardless of whether the roof is vented or unvented in the absence of a vapor barrier between the asphalt shingles and the roof deck. Figures 3a 3c illustrate the phenomenon of intermittent bucking of shingles due to solar driven moisture.

Control construction moisture content with tenting or temporary roofing when necessary. Because the drying capacity of unvented roof assemblies is inherently lower than vented ones it is especially critical to control built in moisture during construction. Removing moisture from roof cavities structural members sheathing and insulation. Air permeable insulation such as fiberglass batts dense packed cellulose or blown in fiberglass can t be used alone for an unvented assembly since these types of insulation can allow moist indoor air to reach the cold roof sheathing leading to condensation or moisture accumulation.

Fix it again unvented low slope roofs are never designed to dry to the exterior because low slope roofing membranes are vapor barriers. Martin s post focuses on the marketing of owens corning 039 s propink unvented attic insulation system. 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. Moisture content in roof structure must be less than 18 prior to covering.

An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot. Figure 2d above right.