Buildings XIII International Conference

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Thermal Performance of the Exterior Envelopes of Whole Buildings XIII International Conference

Abstract: Unvented roof assemblies can bring attic mechanical systems into conditioned space, negating ductwork losses. However, in previous work, unvented roofs with air-permeable (fibrous) insulation, instead of air-impermeable insulation (spray foams) have shown localized moisture accumulation at the ridge. This research is a test implementation of two unvented roof assemblies insulated with air-permeable insulation (netted and blown fiberglass or adhered fiberglass) in hot/humid climates. One test roof is located in Houston, TX and has asphalt shingles; the other is in Orlando, FL, with concrete tile; both are in Climate Zone 2A. Given that localized moisture accumulation and failures occurred at the ridge in previous unvented roofs, a diffusion vent (open to water vapor but closed to airflow) was installed at the highest points in the roof assembly to allow for the wintertime release of moisture. The diffusion vent is an opening at the ridge and hips covered with a water-resistant but vapor-open membrane. As a control comparison, portions of the roof were constructed as a typical unvented roof (self-adhered membrane at ridge). Collected data indicate that the diffusion vent roof shows greater moisture safety and less wintertime moisture accumulation than the conventional, unvented roof design. The unvented roof had winter periods of 95%–100% rh, with other sensors indicating possible condensation; high moisture levels were concentrated at the roof ridge. In contrast, the diffusion vent roofs had drier conditions. In the spring, as outdoor temperatures warmed, all roofs dried well into the safe range (10% MC or less).