Vented Attic Approach Unvented Attic Approach
Vented attic and roof construction has a long history of successful performance. Why change a good thing? As the complexity of attic and roof assemblies increase, the difficulty to construct vented assemblies also increase. The more complex a roof geometry, the easier it is to construct the assembly in an unvented conditioned manner. With complex roof designs, multiple dormers, valleys, hips, skylights combined with cathedral construction with interior soffits, trey ceilings and multiple service penetrations, it is often not practical to construct a vented roof assembly with an AirTight interior air barrier at the ceiling plane. It is more common to locate mechanical systems and ductwork in attic spaces. When such ductwork is leaky, significant problems can occur. “There are significant energy advantages and durability advantages to move the thermal boundary and pressure boundary (air barrier) to the underside of the roof deck.” (Rudd, Lstiburek, & Moyer; 1997) thereby, locating these mechanical systems and ductwork within the building conditioned spaces. In high wind regions, particularly in coastal areas, wind driven rain is a problem with vented roof assemblies. Additionally, during high wind events, vented soffit collapse may lead to building pressurization, window blowout and/or roof loss due to increased pressure in the structure. Unvented roofs, (principally due to the robustness of their construction) generally perform better than vented roofs during hurricanes. In coastal areas, salt spray and corrosion are a major concern with steel frames, metal roof trusses and truss plate connectors in vented attics. Finally, in wildfire zones, unvented roofs and attics have significant benefits in terms of fire safety over vented roof assemblies.