Volume 6, Issue 4 (April 2009)
Compact Asphalt Shingle Roof Systems: Should They be Vented?
Traditional steep-slope roof systems for the residential buildings (single-family homes and multi-building apartment or condominium buildings) consist of asphalt glass-fiber shingles applied over vented or unvented attics. The long-term performance of these roofs is primarily dependent on geographic location, proper roof ventilation, roof slope, material component selection and color, as well as proper detailing and construction. There is a trend towards maximizing interior space in residential dwellings by constructing compact steep-slope roof systems with cathedral or vaulted ceilings without proper venting of the asphalt shingle roof system, which, in turn, may reduce the life of the shingles. The primary reason for venting is to convey water vapor. Build up in the roof, to outside the building. Although few building code jurisdictions are relaxing the requirement for roof ventilation in compact steep slope roof systems, ventilation is still typically required by most building codes. Where allowed by the code, designers and contractors tend to omit the vented air space and favor the use of closed cell spray-applied foam insulations (polyurethane) or permeable insulation combined with vapor retarders to help control the build up of destructive moisture levels within the roof. This article presents the results of numerical case studies created in WUFI® that demonstrate the need for venting in unvented asphalt shingle roof systems with a variety of insulation and vapor retarder materials in a hot (Miami, FL) and cold (Boston, MA) climates. The models utilize historical hourly weather data to simulate the time-varying exterior conditions and examine the effect of natural ventilation and incidental roof leakage on hygrothermal performance of unvented and vented steep-slope roof assemblies. The interior conditions for residential buildings in the models are set at 21.1°C±1.1°C (70°F±2°F) and 35 %±15 % relative humidity. We selected the drying potential and the robustness of the moisture-sensitive roof system materials in response to incidental roof leakage to measure the performance of the roof assembly. Our study showed better durability of vented roof assemblies with permeable insulation in cold climates due to redundancies that can tolerate incidental moisture and provide visual indicators of roof leakage; roof sheathing typically dries in 1-1/2 to 2 months. All of the unvented roof assemblies are intolerant of incidental water leakage and the moisture-sensitive layers (i.e., sheathing and gypsum wallboard (for open-cell polyurethane insulation)) exceed the threshold for decay. In hot, humid climates, the most durable roof assemblies are the vented, open-cell polyurethane systems with shorter drying time of the interior gypsum wallboard when compared to the unvented roof assembly; both the sheathing and gypsum wallboard dry out within 2-1/2 months. In an unvented assembly, the drying time for sheathing is similar but the drying time for gypsum wallboard increases to 6.5 months on average. Alternatively, unvented permeable shingled roofs are a viable option in hot, humid climates, although they are slightly less durable. The least tolerant roof assemblies in either climate are the unvented closed-cell polyurethane roof assembly due to trapped moisture and slow drying of the roof sheathing (up to 12 months in Miami, FL and 27 months in Boston, MA).