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    STP1590

    Thermal Performance Evaluation of Roofing Details to Improve Thermal Efficiency and Condensation Resistance

    Published: 2015


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    Abstract

    For roofing systems with insulation installed entirely above deck, designers typically determine the required insulation thickness from published thermal resistance (R-value in Imperial units and R-value Système International [RSI] in metric) of the roofing insulation. These calculations typically do not consider thermal bridges such as insulation fasteners, penetrations, and other detailing. Increased heat flow at these discontinuities decreases the overall thermal performance of the roofing system and, in cold climates, may also increase the risk of condensation within the roof assembly and surrounding construction.

    Roofing system designers need a better understanding of the impact of thermally inefficient details and methods to improve roofing thermal performance. Computer-based thermal modeling can be used to evaluate heat flow through roofing details by estimating the thermal resistance or transmittance of an assembly. In addition, the models permit parametric analysis to review the impact of minor changes to details, such as thickness and placement of insulation, on reducing heat flow. Models can also be used to estimate temperatures of materials and surfaces within the assembly. This analysis allows designers to compare the temperatures within the roofing detail or assembly with indoor dew point temperatures to evaluate condensation potential. Coupled with a hygrothermal analysis, designers can use thermal models to review and recommend insulation, vapor retarder, and air barrier placement in roofing systems.

    This study builds on previous research by using commercially available three-dimensional heat transfer software to evaluate heat loss through commonly encountered roofing details, such as penetrations, supports, and drains. We also review the condensation risk created by cold materials penetrating the vapor retarder to reach interior spaces. The effect of roofing fasteners and fastening plates through insulation on heat loss and condensation is also evaluated. The overall reduction in total roofing thermal resistance is calculated for a hypothetical building roof. Finally, we explore strategies to mitigate thermal bridging and condensation risk by reconfiguring details and strategically placing insulation.

    Keywords:

    roofing, insulation, thermal bridge, R-value, U-factor


    Author Information:

    Olson, Eric K.
    Simpson Gumpertz & Heger Inc., Waltham, MA

    Saldanha, Cheryl M.
    Simpson Gumpertz & Heger Inc., New York, NY

    Hsu, Jessica W.
    Simpson Gumpertz & Heger Inc., Waltham, MA


    Committee/Subcommittee: D08.20

    DOI: 10.1520/STP159020150021