Impact of Solar Mounting Systems on the Thermal Design of Commercial Roofs

Abstract

Commercial rooftops provide extensive areas representing the desired platform for installing photovoltaic (PV) systems. The combination of roofing assembly and PV system is called photovoltaic roofing assembly (PVRA). Currently, there is a shortage of information about building codes on PV system and roofing assembly integration. The National Research Council of Canada (NRC) developed an industry consortium project to generate codifiable research data on the wind and thermal performances of photovoltaic roofing assemblies. This paper examines the thermal bridging of the PV mounting attachments and its impact on the overall thermal resistance of the roofing assembly. Seven different types of PV mounting fixed attachments were tested in this study. Thermal bridging evaluation of PV mounts was done on the component level as individual mounts were installed through a section of a roofing assembly. The roofing assemblies were designed for a prescriptive thermal resistance of RSI 5.46 m2 K W−1 (R-31 h ft2 °F BTU−1) as per current standard and requirements for Climate zones 4–6. The thermal bridging experiments were conducted on a 1.2 m by 1.2 m guarded hot box at a mean temperature of 24°C. The measured data indicated a decrease in the effective thermal resistance of the PVRA, ranging from 3.3% to 50.0%, compared to the opaque roofing assembly devoid of any thermal bridging. Furthermore, it was found that among fastener-designed PV mounting attachments, the greater the number of fasteners, the more significant the decrease in effective thermal resistance. From the experimental data, chi factors (χ) were developed to support the calculation of point thermal bridging effects on the thermal performance of low-sloped roofing assemblies. Toward codification, efforts are underway to potentially implement these chi factors in the energy codes that could enhance the thermal design of both retrofit and new roof constructions installed with photovoltaic systems.