Advancing the Thermal Design of Commercial Roofs: Impact of Mean Operating Temperature, Thermal Bypass, and Thermal Bridging

Abstract

Thermal impact factors—namely, temperature-dependent R-value (TDRV), thermal bypass, and thermal bridging—are currently not addressed in the thermal design of commercial roofs. Therefore, the National Research Council of Canada has been generating data on these influencing factors for improving the thermal design of roofs. The mean operating temperature (MOT) of the roof assembly directly affects the thermal performance of the insulation. Through an in situ field study, test data were collected on the MOT of the roof assemblies and its relation to the insulation's TDRV. A comparison of two energy transfer theoretical models with the measured data indicated that the conventional approach to roof thermal design underestimates the energy expenditure of the roof assembly. The impact of thermal bypass due to air gaps formed at the insulation joints and fastener thermal bridging was also investigated through various experiments. The effect of gap width, height, and stagger was investigated for thermal bypass. For thermal bridging, experiments were conducted that examined the effect of fastener density, location, diameter, and penetration depth on the thermal performance of the roofing assembly. From the experimental data, psi factors and chi factors were developed to support the calculation of linear thermal bridging and point thermal bridging effects and to fill in the missing gaps in the energy codes for the thermal design of roofs. Because the thermal bridging from rooftop solar mounts is also a concern in the thermal design of roofing assemblies, experiments were conducted on several common solar mounts. The measured data indicated a decrease in the effective thermal resistance of the photovoltaic roofing assembly (PVRA), ranging from 3.3% to 50.0%. This paper summarizes these research findings and demonstrates the effect of these thermal impact factors on roof thermal design through two examples.