Development of chi-factors towards codification of the thermal bridging in low slope roofing assemblies

Abstract

An industry consortium study, Energy Resistance of Commercial Roofs (ERCR), has been developed by NRC to develop scientifically supported performance data on energy resistance of roofing systems that are constructed according to field practices. The ERCR consortium project had two major tasks. In Task 1, the objective was to evaluate the effective thermal resistance of current roof designs and validate their compliance with the energy code requirements and, in Task 2, the aim was to quantify the two thermal impact factors – thermal bridging from fasteners and thermal bypass from gaps between the insulation boards. This paper focuses on the Task 2-thermal bridging from fasteners. More than 100 experiments were conducted to quantify the impact of fastener density, fastener location, fastener diameter, and fastener penetration depth on the thermal performance of the roofing assembly. From the experimental data, the thermal bridging of fasteners was quantified in terms of the “Relative” decrease in effective R-value and Chi-factors (χ). The impact of fastener density revealed that the thermal bridging increases with the fastener density and also with the thermal resistance of the insulation with an average loss in effective R-value ranging from 5% to 14% for the tested configurations. The increase in the fastener diameter or the fastener area (#12 < #14 < #15) increases the overall heat flow through the assembly. In a multilayer insulation layout, fasteners installed in the bottom layer are shielded by the top insulation and can reduce thermal bridging from 30% to 70% relative to the through fastener from the top layer. From the experimental data, chi factors were developed to support calculation of point thermal bridging effects on the thermal performance of the commercial roofs, and also to fill in the missing gaps in the energy codes towards the thermal design of roofs.