Methodology for determining climate-specific effective PIR thermal conductivity and thickness values for energy modeling and code prescription: A case of flat roofs

Buildings generate 12% of Canada’s GHGs; commercial and institutional facilities produce ∼52% of building emissions, making roofs a prime target for efficiency gains. The ability to accurately model the thermal performance of roofs plays a vital role in assessing the implementation of building energy-saving strategies, especially roofs with PIR (Polyisocyanurate) insulation, where its thermal conductivity changes non-linearly with temperature. In this paper, a transient heat transfer model is developed and validated with field experimental data. For the heating-dominated climates considered, the increase in thermal conductivity at lower temperatures of PIR results in greater energy loads when calculated using the temperature-dependent thermal conductivity compared to the code-specified constant 24°C thermal conductivity. To bridge the gap between the two approaches, a method for determining the effective mean insulation operating temperature (MIOT eff ) at which the constant effective thermal conductivity ( k eff ) is representative of the temperature-dependent insulation performance is proposed. The application of the method is demonstrated in five Canadian cities. Finally, charts for determining climate-specific effective PIR thermal conductivity and thickness values for energy modeling and code prescription are presented. • Field tests show PIR insulation performance depends on outdoor temperature. • RC-Network model is able to replicate in-situ roof heat transfer measurements. • Transient heat flow differs using constant vs temperature-dependent conductivity. • Method to find constant-temperature conductivity representing temperature effects. • Design charts give constant-temperature conductivity and insulation thickness.