Near-Roof Air Temperatures: Modelling the Implications for HVAC Performance and Cool Roofs

Conventionally in building performance simulations (BPS), it is assumed that air entering outdoor HVAC equipment is at the outdoor 'ambient' temperature, obtained from a weather file. However, significant spatial variations exist in outdoor air temperature fields, especially within the thermal boundary layers that form near exposed surfaces like roofs. Experiments were conducted at three large-footprint shopping centre buildings, to characterise the above-roof temperature field. An empirical model was derived from the experimental data, and applied in BPS of a shopping centre with rooftop HVAC equipment in seven Australian climates. In these cases, the electricity savings and gas `penalties' attributable to cool roofs would have been underestimated by 44-85% (61% on average) if near-roof air temperature variations had not been modelled accurately.