Thermal performance of building roof elements

The study concerns the evaluation and comparison of the thermal performance of building roof elements subject to periodic changes in ambient temperature, solar radiation and nonlinear radiation exchange. A numerical model, based on the finite-volume method and using the implicit formulation, is developed and applied for six variants of a typical roof structure used in the construction of buildings in Saudi Arabia. The climatic conditions of the city of Riyadh are employed for representative days for July and January. The study gives the detailed temperature and heat flux variations with time and the relative importance of the various heat-transfer components as well as the daily averaged roof heat-transfer load, dynamic R-values and the radiative heat-transfer coefficient. The results show that the inclusion of a 5-cm thick molded polystyrene layer reduces the roof heat-transfer load to one-third of its value in an identical roof section without insulation. Using a polyurethane layer instead, reduces the load to less than one-quarter. A slightly better thermal performance is achieved by locating the insulation layer closer to the inside surface of the roof structure but this exposes the water proofing membrane layer to larger temperature fluctuations.