Numerical analysis of hygrothermal performance of reflective insulated roof coatings

Abstract

The use of thermal insulation in roofing systems has been progressively increased in the last few years especially in emerging market countries to reduce energy consumption due to the more frequent use of air conditioners. However, moisture can meaningfully affect the performance of the roof and cause deterioration of the insulation as well as increasing the risk of mold growth. Therefore, for the prediction of performance and risks in roofing systems, a fully combined heat and mass transport model through roofing systems used in hot and humid climates is needed as it is barely explored in the literature. In this way, a mathematical model considering the combined two-dimensional heat, air and moisture transport through unsaturated roofing systems is presented in this paper. In the porous domain, the differential governing equations are based on driving potentials of temperature, moist air pressure and water vapor pressure gradients, while, in the air domain, a lumped approach is considered for modeling the heat and mass transfer through the cavity between the roof sheet and insulation. Results are presented in terms of heat flux and temperature and moisture content distributions profiles within the porous roofing structure, indicating the impact of moisture adsorption and desorption on the roofing hygrothermal performance due to additional transport mechanisms. The paper shows the importance of using a more complete mathematical model – even when conductive latent loads are negligible due to an impermeable reflective insulation – since the mass transport and phase change mechanisms modify the temperature distribution, which cannot be observed by purely conductive heat transfer models presently used in most building energy simulation tools. The roofing short and long-wave radiation heat transfer processes, including the multi-reflective effect of aluminum foil in the air cavity, are also presented and discussed.