Rainfall is seldom addressed in the analysis of climates for building design, usually neglected for building thermal performance calculations, and there is very little research about its potential cooling effect.
This research aims to estimate and compare the influence of natural rainfall on the thermal and energy performance of a low-rise building in diverse locations of the hot humid tropics to assess its significance. Through whole building simulations in WUFI Plus, a parametric study based on rain intensity is conducted to estimate the hygrothermal performance of a small building (free-running and air-conditioned) at wet, wet-dry and dry locations of the Tropics. Taking a holistic approach, this study observes aspects which are seldom regarded in thermal and energy simulations, like impacts to the outdoor environment and the role of water; and makes a closer approximation to reality than previous research which observes rain events only during daytime and in relation to isolated building components.
The results show that rain can have an important cooling influence, although is usually more appreciable to the exterior of the building than to the interior. When the effect of rain is neglected, the total heat transfer per unit area from all outdoor surfaces to the surroundings can be overestimated by 20% in a rainy month and by 10% in a year; as well as by 7% in a rainy month and by 5% in a year from all indoor surfaces to the indoor air. Maximum temperature reductions due to rain can be 7.4 °C for outdoor surfaces, 1 °C for indoor surfaces, 0.4 °C for indoor air temperature and 0.5 °C for indoor operative temperature. The differences in thermal and energy performance result from consumption of relatively small amounts of rainwater (up to 67 kg in a rainy month and up to 448 kg in a year), which gives an idea of the potential of the rainwater surplus in the humid tropics for natural cooling in buildings and urban sites.