Solar gain and building envelope: The surface factor

Harnessing the appropriate radiative characteristics of buildings' external surfaces is a highly effective way to reduce internal space heating and cooling demand. Building energy-dynamic simulations are created in order to compare various external coatings with reference to a social housing case study in a typical Mediterranean climate (Naples, Italy). An innovative index, the surface factor (SF), is proposed to inform the choice of the most suitable surface coatings as a function of local climatic conditions and thereby reduce overall energy demand. For cities with high solar gain and/or low winter degrees-day (SF > 0.65), high-reflective and/or high-emissive coatings (i.e. 'cool paints') are suitable, while low-reflective and/or low-emissive external coatings are more suitable for cooler climates (SF < 0.4). The simultaneous interactions of wall mass, surface radiative characteristics, and indoor ventilation on the summer cooling energy demands are evaluated and substantial heating or cooling savings can be achieved, particularly for existing buildings. For regions in which outdoor night temperature is lower than the indoor value, high thermal inertia walls are viable only when coupled to night ventilation. Diurnal ventilation coupled to massive walls was found to be energy inefficient.