Natural aging of cool walls

Wall finishes with high solar reflectance and thermal emittance, commonly known as cool walls, can reduce the exterior surface temperatures of façades, and consequently the building cooling energy needs and power demand, and lower the sensitivity to degradation. Aging, though, may affect their performance. To investigate this risk, we exposed for four years in Milan, Italy, two series of façade finish coats, white and beige, facing north and south, in vertical and vertical-sheltered position, and we measured their solar spectral reflectance and thermal emittance before and after aging. The solar reflectance of the white finish coats drops from 0.75 to 0.55 in four years, and from 0.46 to 0.38 for the beige coats, while the thermal emittance is unchanged. Then, for a typical residential building with white walls, we computed that the cooling energy needs increase with walls aging by 5% and 11%, respectively, with or without exterior wall insulation. The exterior surface temperature is increased even by 6 °C, and the number of sudden surface temperature variations in one hour is boosted. Finally, the moisture content in the external layers is reduced, showing the impact on the heat and mass balance because of the uncertainty in solar absorption due to aging.