In recent years water on external walls has become a major subject of research because it promotes soiling and microbial growth. The increase in surface moisture of façades can be largely attributed to better thermal insulation and lower thermal capacitance of external walls, leading to frequent condensation of outdoor air caused by long-wave sky radiation. In this paper, the principal moisture loads on external walls, wind-driven rain and exterior condensation, are investigated and the hygrothermal response of differently painted façades to these loads are evaluated. It turns out that the most effective driving-rain protection may show the poorest performance when exterior condensation occurs. Therefore, both characteristics have to be considered jointly. Relying on known results, the best way of dealing with both loads seems to be a moderately water-permeable surface coating.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings X – December 2007

Units: SI