Rainwater runoff from building facades: A review

Rainwater runoff from building facades is a complex process governed by a wide range of urban, building, material and meteorological parameters. Given this complexity and the wide range of influencing parameters, it is not surprising that despite research efforts spanning over almost a century, wind-driven rain and rainwater runoff are still very active research subjects. Accurate knowledge of rainwater runoff is important for hygrothermal and durability analyses of building facades, assessment of indirect evaporative cooling by water films on facades to mitigate outdoor and indoor overheating, assessment of the self-cleaning action of facade surface coatings and leaching of particles from surface coatings that enter the water cycle as hazardous pollutants. Research on rainwater runoff is performed by field observations, field measurements, laboratory measurements and analytical and numerical modelling. While field observations are many, up to now, field experiments and modelling efforts are few and have been almost exclusively performed for plain facades without facade details. Field observations, often based on a posteriori investigation of the reasons for differential surface soiling, are important because they have provided and continue to provide very valuable qualitative information on runoff, which is very difficult to obtain in any other way. Quantitative measurements are increasing, but are still very limited in relation to the wide range of influencing parameters. To the knowledge of the authors, current state-of-the-art hygrothermal models do not yet contain runoff models. The development, validation and implementation of such models into hygrothermal models is required to supplement observational and experimental research efforts.status: publishe