Rain on building façades

Rain is one of the main causes of moisture damage to the building envelope, leading to problems such as rain penetration, frost and salt damage, discoloration by leaching, soiling by differential washing, etc. The potential of deterioration due to rain depends on the façade material, the junction of building envelope components, the overall geometry of the building, but also the presence or absence of modulation on the façade. In this paper, Computational fluid dynamics (CFD) simulations are used to analyze these effects and illustrated with examples taken in Sicily. It is firstly shown that buildings protect themselves from driving rain due to the wind blocking effect, but that façade openings can lead to increased deposit of driving rain due to occurrence of high wind velocities originating from pressure shortcuts. The importance of façade modulation by cornices or roof overhangs for shelter and deflection of rain is next demonstrated. It is further highlighted that these modulations should include drips to shed rain water running down the façade. Façades without moisture buffering by capillary action of façade materials are found to be very sensitive to staining due to run-off of dirty water from horizontal modulations. Finally, an example illustrates the sensitivity to rain penetration and degradation of face-seal façades.