Driving rain on building facades

Moisture is one of the most important factors affecting building envelope durability and performance, especially in cold climates. Understanding and predicting moisture movement within and through the envelope is therefore of fundamental importance. Although computer modelling of building envelope hygrothermal performance has made great strides in the last decade, one of the largest sources of moisture remains relatively poorly understood : driving rain. The Building Engineering Group has conducted a number of studies of driving rain deposition, drainage penetration, and absorption both in the lab and in a full-scale natural exposure and test facility. This paper presents some of the results from the detailed field monitoring of temperature and moisture, especially in brick veneers, of wall panels exposed to the climate of Ontario as well as laboratory tests. The nature of driving rain is examined in some detail and theoretically-derived predictions are compared to field measurements. The coincidence of wind and rain especially with regard to seasonal and directional variations are considered. Driving rain deposition and wetting of vertical above-grade building envelopes is discussed and field results are compared to other research. The effects of rainfall intensity, rainfall duration, wind direction, previous wetting history, surface texture, and water absorption are discussed and some practical guidelines for the assessment of the driving rain moisture load are provided.