C029 -- Comprehensive Study of Moisture Risk on Building Facades Based on Spatial Distribution of Wetting and Drying PDF

Moisture is one of the main factors affecting the durability of buildings. Moisture conditions within buildings result from their wetting and drying behavior. Wind-driven rain (WDR) is often the largest moisture source for wetting of building envelopes whereas evaporation is the main mechanism that removes water from porous building materials. Moisture-induced damage risk of building envelopes is mostly evaluated with one-dimensional hygrothermal simulations. In these studies, WDR load on building facades is typically estimated with semi-empirical methods, leading to uniform WDR intensity across large parts of facades with large uncertainties. Similarly, the convective moisture transfer coefficient (CMTC), a key parameter for the calculation of evaporation rate, is commonly calculated with simple empirical equations and taken as a surface-averaged value. This simplification underestimates the evaporation rate for regions of the facade that are more exposed to wind and overestimates for those that are less exposed. The balance of WDR load and CMTC determines moisture risk at the different parts of a building facade. The regions with a higher WDR load typically indicate a higher moisture risk whereas the regions with a higher CMTC indicate a lower moisture risk. When the actual spatial distributions of WDR and CMTC are not considered, the moisture risk of facades cannot be accurately evaluated in detail. Additionally, locations with the highest moisture risk cannot be determined. In this study, an integrated model that considers air flow around a building, WDR, radiation, moisture and heat transport in the building envelope is used to analyze moisture risk on a building facade. The spatial distributions of both WDR load and CMTC on the building facade are obtained with computational fluid dynamics (CFD) simulations. There are large spatial variations of WDR and CMTC on the facade. Generally speaking, locations with higher WDR load have also higher CMTC. Therefore, the location with a higher WDR load may not always be the location with a higher moisture risk. The moisture level on the facade is affected by both WDR and CMTC. Both wetting and drying components need to be accurately determined to ensure a reliable evaluation of moisture risk on building facades.