While the numerical simulation of moisture transfer inside building components is currently undergoing standardisation, the modelling of the atmospheric boundary conditions has received far less attention.
This article analyses the modelling of the wind-driven-rain load on building facades by partial simplification of a complex CFD-based method along the lines of the European Standard method. The results indicate that the directional dependence of the wind-driven-rain coefficient is not of substantial importance. A constant wind-driven-rain coefficient appears to be an oversimplification though: the full variability with the perpendicular wind speed and horizontal rain intensity should be preserved, where feasible, for improved estimations of the moisture transfer in building components.
In the concluding section, it is moreover shown that the dependence of the surface moisture transfer coefficient on wind speed has an equally important influence on the moisture transfer in building components.
Knowledge of the hygric behaviour of building components is of serious importance for different building physics issues. Insight into a building component's hygric behaviour is evidently needed when analysing durability problems of existing components, or when assessing the expected performances of newly developed components. The moisture transfer inside building components also affects the interior climate though, and hence plays a role in interior air quality and energy performance. The moisture transfer inside the building components moreover determines the moisture regime at the exterior surface, and thus influences esthetical appearance, by playing a part in soiling phenomena and algae formation.
Until recently, the Glaser method was accepted as the standard [1] calculation tool for such evaluation of building components’ hygric behaviour, but its restrictions—stationary, no liquid transfer, no air transfer—make it only rarely reliably applicable. Presently, application of hygrothermal simulation programmes for the evaluation [2] and [3] or optimisation [4] and [5] of the hygric performance of building components is becoming general practice.
Currently, numerical simulation of moisture transfer in building components is undergoing standardisation [6], to which a quality assessment methodology was recently added [7]. Both are restricted though to moisture and heat transfer inside permeable building components, and do not thoroughly discuss the atmospheric boundary conditions. The dependability of hygrothermal simulations under atmospheric excitation cannot be guaranteed, however, without an accurate modelling of these phenomena. This article concentrates on the atmospheric moisture load, presumed the largest and most important uncertainty.
The ensuing study reveals that wind-driven rain is the main moisture source for permeable building facades and investigates the required level of detail in implementations of wind-driven rain as a boundary condition in hygrothermal simulations. This study focuses on the long-term ‘moisture response’ of building facades, which is represented by the variation of average and surface moisture contents over the course of a year. The yearly wind-driven rain and evaporative drying amounts are, however, also incorporated in the comparison. Other ‘shorter-term’ wind-driven rain-related phenomena, such as run off and water penetration, are discussed only secondarily.
This analysis, based on hygrothermal simulations of building components, piecewise simplifies a complex CFD-based formulation [8] for wind-driven rain along the concepts of the European Standard on wind-driven rain [9], in order to evaluate the acceptability of the differing features of both approaches. More specifically, this article investigates the significance of the dependency of the wind-driven-rain coefficients on rain intensity, wind speed and wind direction. In the final paragraph, the resulting conclusions are put into a larger perspective, by sketching the effect of the surface moisture transfer coefficients on evaporative drying, the primary moisture removal mechanism for permeable building facades.
In this article, the sensitivity of hygrothermal simulations of building facades to the level of detail in modelling the wind-driven rain has been analysed. For the locations investigated here (Essen and Bremerhaven), wind-driven rain appears to be the main moisture supply for permeable facades, while evaporative drying appears to be the primary loss mechanism. The study mainly focused on the correct modelling of the wind-driven rain.
The modelling results indicate that for most locations on facades with most orientations in most climates, the directional dependence of the wind-driven-rain coefficient is not of substantial importance when assessing the long-term moisture response of permeable building components. The directional αθ(Rh,U, θ–ϕ) can be replaced by α⊥(Rh,U′), to be used in combination with the projected wind-driven-rain index U cos(θ–ϕ)·Rh.
The use of a constant View the MathML source, on the other hand, appears to be an oversimplification. Implementation of the whole α⊥(Rh,U′) relation is recommended where feasible. To avoid complex and time-consuming CFD-simulations before any hygrothermal simulation, the idea of a wind-driven-rain catalogue was reiterated.
In a final paragraph, it has been shown that the moisture responses of building facades are as sensitive to the modelling of evaporative drying, specifically the surface moisture transfer coefficient. Whereas reliable prediction methods exist for α⊥(Rh,U′), these are lacking for the hm,e(U,θ–ϕ) dependence. Further research on this topic is hence encouraged.
