Effects of considering moisture hysteresis on wood decay risk simulations of building envelopes

Abstract

Wood decay risk assessment of building envelopes is commonly performed by engineers, architects and practitioners using results of Heat and Moisture Transfer (HMT) simulations and damage models. The commonly accepted HMT models use bijective sorption functions, accepting that materials reach hygrothermal equilibrium with the humidity contained in the air of the material’s pores and of the environment at a single MC. On the other hand, due to moisture hysteresis, equilibrium can be reached at different MCs for the same air condition, depending on previous equilibrium states. The aim of this work is to quantify the effect of considering hysteresis in HMT simulations and to evaluate its propagation in the risk assessment procedure for the case of wood decay. The software MATCH is used, implementation of an HMT model with hysteresis. Three timber walls are simulated in seven locations (Bolzano, Copenhagen, Hong Kong, Ottawa, Shanghai, Udine, and Vienna), first with hysteresis and then with simplified bijective sorption functions (adsorption, desorption, and mean sorption curve). MC and temperature time series are used to perform wood decay risk assessment with two damage models. The results show that the influence of hysteresis can be relevant, and that the choice of the sorption curve used in the simulations should be discussed. For the case of a CLT wall in Shanghai, simulated using the adsorption curve, a mean difference of 1.6% MC is found from the hysteresis case. This resulted in a difference of 0.7 decay rating in 10 years and 6% mass loss in 30 years.