Dynamic modelling of moisture absorption and desorption in buildings

Abstract

Moisture absorption and desorption by building materials in the hygroscopic range have been theoretically modelled. The Biot number which can be defined as the ratio between the material moisture resistance to the convective mass transfer resistance has a significant meaning in relating where the greater resistance to moisture transfer occurs. Using Biot number, the dynamic moisture behaviour of building materials within the indoor environment has been classified into three main categories. At high Biot number ( Bi → ∞), the material surface attains instantaneous moisture equilibrium with the surroundings. At low Biot number ( Bi → 0), the material moisture behaviour can be described through a lumped-parameter modelling. For most materials in buildings, moisture interaction with the surroundings occurs through a thin layer of material surface, and the amount of moisture absorption or desorption is mainly determined by the material surface moisture conditions. In order to evaluate material surface moisture conditions, the governing moisture transfer equation is solved via an approximate analytic technique (i.e. the moment method) in conjunction with numerical formulation. Both the dynamic and the alternating nature of the absorption/desorption processes can be modelled by this proposed analytic-numeric method. Comparison with experimental results and numerical solutions shows satisfactory agreement with the proposed model. Using this model the dynamic effect of moisture absorption and desorption by interior materials on indoor air humidity can be modelled.