Influence of variability in hygrothermal properties on analytical results of simultaneous heat and moisture transfer in porous materials

Abstract

Depending on the data source used, the material hygrothermal properties that are used in the numerical analysis of simultaneous heat and moisture transfer will not be consistent. Differences in measurement methods and the individuality of specimens account for this. It is necessary to choose values from these different physical property sets to conduct a numerical calculation, which can cause the calculated results to differ. The subsequent range of variation in the calculated results should be quantitatively evaluated. In this study, the physical properties of several types of porous building materials were first gathered from four databases. The data were then categorized based on the kind of material and compared in terms of each physical property (density, porosity, specific heat, moisture capacity, thermal conductivity, and vapor permeability). The density, porosity, and specific heat varied by 10% on average, and the moisture capacity, thermal conductivity, and vapor permeability varied by 20% or more for all types of materials. In particular, the vapor permeability of plywood and moisture capacity of gypsum board differed by 50%. The influence that these physical property value variations had on hygrothermal calculation results was then quantitatively demonstrated for moisture and heat flow rate under a step change in the relative humidity or temperature of indoor air for a single layer wall. The moisture and heat flow rate into a single layer wall fluctuated by approximately 10%–40% due to differences in the vapor permeability and moisture capacity of the materials. For all types of materials, moisture was transferred more slowly than heat. Therefore, differences in moisture property values, such as vapor permeability and moisture capacity, influenced the results more significantly. Moreover, the moisture flow was accompanied by a phase change. The differences in moisture property values thus affected the heat flow.