Ground-coupled heat and moisture transfer from buildings

Abstract

A two-dimensional finite element heat and moisture transfer computer program and a companion two-dimensional heat transfer computer program were developed to study the ground-coupled heal transfer from buildings. Ground-coupled heat transfer can account for over 50% of the heat loss from a well insulated building in a cold climate. Soil thermal conductivity is a strong function of the soil moisture; therefore, accurate analysis of the ground-coupled heat transfer requires knowledge of the moisture content of the soil. The moisture transfer model developed in this work is based on a mechanistic approach with temperature and matric potential as the independent variables. The model includes a detailed treatment of the ground surface heat and moisture balances and models freezing of the soil. The finite element formulation uses the Galerkin weighted residual method. The highly non-linear equations are solved using a modified Picard iteration technique. The effects of moisture added to the ground surface and of water table depth on the heat transfer from a slab-on-grade and a basement are investigated. The effect of the moisture added to the surface is largest in the summer and larger for uninsulated floors and basements. Basement walls are sensitive to the conditions at the surface and are the most affected by the surface moisture. Basement floors are relatively unaffected by the short-term variations at the surface, but they are closely tied with the deep ground conditions, such as ground water. Comparison of annual simulations from the heat and moisture transfer model and the heat transfer model produced agreeable results when an appropriate values of soil thermal conductivity and evapotranspiration were chosen. Using seasonal values of soil thermal conductivity for heat conduction models can distort the daily results even though the annual results may appear to be correct.