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Abstract: Combined multidimensional analysis of heat, air and moisture transport through porous building elements is barely explored in the literature due to many difficulties such as modeling complexity, computer run time, numerical convergence and highly moisture-dependent properties. In this way, a mathematical model considering a combined two-dimensional heat, air and moisture transport through unsaturated building upper corners is presented. In order to improve the discretized model numerical stability, the algebraic equations are simultaneously solved for the three driving potentials: temperature, vapor pressure and moist air pressure gradients. In the results section, the convective effects caused by air stagnation are analyzed in terms of heat flux and mould growth risk for different boundary conditions, showing the importance of a detailed hygrothermal analysis – which is normally disregarded by simulation tools – for accurately predicting building energy consumption, indoor air quality, thermal comfort or mould growth risk. [Copyright &y& Elsevier]

The article presents a theoretical model to describe coupled heat and moisture transfer in porous media which finds relevance in building sciences where thermal comfort and energy efficiency are of high interest and in geophysics where accurate prediction of subsurface water content and temperature distributions has been a major focus. The Philip and de Vries based model (Philip, J.R. and de Vries, D.A., 1957. Transactions - American Geophysical Union, 38, 222-232) was applied to four cities with strikingly varying climatic conditions using hourly weather data for each city. At the topmost layer in January, daily average soil temperature was highest in Singapore and lowest in Seattle while the daily average soil moisture content was highest in Singapore and lowest in Johannesburg. In July, the daily average temperature was highest in Singapore and lowest in Curitiba while the daily average moisture was highest in Curitiba and lowest in Johannesburg. [ABSTRACT FROM AUTHOR]

Many of the now well-known building energy simulation programs use the response factor method developed in the early 1970s by Stephenson and Mitalas. These are TRNSYS, EnergyPlus, Blast, and DOE-2, to name but a few. Others, such as PowerDomus, ESP-r, and BSim, perform finite-volume or finite-difference calculations to solve the heat and mass transfer through the building envelope. These two different approaches are known to have strengths and weaknesses. The main objective of the present exercise is to compare the prediction of both methods. A two-step procedure is employed here. The first deals with the pure thermal problem, i.e., without moisture calculation. Three different cases of increasing complexity are studied and compared to analytical solutions. The second step focuses on the moisture problem alone by comparing the responses obtained with a two-layer buffer storage model and a finite-volume discretization for moisture transfer. Results show that time step values are determinant even for pure thermal cases where the classical value of 1 h can lead to notable errors. For problems with moisture sorption in the wall, it has been shown that grid refinement is a very decisive parameter, while the time step has to be set, to unusually small values, to achieve a good response. [ABSTRACT FROM AUTHOR]

Abstract: In order to precisely predict ground heat transfer, room air temperature and humidity, a combined model has been developed and conceived to calculate both the coupled heat and moisture transfer in soil and floor and the psychrometrics condition of indoor air. The present methodology for the soil is based on the theory of Philip and De Vries, using variable thermophysical properties for different materials. The governing equations were discretized using the finite-volume method and a three-dimensional model for describing the physical phenomena of heat and mass transfer in unsaturated moist porous soils and floor. Additionally, a lumped transient approach for a building room and a finite-volume multi-layer model for the building envelope have been developed to integrate with the soil model. Results are presented in terms of temperature, humidity and heat flux at the interface between room air and the floor, showing the importance of the approach presented and the model robustness for long-term simulations with a high time step. [Copyright &y& Elsevier]