Development of a Simplified Heat and Moisture Transfer Model for Residential Buildings

Kim, Joowook

Graduate Thesis Or Dissertation

Development of a Simplified Heat and Moisture Transfer Model for Residential Buildings Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/0g354f409

Abstract

A large number of building energy simulation programs are used to evaluate indoor conditions and energy performance of buildings. Every building energy modeling tool calculates the heat and moisture exchange among the outdoor environment, the building envelope, and indoor environments as a fundamental component of heating, ventilating, and air conditioning (HVAC) loads. While all tools account for moisture flow through the envelope by infiltration, almost no users calculate moisture diffusion through the envelope or moisture storage in the building due to excessive computation time and unavailable material properties. However, research has shown, and work in this dissertation has confirmed, that significant errors in energy consumption predictions occur when moisture exchange is ignored, especially in hot and humid climates. This dissertation explores the impact of moisture exchange in building energy models and develops recommendations for improving simplified moisture exchange calculations under circumstances where the errors are the greatest. A research model has been developed to model detailed heat and moisture transfer in building envelopes, based on the HAMT model in EnergyPlus, which allows for the examination of both vapor and liquid transport mechanisms. The detailed model is validated through international benchmark tests and is used as a reference model to test simpler methods. Models for conduction transfer function (CTF) and effective mean penetration depth (EMPD) methods have also been developed.The research explored the opportunities to improve existing simplified moisture exchange models, specifically the heat and moisture transfer function (MTF) methods and the EMPD method. The MTF method accounts for transport of moisture vapor, but ignores liquid transport mechanism. An analysis was conducted to assess the impact of liquid transport on overall heat and mass transfer under various external environmental conditions for walls made of different building construction materials. Results indicate that liquid transport mechanism can have a significant impact on moisture flow through some construction materials, specifically concrete and gypsum, at high relative humidity. However, sensible and latent fluxes on the inside surface of multi-layer walls are not significantly impacted by the presence of liquid transport in the heat and moisture transfer calculations model. It is concluded that changes to the MTF model to account for liquid transport are not warranted. The EMPD model accounts for cyclic moisture storage at the surface of building materials and assemblies. As an extension of the conventional EMPD method, which accounts for buffering only on the inside surface, a two-sided moisture penetration depth model is proposed and evaluated how much the EMPD model at the external surface would influence thermal and moisture transfer at the inside surface. The results indicate that the two-sided EMPD model has no significant impact on inside sensible and latent heat fluxes.During the research, it was observed that moisture transport through building envelope assemblies, as opposed to moisture buffering in the assemblies, is usually very small. In fact, in most cases, it may not be necessary to account for moisture transport through the assemblies. This thesis proposes a simple assessment method to evaluate the sensitivity of HVAC loads to moisture transfer through constructions under different external conditions (temperature, humidity, and solar radiation). The methodology uses the ratio of latent heat flux and sensible heat flux, which is estimated using the mean values of temperature, vapor pressure, and sensible and latent resistances. The method is validated using the detailed heat and mass transfer model. The method can be used as a pre-simulation screening tool to assess whether moisture flux is expected to be significant.The one typical building construction assembly that

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