Combined heat and moisture transport modeling for residential buildings

Zhipeng Zhong, Purdue University

Abstract

Residential buildings are meant to provide a safe, healthy, and comfortable indoor environment for occupants. However, many residential buildings suffer from a variety of moisture problems. Unfavorable indoor relative humidity can make occupants uncomfortable. What’s more, high humidity within building envelopes can lead to deterioration of material, and cause some serious health problems due to the growth of mold and mildew. This research addresses modeling of building envelope transient heat and moisture transfer for structures used in residential buildings. The wall model development is based on a previously developed one-dimensional model called MOIST 3.0, which incorporates water vapor diffusion and water liquid capillary transfer. An important aspect of the current effort has been the development of a wetted surface model that allows consideration of the moisture transfer caused by wind-driven rain, where capillary liquid water transfer is an important mechanism. In order to validate the MOIST model for heat/vapor/capillary transfer, an experiment was devised and carried for drying of a sugar-pine panel. The panel was initially soaked until saturation and then exposed to typically ambient conditions and surface radiant heating with controlled air flow. The experiment was designed to simulate the effects of solar-driven moisture transfer that would follow rain. An automated weighing system was used to trace the overall wood moisture content and moisture pin-pairs measured the local moisture content within the test specimen. During the transient drying test, radiant heating was projected on one surface then switched to the other so as to develop a better understanding of the nature and significance of solar-driven inward vapor diffusion. There was relatively good agreement between moisture content predictions and measurements from the moisture pin-pair sensors and very good agreement of the overall moisture content from the weighing system. Another contribution of this research is a simplified model for predicting transient heat transfer in ground-coupled floor slabs. The model simplifies the traditional two-dimensional approach by employing two parallel one-dimensional transient heat transfer paths. The model incorporates correlations for the effects of ground soil properties and edge insulation that were developed for two common floor configurations. Hourly heat flux predictions compared very well with predictions from a two-dimensional finite-element program for these two geometries. The method can be integrated within hourly simulation programs so that fast estimation of transient heat transfer for the indoor air to the slab-on-ground can be realized. A major contribution of the research is the coupling of the detailed envelope model with a whole building model that allows investigation of the impacts of envelope design on occupant comfort, energy use, and wall material conditions that can lead to mold growth, insulation degradation, etc. In addition to the external wall model, the whole building model incorporates several individual models that were developed, including: (1) weather data treatment including wind driven rain and solar radiation, (2) air infiltration and inter-zonal air flow, (3) indoor heat and moisture generation, (4) heat transfer through slab-on-ground floors, (5) indoor moisture storage within furnishings and other soft materials, and (6) HVAC equipment. The model allows a detailed analysis of indoor/attic air conditions and moisture information for building materials. The original MOIST model only considered individual walls and assumed constant indoor air conditions. In order to evaluate the importance of coupling the wall analysis to indoor conditions, results of the whole building analysis were compared with a single wall analysis performed with constant indoor conditions. The two approaches gave significantly different predictions for moisture levels within materials located near the indoor space but gave essentially the same results for layers located away from the interior space. The stand-alone wall analysis resulted in relatively stable moisture for the interior gypsum layer because of the constant indoor boundary condition, whereas the interior surface for the whole building analyses varied over a relatively large range. Although the moisture levels would not cause mold or material damage problems for the case study considered, the stand-alone wall analysis would not identify potential problems that could occur due to more significant indoor moisture gains that could potentially occur, such as moisture gains within a bathroom not having an exhaust fan. The whole building model has the potential for evaluating moisture problems caused by indoor conditions and also can consider the impacts of design choices on indoor air moisture levels. On the other hand, the standalone model enjoys much lower computational cost and may be adequate for evaluating many moisture problems caused by ambient effects. The whole building analysis tool was used to perform some case studies. Moisture performance of building envelopes was analyzed in some featured climates (heating climate, mixed climate, and dry or humid cooling climate) with representative building constructions. It was shown that for heating climates, a vapor retarder should be placed close to the room side so as to prevent vapor excursion from the relatively warm and moist indoor air. For humid cooling dominated climates, a vapor retarder should be positioned close to the ambient side to stop vapor incursions. On the other hand, for hot and dry climates it is not necessary to use a vapor retarder. For mixed climates, a conventional wall structure without a vapor retarder can work adequately. However, a sandwiched structure with vapor retarders on both sides of a low-permeability insulation is another option because it can handle either vapor incursion from the ambient during summer or vapor excursion from the indoor air during winter.

Degree

Ph.D.

Advisors

Braun, Purdue University.

Subject Area

Civil engineering|Mechanical engineering

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