Aalborg Universitet Whole building hygrothermal simulation model

A whole building hygrothermal model has been developed on the basis of an existing detailed model for thermal simulation of buildings. The thermal model is a well-proven transient tool for hour-by-hour simulation of the thermal conditions in multizone buildings. The model has been expanded with new capabilities for transient simulation of indoor humidity conditions, taking into account the moisture buffer capacity of building components and furnishings and the supply of humidity from indoor activities. Also integrated in the model are transient calculations of the moisture conditions in the layers of all the external building envelope components. The advantage of the new model is that both the boundary conditions for the envelope and the capacity of building materials to buffer the indoor humidity are considered in the same calculation. The model considers the latent heat effect associated with the absorption or evaporation of moisture, and it takes into account the way in which moisture in the building materials affects their thermal conductivity. The paper presents the principles for the model and some applications and calculation results. The model is validated against experimental data from a full-scale test cell. In the test cell, it is possible to control the release or withdrawal of humidity from the indoor space and measure the response in humidity of the air and the moisture content of building materials in the room. A sequence of experiments has been conducted using different interior materials to provide source data for the effect of moisture absorption and release. Examples of comparisons between simulated and measured data are presented. INTRODUCTION Humidity in indoor spaces is one of the most important factors influencing indoor air quality. Many health-related problems in the indoor environment (e.g., sick building syndrome [SBS] can be associated with high indoor humidity and “damp buildings” [Clausen et al. 1999]). The humidity level in a building depends on a combination of factors such as moisture sources, ventilation and air movement, reservoirs and sinks, heating, insulation, external conditions, as well as building materials and occupants. Among these, the moisture buffering effect of the materials in a building is an important factor. There is a general interest in exploiting the moisture buffering effect of building materials, such as wood, to dampen the cyclic excursions of indoor humidity. However, this effect is often disregarded by building designers and engineers. Ventilation with fresh air is a way to improve the problems of high indoor humidity, but ventilation requires energy to condition the air and to run the fans of the ventilation systems. So there is interest in being able to design buildings for a suitable balance between moisture supply and required ventilation. However, the humidity condition of indoor air is not just a result of moisture supply from current activities and the actual ventilation rate. It also must be considered that many building materials and interior furnishing are hygroscopic, so they act as buffers for the indoor humidity. Several approaches to model the indoor humidity conditions have been developed in previous research. The approaches vary from simple steady-state models that completely disregard the indoor moisture buffering (Loudon 1971), over empirical models that acknowledge but physically do not describe the buffering effect (Tsuchiya 1980), to more 572  2003 ASHRAE Carsten Rode is an associate professor in the Department of Civil Engineering and International Center for Indoor Environment and Energy, Technical University of Denmark, Lyngby, Denmark. Karl Grau is a senior researcher with Danish Building and Urban Research, Denmark. physics-based models that consider the buffering in a surface layer of the building elements (e.g., the effective penetration depth models by Kerestecioglu [1990] and Cunningham [1992]). Furthermore, two different moisture balance models are implemented in version 15 of the TRNSYS simulation system—the effective capacitance humidity model and the buffer storage model (Klein et al. 2000). Typically, previous models have predicted the indoor humidity without consideration of the materials in the deeper layers of building constructions and furnishing, although they set the boundary conditions for the indoor humidity variations. In contrast, several detailed models have been developed for the moisture conditions of exterior building envelope constructions. High indoor humidity is among the most important reasons for harmful accumulation of moisture in the building envelope, and it can be a direct or indirect reason for extra heat flow though the enclosure of buildings. This has been realized in an international research project—heat, air, and moisture transport in insulated envelopes (HAMTIE), IEA ECBCS 24—where one subtask was devoted to describe the indoor and outdoor climatic influence on the envelope (IEA 1996a). Another subtask of the same project dealt with calculation methods to predict the moisture conditions within the envelope structures (IEA 1996b). The calculation methods all assumed the indoor climate to be prescribed as one of the preconditions for the calculations, disregarding the influence that building components have on the indoor humidity. It is only recently that models have been developed that make detailed and integrated prediction of both the indoor humidity and the moisture conditions in constructions. Hagentoft (1996) presented an unpretentious model that calculates the moisture conditions both in building constructions and in an indoor zone. Kurnitski and Vuolle (2000) have constructed a transient model for heat, air, and moisture conditions in building envelopes in a modular simulation environment where room moisture and temperature balances are also calculated. A recent development has been made to complement the DOE EnergyPlus program (Crawley et al. 2000) with whole building moisture calculations. However, EnergyPlus is based on a response factor method, which may not have been optimal for calculating transient moisture transfer, as this is a nonlinear phenomenon (Liesen and Pedersen 1999). Finally, a model for calculation of indoor air conditions has been developed from an existing tool for moisture analysis of building envelope parts (Simonson and Salonvaara 2000). Moisture conditions cannot be predicted without knowing the thermal conditions. It is quite obvious, therefore, to develop a model for prediction of whole building moisture conditions as an extension to an existing model for a detailed thermal analysis of buildings. Such a model already predicts the thermal condition of the indoor environment and all the adjacent building components. Normally, the thermal calculation models are rather elaborate themselves, their thermal predictions have already been validated, and they may have extensive user interfaces. One such model is included in an existing tool for integrated building simulation (Wittchen et al. 2000). There is a great need to experimentally verify the predictions of whole building hygrothermal models. Some relevant experimental investigations are reviewed by Mitamura et al. (2001) and by Virtanen et al. (2000). In this paper, we describe an experiment using a full-scale test cell to investigate the moisture buffering effect of building materials. The results are used both to characterize the buffering capacity of different materials and to verify predictions of the numerical model. The term “moisture buffering effect” will be used to indicate the ability of building materials to decrease humidity variations in indoor spaces.