Modelling of indoor air humidity transient behaviour: Effect on exterior wall moisture performance

Indoor air humidity is an influential parameter in determining the quality of the indoor environment, building energy consumption as well as the performance of the exterior building envelope. The main objectives of this study are to predict indoor humidity transient behaviour within buildings and to investigate its effect on exterior wall moisture performance. To achieve these objectives, the study incorporates four main developments. First, evaluation of moisture absorption and desorption by interior materials. Second, prediction of air humidity behaviour in single-zone and multi-zone enclosures. Third, theoretical description of multi-layer walls moisture response under transient boundary conditions, and their interaction with the indoor environment. Fourth, development of simulation tools and case studies for theoretical evaluation and analysis. Moisture absorption and desorption processes within buildings can have considerable impact on air humidity. A theoretical model for evaluating moisture absorption and desorption by interior materials is proposed. The model has been incorporated into single-zone and multi-zone indoor humidity evaluation models. Based on these models, computer programs were developed to theoretically study indoor humidity response to the various moisture transport processes in different enclosures. The study has revealed the importance of the involved processes and the wide variation of their influence in determining indoor humidity behaviour. In addition, results have indicated the uniqueness of individual zones air humidity behaviour and its dependence on their relative location along the building air flow path.; Wall moisture response has been modelled by a system of governing equations describing the transient simultaneous heat and moisture transfer through the wall system components. The governing equations were numerically formulated using the implicit finite-difference scheme, and the resulting heat and mass balance equations were simultaneously solved. Based on these mathematical developments, a computer model was developed and used in conjunction with the indoor air humidity prediction model to evaluate the impact of indoor humidity behaviour on wall moisture performance. Moisture behaviour of non-cavity and cavity wall systems has been theoretically evaluated under different indoor air humidity variational patterns. Results indicate that modifications in indoor humidity behaviour imposed by the air leakage and the indoor moisture generation processes were found to be the most influential. The impact of the other moisture transport processes was found to be relatively less appreciable. The indirect relationships between the seasonal moisture accumulation and the space physical characteristics were studied through a parametric evaluation approach. The relationship between the level of moisture accumulation in the wall components and the air leakage coefficient of the exterior walls was found to be a negative decaying relationship, while it is a positive decaying relationship with the indoor moisture generation rate. Depending on the type of wall and its interaction with the indoor environment, moisture behaviour of the exterior wall can be greatly altered in response to changes in indoor air humidity. Hence, its transient behaviour has to be considered for more accurate and realistic evaluation of wall moisture performance. In addition, modifying indoor humidity behaviour can be seen as a potential measure for improving short term as well as long term moisture performance of exterior walls.