Modelling of air barrier characteristics in predicting the moisture performance of building envelopes.
Abdul-nabi, L.
1996 M.A.Sc., Concordia University
Abdul-nabi, L., (1996), "Modelling of air barrier characteristics in predicting the moisture performance of building envelopes.", M.A.Sc., Concordia University.
Interstitial condensation is recognized to be the cause of many problems in building envelopes. In multi-layered constructions, the emphasis often has been placed on vapour barrier requirements based on moisture diffusion. Several recent studies have shown that diffusion is only a minor factor in interstitial condensation and that airflow is much more significant in driving moisture through the envelope. For designers, there are no simple methodologies available yet to consider the air leakage phenomenon in the design of envelope components. When airflow is incorporated in simple moisture prediction models, its effect is calculated separately and then added to moisture diffusion effect. This method can lead to important overestimation in results and constitutes a major drawback of the simple models. Another limitation to their use is that they do not calculate the air flow rate through the analyzed section, therefore the user is required to provide input data like the air velocity and flow direction or equivalent leakage area of the section, each of which is difficult to assess.
The present work develops a simple model that predicts condensation in walls due to the combined effect of vapour transfer by diffusion and convection. The flow rate is calculated by making use of the air permeability properties of the materials and elements making up the different layers of the envelope. The proposed methodology to calculate airflow rate through the envelope has been implemented in a computer model used in analyzing a number of common wall stations on a yearly basis. Some of the widely adopted design guidelines related to moisture control have been also investigated.