Understanding Vapour Permeance and Condensation in Wall Assemblies

The use of polyethylene vapour barriers is well integrated into building codes and the Canadian construction industry, a result of significant investment in research and training. Their use has resulted in significant improvement in building envelope air tightness when properly detailed. Contractors and inspectors have developed a strong understanding of the details and practices required to achieve tight and reliable enclosures. However, some groups have expressed concerns that polyethylene vapour barriers may reduce drying and entrap moisture. In particular, problems have been encountered with below-grade walls where inward drying of initial construction moisture within the concrete foundation walls is trapped by polyethylene sheets. Problems have also been identified in above-grade walls where absorptive and non-ventilated claddings are employed. A research program is currently underway to determine the significance/insignificance of potential moisture problems due to plastic sheeting in above-grade and below-grade wall assemblies. The research is aimed to outline cases where performance can be improved, and changes that could reduce inappropriate use. Finally, the research aims to address the benefits/risks with polyethylene sheeting with clearer delineation of the situations in which it is necessary, potentially damaging, and unimportant. This paper presents key findings from the research work and field test results collected to date within the following framework: • The literature review; • Field testing of four common below-grade wall assemblies (with and without polyethylene sheeting) in a southern Ontario home; • Field testing of six common above-grade wall assemblies (with and without polyethylene sheeting) in the University of Waterloo test exposure facility (BEGHut); • Comparison of the field testing data and computer models, to provide validation of the model against this installation. • Extending the test results to broader practice across Canada through computer modeling. Future papers will provide more detailed analysis and computer model verifications after the project is complete.