Effect of the air and vapor permeance of exterior insulation on the flow of moisture in wood frame walls in a cold climate

During winter and spring 2008, National Research Council Canada researchers in partnership with Natural Resources Canada and Canada Mortgage and Housing Corporation compared the hygrothermal performance of three RSI 3.5 (R20) 38 mm x140 mm (2x6) wood frame wall assemblies in a field exposure facility in Ottawa, Canada. Two of the walls were retrofitted with RSI 1.76 (R10) exterior insulation: one with mineral fibre, and the second with extruded polystyrene (XPS). During this first year of the field study, the potential for moisture accumulation behind the XPS insulation at the surface of the wood-based sheathing board was observed. To further study this phenomenon, the wood frame wall without retrofit and the wall with XPS insulation were left in place and monitored for a second year. This paper discusses the results from the two years of the field trial, with particular attention on the flow of moisture across the wall assemblies, and the resulting humidity of air at the exterior surface of the sheathing board.
ICBEST 2010: International Conference on Building Envelope Systems and Technologies: 27 June 2010, Vancouver, Canada