An extensive investigation of moisture problems in wood frame houses revealed that one of the most common problems was the formation of mould and mildew on inside wall surfaces of exterior corners (Van Poorten, 1983). The majority of the problem houses were of recent construction (around 1975), electrically heated yet "naturally ventilated." Clearly, inadequate ventilation was one of the contributing factors.It was not possible to explain this occurrence solely in terms of thermal bridging, two-dimensional heat flow, hindered air circulation or lack of radiant heat gain. It was then suggested that additional cooling of wall surfaces resulted from defects in the sheathing which allowed wind to cool the wall construction in corners where a rapid change in wind pressure occurred. It was also suggested that in future housing, such problems could be largely avoided by providing airtight sheathing at building corners by moving the air barrier, customarily located on the room-side of the wall insulation, to the weather side where it would perform two functions (Timusk, 1983).A full-scale comer of a wood frame wall was built and evaluated for wind cooling effects in the laboratory. According to prediction, significant additional cooling did take place when defects in wall sheathing did exist and when the corner was exposed to a typical wind pressure gradient.The effectiveness of an externally-located air barrier was further demonstrated in a prototype house. Its installation and inspection was facilitated by locating it where it is not penetrated by joists, partition walls and electrical wires.