Within months, pale grey bands are appearing on the walls. As early as 1958, staining and discolouration of painted drywall surfaces along joint lines was shown to occur in the presence of atmospheric hydrogen sulfide, a common air contaminant 1 . Heavy metals in the joint compound were suspected. From 1987 to 1989, several houses in Alberta and British Columbia were found to show this phenomenon, and the problem was investigated here in Canada by both the Alberta and National Research Councils. The conclusions drawn from these surveys led the Canadian Home Builders' Association and the Canada Mortgage and Housing Corporation to commission, in 1990, a study of more technical scope to be performed by Institute for Research in Construction.
Extensive research culminated in a report, a summary of which is presented here.
Identifying the culprit A number of homes in Calgary and Edmonton were studied. In all cases, latex paint had been used. In order to determine the cause of staining, sections of drywall were removed to the laboratory. Physical, biological, and chemical testing was undertaken. No physical changes between stained and unstained materials were observed, nor was any organism shown to cause the problem, although mildew was initially suspected.