An older masonry building near Chicago developed mold and mildew problems when gypsum wallboard and furring strips were placed on the interior. The interior surface of the gypsum wallboard drywall was covered with vinyl wall covering. In time, pink stains formed on the vinyl wall covering. When we peeled back the wall covering, we found that the gypsum wallboard was covered with mold and mildew. I have seen similar problems in buildings in the southeastern United States. This building, however, is in a northern climate. Why is this occurring?

The mold and mildew are growing behind the vinyl wall covering because the space between the vinyl and the gypsum wallboard is holding moisture. The paste for the vinyl, along with the paper on the gypsum wallboard, feeds the mold and mildew. The moisture's source must be inside the masonry wall, not in the room, because the vinyl wall covering functions as a vapor retarding membrane. It is likely that this problem is occurring during the summer months when air conditioning is cooling the interior.

The exterior masonry walls must be holding considerable moisture. When sunlight heats the masonry, air within the wall system will be especially hot and humid. This hot, moist air distributes moisture throughout the furring space between the gypsum wallboard and the masonry's back face. The moisture will condense on the exterior face of the vinyl wall covering. Many older masonry walls were built as multiple-wythe barrier walls. Water that penetrated the outer wythe often found its way into the inner masonry wythes and was absorbed. Over time, this water dissipates to the exterior and the interior. In the past, these walls were often covered with portland cement plaster and paint. Many paints allow the water vapor to readily pass through into the interior. Even if moisture collected in the wall, the portland cement plaster did not provide a food source for mold and mildew. In these cases, water can remain in these walls for a long time, causing relatively minor problems, such as peeling paint.

Covering the inside face of these walls with vinyl wall covering significantly changes how they perform. The vinyl wall covering does not allow water to evaporate to the interior, and the gypsum wallboard and paste provide a food source for mold and mildew.

I typically would not recommend putting vinyl wall covering on the inside face of older masonry walls. Even with complete tuckpointing of the wall's exterior face, it may not be possible to reduce the amount of moisture penetrating the masonry walls to a level sufficient to prevent mold and mildew problems. There are special types of wall covering that are as durable and easy to clean as vinyl, which have a fairly high water-vapor permeance and, so, allow water vapor to pass through. If a wall covering is necessary, I recommend using one with a perm rating greater than 5.