I have seen what appears to be mold growing on the surface of some painted concrete masonry in our basement, especially in low-light areas. Is concrete masonry such as this particularly susceptible to mold growth?

Concrete masonry itself is inorganic and does not support mold growth. Although it is possible for mold to grow on a paint film, it is more likely feeding on dust particles trapped in the film. The surface of the paint can attract dust from the air, especially when the surface is wet from condensation. The dust particles contain considerable organic materials. Similarly, mold can grow on unpainted masonry surfaces that remain moist for long periods of time and attract dust from the air.

Mold growth on the surface of painted masonry is probably not related to moisture within the wall, but rather from moisture on the surface of the paint. Moisture can condense on cool masonry surfaces in humid spaces, such as basements. Using paints with mold inhibitors does not prevent this problem from occurring because they don’t stop mold from growing on the dust.

Eliminating the sources of moisture and dehumidifying the air within the basement are probably the most effective ways to prevent condensation and subsequent mold growth from occurring.