The paper discusses investigation of the impact of the building environment on the ability of building materials to support microbial growth, using static chambers with defined relative humidity, temperature, and light conditions. The ability of fungi to grow on materials is well established, given sufficient moisture and nutrition. Four types of ceiling tile were investigated: one aged, two mineral fiber/crystalline silica, and one fire-resistant. Ceiling tile pieces were placed in the chambers, allowed to chamber-equilibrate, and inoculated with either Penicillium glabrum or Aspergillus versicolor. The critical bulk moisture content for organism growth varied depending on both tile type and test mold. P. glabrum counts increased three orders of magnitude in 28 days. A versicolor did not grow as well. Organism related differences are apparent.