Mold growth on the damp internal surfaces of buildings is estimated to occur in some 2.5 million dwellings in the UK. As moisture is regarded as the key factor in promoting mold growth in buildings, a study was carried out to determine the moisture requirements for growth of molds isolated from domestic dwellings, with the intention of providing building operators and practitioners with information on critical internal humidity levels. Nineteen species of molds which occurred in at least 10% of domestic air samples were assessed for their ability to grow at different levels of water activity (A_w: A_w=RH/100). Certain of these were selected for more detailed studies of their A_w requirements for growth on a range of building substrates including paint and woodchip wallpaper. The lowest A_w level recorded for growth on malt extract agar was 0.76 (76% relative humidity) for <i>Aspergillus repens</i>, while for <i>A. versicolor</i> and several <i>Penicillium</i> species the minimum value was 0.79. On woodchip wallpaper painted with an emulsion paint the A_w minimum for <i>A. versicolor</i> and <i>P. chrysogenum</i> was also found to be 0.79 at 25°C, although at a temperature of 12°C, which was closer to that found for wall surfaces in practice, the A_w minima were elevated to 0.87 for these two molds. In general the study showed that increasing the temperature and the nutritional status of the substrate led to a reduction in the A_w requirements of the molds. In building practice therefore, the aim must be to maintain relative humidity and surface temperature conditions within dwellings at levels such that mold susceptible substrates remain below A_w 0.80 to avoid development of mold growth.