Since some ten years ago many rendered facades of residential buildings in Israel have developed patterned stains. Stains develop where the walls are composed of infill blocks, or otherwise insulated substrates, whereas the parts, which are composed of concrete (columns, beams and edges of floors), remain entirely clean. Samples taken from infected buildings have shown that the stains stem from the selective growth of some seven morphological groups of fungi, as well as one or two sorts of algae.

Rendered surfaces of walls (with 24 combinations of various construction features, renderings, and surface coverings), exposed to laboratory conditions of 25oC & 75%RH and with the other side at 100% humidity, were inoculated with cultivated suspensions created from the fungi and algae taken from the infected walls. No growth occurred under these conditions, neither when the walls were exposed in an exterior exposure site. Measurements of the drying process of the walls have indicated that the substrate below the rendering did not dry out throughout the entire period. Hygrothermal calculations have shown that under the laboratory controlled exposure conditions the surface itself dries out very fast, whereas the substrate wall may remain wet for a prolonged period. However, no significant effect of substrate's wall structure on the rendering's moisture content evolution could be observed. Many of the same surface covering materials proved to be prone to fungi growth when inoculated and exposed to optimal growth conditions of 25oC & 95%RH.

Simultaneous heat & mass transfer analysis of the same walls, when exposed to typical winter and summer temperatures inside dwellings, and the more realistic dynamic conditions that occur outside (addressing a typical meteorological year with the representative daily temperature, humidity, cloudiness, solar radiation, and sky temperature), have shown that significantly different moisture content evolutions occur in the same renderings due to different substrate walls. As significantly smaller moisture contents were obtained in the renderings above a concrete substrate (compared to various thermally insulated walls), these results could explain the patterned staining.

• fungi: molds in buildings

1. A model for moisture-content evolution in porous building elements: treatment of irregular regions
2. Air leakage of curtain walls -- diagnostics and remediation
3. Research and development needs for better implementation of the performance concept in building