Issue link: https://hi.iaq.net/i/630729
144 Appendix C Either surface temperature or vapor pressure can be the dominant factor in causing a mold problem. A surface temperature-related mold problem may not respond very well to increasing ventilation, whereas a vapor pressure-related mold problem may not respond well to increas- ing temperatures. Understanding which factor dominates will help in selecting an effective control strategy. Consider an old, leaky, poorly insulated building. It is in a heating climate and shows evidence of mold and mildew. Since the building is leaky, its high natural air exchange rate dilutes interior airborne moisture levels, maintaining a low absolute humidity during the heating season. Providing mechanical ventilation in this building in an attempt to control interior mold and mildew probably will not be effective in this case. Increasing surface temperatures by insulating the exterior walls, and thereby reducing relative humidities next to the wall surfaces, would be a better strategy to control mold and mildew. Reduction of surface temperature- dominated mold and mildew is best accomplished by increasing the surface temperature through either or both of the following approaches: ■ Increase the temperature of the air near room surfaces either by raising the thermostat setting or by improving air circulation so that supply air is more effective at heating the room surface. ■ Decrease the heat loss from room surfaces either by adding insulation or by closing cracks in the exterior wall to prevent wind-washing (air that enters a wall at one exterior location and exits another exterior location without penetrating into the building). Vapor pressure-dominated mold and mildew can be reduced by one or more of the following strategies: Above: In this building, mold and mildew spots appeared on drywall joints on the interior walls. When the wall was cut open, mold growth was visible in the wall cavity and the structural steel showed corrosion. The problem was caused by construction moisture trapped between the interior finish and the exterior sheathing. The solution was to modify the exterior wall so that moisture could vent to the outdoors. Below: This is visual evidence of air movement through the building shell. The water vapor in the warm, humid indoor air has condensed and frozen on the exterior wall.