The Falcon Group Engineers, Architects and Energy Consultants
A N D R E W A M O R O S I
A side from proper construc- tion, the most essential part of preventing moisture damage to a building from condensation is to keep in door relative humidity at ef- fective levels during the winter season. Hu- midity levels consistently below 30% may cause respiratory problems and shrinking of wood furniture, flooring or trim, readin- gs of 30 or higher appear to prevent these problems. When indoor heat and humidity is excessi- ve during the winter season, condensation conditions begin to appear. It is difficult even with proper wall construction and va- por retarders to construct a building that will not have condensation problems when indoor humidity exceeds 40 percent. In general terms, the interior warm air con- tains moisture that will migrate by diffusion to the lower pressure or colder air (exte- rior). The warmer or more moisture-laden air, the more diffusion that occurs and with that more condensation. When a house is retrofitted with insulation without the benefit of vapor retarders and air leaka- ge control, an even lower humidity may be required. Persistent condensation on double-glazed windows is a good indicator that relative humidity is too high and may cause damage to the exterior finish. Some interior moisture control is possib- le by using exhaust vents in kitchens and bathrooms. These may be manually con- trolled by a conscientious homeowner or automatically controlled by humidistats that turn on the fan when relative humidity exceeds a predetermined level.
A more positive measure is to connect a small duct from the outdoors to the return side of a forced-air heating system, so that fresh air is drawn into the house whenever the system is operating. A damper placed in this duct will allow the homeowner to control incoming air. While ventilation re- sults in energy requirements to heat the incoming air, it is still much more energy efficient than purposely building with un- controlled natural air leakage. With natural leakage, the greatest air exchange oc- curs during the coldest weather when the least air exchange is needed for humidity control because incoming air is very dry. By controlling the ventilation to the actual amount needed the benefits of energy effi- cient construction are preserved. An alternative to direct ventilation is the use of an air-to-air heat exchanger. This type of device removes a large portion of the heat from air being exhausted and transfers it to incoming air from outdoors. However, the initial cost of this equipment has to be compared to savings in opera- tion. Dehumidifiers have also been used in extreme cases, but they generally are not effective in drying the air below about 50 percent relative humidity, and they are major consumers of energy . Protection of Walls The control of moisture during the winter in wall cavities is typically by the use of vapor retarders near the warm (interior) face of the wall, reducing the vapor transfer into the wall. Air leakage into the wall cavity adds more moisture into the wall then dif- fusion of water vapor through the materials of the wall. Moisture laden air can enter the
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