Q. We have a problem of condensation on the inside of our windows. Last winter we applied expensive thick plastic to overlay the windows inside. They don't fit tight and the condensation is worse. Now some black mold is showing up. We are told the house is too well insulated. Mold never occurred before in 30 years. What is the answer? D.L. Milne Seattle, Wash.
A. Condensation is caused by moist interior air suddenly being cooled below its dew-point temperature. This cooling normally occurs at a highly conductive surface such as single-pane window glass and aluminum or steel frames without thermal breaks to the outside.
Installing a sheet of plastic over a window on the inside creates additional insulation as well as a thermal break which inhibits condensation. However, condensation may still occur on the prime window due to moist air trapped between the prime window and the interior plastic; or if the interior window is not well-sealed, moist room air circulates into the dead air space, causing condensation.
If the prime window is very tight, the moisture cannot excape and condenses on the prime window glass. A loose prime window allows outside air to circulate in the air space. In winter this air has a low moisture level and it dries out the air space.
If the interior window is well sealed, use a desiccant (drying agent) in the air space. Obtain a desiccant such as selica gel, and place it between the two units.
Interior plastic inside storm windows should reduce condensation. However, in some cases such overlays aggravate the condensation problem when installed against a well-sealed prime window. Each installation is different. Thus, remedies may vary.
Reduce interior humidity in practical ways, use dehumidifiers, install and use exhaust fans in bath, kitchen, and laundry. Keep crawl spaces under the house dry. Outside, compel rain water to run away from the building.
Popular energy-saving means and materials have indeed made dwellings so airtight as to encourage increased humidity.
For more specific recommendations, contact a company that sells plastic storm-window overlays. One firm in the field is Plaskolite, Inc., P.O. Box 1497, Columbus, Ohio 43216, telephone (614) 294-3281.
If you have a question on maintaining your house, send it to the real-estate editor, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115. Forrest M. Holly is a longtime California builder now living in Arizona.