Publication date: 01/05/2008

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Condensation starts when cold air meets warm

As soon as we get the first long cold snap I get questions about wet walls and windows.

Furnaces, clothes dryers and ventilation fans all expel air from homes. This air has to be made up to restore equilibrium, or the house becomes "negative."

Many homes today suffer from negative air. This wasn't a problem 20 years ago because homes were more loosely built. Newer homes have house wrap and better wall insulation. This makes the homes tighter and more energy efficient, but cuts down on air infiltration. The same effect happens in older homes with added insulation or new energy efficient windows.

Since nature cannot stand a vacuum, the air needed to reach equilibrium is pulled in through every available opening. The most common openings are fireplace chimneys, and gaps around walls, doors and windows. During the winter, incoming drafts chill walls and windows.

Air comes in all the time, but we don't notice it during the day because we are active, and warm air from the furnace is circulating around the house. Two places that have little air circulation are closets and closed rooms. Closets seldom have warm-air ducts, and second-story bedrooms or rooms farthest from the furnace often receive little or no warm air.

If the closet or closed room has an outer wall, the temperature of the air next to that wall becomes cold. Cooling air loses its ability to hold water, and condensation forms on the wall.

The longer the air remains still, the lower the temperature becomes and the more moisture collects on the wall. The smaller the room is, the more dramatic the temperature loss and the greater the probability of condensation. Sometimes clothing actually gets damp.

Condensation around windows is even more dramatic. The R-factor (insulation value) of windows is much lower than walls. This means that at any given time during the winter, windows are colder than walls. The air temperature next to windows is the coldest in the room.

Night is usually the coldest part of any 24-hour period. Conscientious homeowners often lower the thermostat and pull curtains or blinds before going to bed.

While this is energy efficient, air circulation is decreased because the furnace blower motor is not working as much. Pulling the blinds and curtains traps enclosed air and the temperature next to the window drops precipitously, causing condensation.

As cold air loses volume, warm, moist air is pulled into the window to rapid chill the area. The cycle continues all night. As more and more warm, moist air loses temperature, it releases moisture content and window condensation increases.

The next morning when blinds are raised and curtains pulled open, the homeowner finds standing water on the windowsills.

Many then call or write me about shoddy windows. Remember, windows don't make water.

One of the easiest ways to combat the problem is to make sure your house does not have negative air so that cold air is not sucked into the house. To do this, install an air makeup unit such as the Xavier EQUALIZ-AIR, (734) 462-1033, www.equaliz-air.com, or the Skuttle Model 216, (888) 758-8853, www.skuttle.com. A more sophisticated technique is to have a heating contractor install a Heat Recovery Ventilator (HRV) that uses the heat and humidity from outgoing air to heat and humidify incoming air.

A second line of attack is to make the inner edge of walls and windows warmer. To do this, increase insulation and reduce air infiltration with cellulose or foam insulation. Caulk and seal all wall penetrations.

Single pane windows are the coldest. Double and triple glazing increases the R-value and decreases the U-value of the glass, making the surface of the glass coming in contact with room air warmer. This decreases condensation.

The best way to improve single-pane glazing is to add another layer of glass by installing interior storm windows by companies such as Innerglass, (800) 743-6207, www.stormwindows.com.

The more efficient the glazing, the less need to trap air by pulling blinds and drawing curtains.

If you have a two-stage furnace, you can keep the blower motor running at all times, increasing air circulation. Keep doors to problem closets and rooms open or install fans to eliminate stratified air.

Finally, keep interior humidity in line with outside air temperature.

Buy a hygrometer and track inside and outside temperatures and relative humidity. Jimmy Kogut, the inventor of the Equaliz-Air, says that if your home is 72-degrees Fahrenheit, the inside humidity should never be over outside air temperature plus 25. So if it is 15 degrees outside, inside humidity can't be more than 40 percent or you will have condensation.

With a little effort you can win the battle against wet walls and windows.

Note: This article was accurate at the date of publication. However, information contained in it may have changed. If you plan to use the information contained herein for any purpose, verification of its continued accuracy is your responsibility.

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