Managing Window Condensation

By Hannah McKenzie | February 21, 2013

Q: How can I address condensation on my windows? Are energy-efficient windows a good idea?

A: In North Carolina and most of the U.S., you can expect window replacements to pay for themselves with energy savings in 40 to 80 years. Replacing the windows might improve your comfort level by making your home look better and may also help with condensation and saving some energy. However, replacing your windows is unlikely to save enough energy to quickly repay the costs, unless your current windows are extremely leaky (i.e., missing a bunch of glass) or if energy prices escalate. On the other hand, if you are replacing your windows anyway, it is worth paying a little extra for the energy-efficient ones. The energy savings will in fact pay for the difference between standard windows and energy-efficient windows very quickly.

You can probably address condensation issues without replacing the windows, though. Warm, moist air coming into contact with a cold surface causes moisture to condense on windows. During the winter, if there is moisture in the air and it hits the cold window, the water condenses and runs down the glass. If the window surface is warmer, it is less likely the moisture will condense on it.

To fix this issue, you’ll first need to figure out where the moisture is coming from. In most homes, it comes from cooking, bathing and doing laundry. Otherwise, it can result from a damp or wet crawl space or basement, a leaky roof or plumbing leaks. It can also come from homes with unvented combustion devices, such as heaters, fireplaces, gas ranges and ovens.

Before replacing the windows due to condensation, check the attic for signs of a roof or plumbing leak, and check the ductwork attached to the bathroom and kitchen exhaust fans, if you have them. Are the ducts still connected? Do they go all the way to the outside, through the skin of the building? Can they still move air?

Next, go into the crawl space or basement to look for leaks and signs of moisture. If there is a lot of moisture, chances are some of it is getting into the house and condensing on cold surfaces like windows.

Now, go into the bathroom and turn on the exhaust fan. Check the fan by putting several squares of toilet paper against the grill. If they stay there, you know the fan is moving some air through it. If the paper just falls, then the fan is not moving air and you need to figure out why. Bathroom exhaust fans that don’t work well (or aren’t used) are a common source of moisture in houses. Often, replacing an old, noisy, inefficient bath fan with a new, quiet, energy-efficient model will solve the condensation problem if the fan is used.

Similarly, if you do a lot of cooking, and particularly if you have a gas range, an exhaust fan or range hood in the kitchen ducted to the outside can help control moisture. A so-called “recirculating” range hood does not remove moisture from the air. It needs to be ducted through the skin of the building, all the way to the outside.

Finally, be sure to check for so-called “unvented” heaters or fireplaces. These are used more in severe winters and could account for excess moisture. The typical “ventless” heater or fireplace puts about a gallon of water into the air approximately every three hours. This water often ends up on cold windows, causing condensation and puddles of water on sills. It can also condense on cold walls, causing mold growth. “Ventless” heaters or fireplaces may also give off carbon monoxide and other harmful gases, so they should not be used in homes.