Buildings

Window Condensation

Q: A number of times this winter I noticed the windows in my home would sweat, causing water puddles to form on the window sills. This has happened in the past, but it has been much worse this year. A friend recommended a window expert, Sam, who explained the problem is our old, single pane windows get so cold that water condenses on them. He said the solution is to replace the windows with new, energy-efficient ones that don't get as cold. According to Sam, not only will they solve the condensation problem, but they'll pay for themselves in no time with the energy savings. How long will it really take to get that money back in energy savings?

A: In North Carolina and in 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. It may also help the condensation problem as well as save some energy. But replacing your windows is unlikely to save enough energy to quickly repay the window costs unless your current windows are extremely leaky (e.g. missing a bunch of glass) or if energy prices escalate. On the other hand, if you are replacing your windows for other reasons, it's always worth it to pay 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.

The good news is you can probably solve the condensation problem without replacing the windows. Sam is correct about the condensation. Warm, moist air is coming into contact with a cold surface and is causing moisture to condense on the window. It's just like having a cold can of Cheerwine on a hot summer day. When warm, moist air hits the cold can, the moisture in the air condenses and runs down the can. In your home 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. In other homes, moisture can be a result of 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.

So before thinking about replacing the windows, go into the attic and look for signs of a roof or plumbing leak. Also, check the duct work 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? Was the exhaust fan duct crushed by the cable guy, or can it still move air?

Next, go into the crawl space or basement and again, 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, at least you know the fan is moving some air through it. If the paper just falls down with the fan on, then it's not moving air and you need to figure out why. In my experience, bathroom exhaust fans that don't work well are probably the second most common source of moisture in houses. The first most common source includes homes that don't have an exhaust fan or that have one but no one uses it. The exhaust fan should run during and 20 minutes after a shower.

Many homes were built without bathroom exhaust fans, which building code allows as long as there is a window that opens in the bathroom (as if any of us are going to throw open the window as we step out of the shower when it's 22 degrees outside).

Very 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 the moisture. A so-called "recirculating" range hood does nothing to remove moisture from the air. Again, it needs to be ducted through the skin of the building, all the way to the outside. A good range hood that is fairly quiet is much more likely to be used than a cheap noisy one. Spending a little more money on a better investment calls for more return in the end.

Finally, be sure to check for so-called "unvented" heaters or fireplaces. These are used a lot more in a severe winter like this one and could account for having a lot more moisture in the house than normal. The typical "ventless" heater or fireplace puts about a gallon of water into the air every three hours or so. This water often winds 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 gasses, so they should never be used in homes.