This is a common question asked by many homeowners annually in the winter when the temperature drops. More often than not, people with homes that are less than 20 years old ask the question, or people with homes that are much older with newly installed windows. While the simple answer would be to just blame the windows, it is actually much more complicated than that.
For those of you that are not meteorologists, Wikipedia defines the dew point as “the temperature below which the water vapor in a volume of humid air at a constant barometric pressure will condense into liquid water.” Have you ever had an ice-cold glass of lemonade outside on a warm, humid day? Did you notice how the outside of the glass accumulated moisture? The reason that the water collected on the outside of the glass has everything to do with the dew point.
“So what does this have to do with my windows?” you might ask.
When discussing window condensation with homeowners, I always tell them there are two parts to solving their window moisture problems. One part is my job, and one part is their job.
The first part (my job) is to make sure that the home has windows with a high quality glass package that will keep that glass temperature as high as possible. It is important to set realistic expectations at this point. When the temperature outside is 0° and the inside temperature is 70°, it is unrealistic to expect your inside glass temperature to be 70°. Typically, depending on the quality of the window, the inside glass temperature will be somewhere between 30° and 55°. Obviously, the warmer it is, the better. This is important because, if that glass drops below the dew point, you will see moisture appear on the glass.
The second part (the homeowner’s job) is to control the humidity levels in the home in order to keep the dew point as low as possible. There are many sources of humidity in the home, including humidifiers, heating systems, cooking, showers and baths, plants, pets, and even our bodies. All of these things contribute to the amount of moisture in the air inside our homes. As newer homes have been built tighter with less air leakage, and older homes are being retrofitted with tighter energy efficient products, this moisture cannot escape to the outside like it used to be able to.
So what can a homeowner do?
During the winter months, carefully monitor the humidity levels in the home. The following chart shows where your indoor humidity levels should be based on the outdoor air temperature.
| Outside Temperature (°F) | Inside Relative Humidity (%) |
| -20 | 15-20% |
| -10 | 15-20% |
| 0 | 20-25% |
| 10 | 25-30% |
| 20 | 30-35% |
Another solution that has proven to be very effective is to use a heat recovery ventilator (HRV) to exchange the moist inside air with the dry outside air. These systems work by drawing the warm moist air from the house into the unit through your cold air returns, sending it to the outside of the home through a duct, and then drawing cool dry air from the outside back into the house to replace it. An HRV can be purchased from your local heating and cooling contractor, who will typically do the installation and adjustments required.
In addition to maintaining lower humidity levels, open any heavy curtains and move furniture away from windows that may block air circulation. Also, keep plants out of your bow and bay windows in the winter. Be sure to use an exhaust fan in any bath or shower areas to take the moisture outside of the home, because that moisture doesn’t just stay in the bathroom.
If all of this sounds like a lot of work, or if you like to have a little more humidity in the air than what is listed above, you should expect to see some moisture on your glass. If you have wood windows, you can expect this moisture to seep into the sash frames and damage the finish, rot the wood, or grow mold along the edge of the glass.
However, if you have windows that are made of fiberglass or vinyl, this moisture will typically dry up once the weather outside warms up or the humidity level in the home drops, without causing damage to the finish or function of your window.
To alleviate the issue of window condensation, consider installing Infinity Fiberglass Replacement Windows. Contact Callen to schedule a free consultation.
by Mike Wood, Sales Manager
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