I have received several emails from readers wondering why their windows “sweat” during the heating season here in Florida. Like most building science questions there are the quick answers but then you must probe deeper to find the causes of the problem. The quick answer for windows sweating, condensation is the technical word, is warm air is able to hold more humidity than a cold window (relative humidity is the moisture saturation in the air). As the warm air meets the colder temperatures near and on the window, the colder air and window surface cannot hold the same amount of moisture. When the temperature gets low enough the air is at 100% humidity the water vapor turns to liquid. This is called the “Dew Point.”
Relative humidity is called relative because it changes with the temperature of the air mass. For example, 90° and 67% relative humidity will have the same amount of moisture as 110° and 33% relative humidity. The warmer air has a greater capacity to hold moisture. Therefore the percentage of saturation is less.
Often you will hear the weatherman refer to “dew point.” Dew point is the temperature when the air mass is fully saturated; relative humidity is 100% causing the water vapor to become a liquid. Studies have been done and charts made that show the dew point for various combinations of temperature and relative humidity. They are called psychometric charts if you’d like to read more. So, at 72° with relative humidity at 50% inside your home the dew point is 52.4°. Say that is the situation in your home and someone gets a shower and doesn’t use the exhaust ventilation and the home is now 72° with 55% humidity. Now your dew point is 55°! So if your window surface temperature is 55° or below, you will have condensation. If you have a home temperature of 70° with 40% relative humidity your dew point is 44 degrees. That means if your window surface temperature is 44* or less, your windows will sweat.
Here are some things to improve the situation. An extreme measure would be to replace the windows with an insulated glass and vinyl frame. The inner pane rarely will get cold enough to allow condensation but it still can happen in extreme conditions. Our single pane, metal windows will have more condensation issues that cannot be remedied short of replacing them. Using your kitchen and bathroom ventilation fans will help keep the relative humidity lower. Raising your temperature a couple of degrees will also help, because the warmer air can hold more moisture it lowers the percentage of humidity in the air thus lowering the dew point. A dehumidifier may help keep your relative humidity in the 40% -50% range, which is optimal all year round.