This past month, I have received two phone calls about the same issue, moisture buildup on the glass of exterior windows. A buildup of moisture is a common occurrence during winter months in colder climates. In many cases, a small amount of moisture is not harmful. However, in some cases the moisture can lead to window damage or even the growth of mold. In order to address this issue, we first have to understand the cause.
When examining the thermal performance of a window, we find that the greatest rate of heat transfer occurs around the perimeter of the widow where the glass meets the sill. As a result, the glass surface temperature in these areas is closest to the temperature outside. Moisture builds up in these areas because the surface of the glass reaches a temperature at or below the dew point temperature in the space.
DEHUMIDIFICATION WITH VENTILATION
In order to prevent this buildup of moisture, there are two practical solutions. The first option is to improve the thermal performance with a window replacement. New high efficiency windows can slow the rate of heat transfer and raise the surface temperature of the glass. This can be an expensive option and may not make sense in most cases. The second option is to lower the dew point in the space through dehumidification.
Dehumidification is the process in which water vapor is extracted out of the air. This can be accomplished actively with the use of a mechanical dehumidifier or passively by introducing dry air into a humid environment. When I was asked how to address the issue of moisture building up on the glass of exterior windows, in both cases I recommended that we dehumidify the space passively with ventilation (outside air).
In Kalispell, Montana, the average outdoor humidity for the month of January is around 80% RH. In order to maximize indoor air quality, ASHRAE recommends that indoor space humidity levels should be kept no greater than 65% RH. Now the question becomes, how do we dehumidify a 65% RH space with 80% RH outside air? This can be answered with a simple experiment.
When I conducted this experiment on October 10th, the outside air temperature was 44.3 deg F with a relative humidity of 65.5%. These readings were taken using a basic, inexpensive temperature/humidity meter. The meter was placed outside until the reading levels stabilized. Once stable, the meter was placed in a one gallon zip lock bag along with excess air around the meter. The ziplock bag was tightly sealed then transported in indoors where it rest for 15 minutes. At that time, the readings had shifted to a temperature of 69.0 deg F and a relative humidity of 44.2%.
One would anticipate that the temperature of the air trapped in the ziplock bag would increase after some time indoors. As for humidity, the shift is a mystery without proper explanation. The heat that was added to ziplock bag was completely sensible as opposed to a mix of sensible and latent. That is, indoor air did not mix with trapped outdoor air in the bag, there was no moisture added or taken from the bag. Any time sensible heat gain is realized, there is decreased relative humidity. This can be further explained with the use of a psychrometric chart.
Now let’s reexamine the window moisture problem. If we introduce enough outside air to a space, the mixing in time will dehumidify the air. Through dehumidification, we have now lowered the dew point of the space. And by lowering the dew point, we have now prevented moisture from building up on the window’s cold glass.