Back to basics: the causes of condensation on fenestration
A conversation about condensation on fenestration cannot occur without an understanding of the concepts of dew point and humidity.
Dew Point
Air retains different amounts of water vapor depending on its temperature because of changing evaporation rates. As warm water vapor-laden air is cooled, the amount of water vapor it can retain is reduced. When temperature is where the air can no longer retain the original amount of water vapor, it has reached a saturation point. Water vapor is then removed from the air through condensation, forming liquid water droplets (dew). The temperature at which this condensation first forms is called the "dew point." This is the temperature at which air, with a given moisture content, must be reduced to reach its saturation point. The relative humidity of the air is 100% at this point.
The dew point temperature measures how much water vapor the air holds at any given time. The higher the dew point, the more water vapor is being held in the air. This means that the air does not have to be cooled as much for saturation and condensation to occur. The less water held, the lower the air temperature (dew point) at which condensation (dew) occurs.
Relative Humidity
Relative humidity, as the term suggests, is a relative term. It is dependent not just on water vapor content but also on the temperature of the air. It measures the amount of water vapor in the air relative to the amount needed to achieve saturation at a specific temperature. It is calculated as a percentage. The closer it is to 100%, the closer the air is to saturation. When the air and dew point temperatures are close, the relative humidity will be high because air will be close to its saturation point. The relative humidity will be low when the air temperature is much higher than its dew point temperature.
Implications for Windows
With fenestration, cold room-side surfaces will tend to cool the air nearby. Condensation will occur on fenestration’s interior surfaces when surface temperatures dip below the dew point of the room’s air.
Surface temperature of the fenestration system is not uniform. Colder areas occur where there is direct thermal bridging between interior and exterior elements (see figure above). These areas typically are at the frame and edge of glass, where less attention is typically paid to improving thermal performance. These areas have a higher risk for condensation.
Choosing frames with wide thermal barriers and insulating glass with a warm-edge spacer are two strategies that can reduce thermal bridging, maintain higher interior surface temperatures and reduce condensation risk.