The dew point is a temperature. In my article on the psychrometric chart, I defined dew point as “the temperature at which moisture in the air begins to condense on hard surfaces.” Dew point can be measured or calculated for a specific point in time at a particular location, indoors or outdoors. No matter where it’s measured, the dew point can never be higher than the air temperature.
Here are a few other definitions for “dew point”:
- The dew point is the temperature to which air must be cooled to become saturated with water vapor.
- The dew point is the temperature at which the water vapor in a sample of air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates.
- The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%.
Preventing moisture accumulation in sheathing
If you are a designer or builder who cares about the moisture performance of your walls, you’re likely to encounter the term “dew point” in any discussion of wintertime moisture accumulation in wall sheathing. If we know the dew point of a home’s interior air, and we know the temperature of the wall sheathing, we know what will happen if any of that warm interior air contacts the wall sheathing. For example, if the dew point of the interior air is 40°F, and the wall sheathing is at 32°F, we know that the sheathing is cold enough to accumulate moisture when the interior air comes in contact with the cold sheathing.
In this situation, we might say that “the sheathing is below the dew point of the interior air.”