What is Condensation?

Condensation occurs when air is cooled to temperature at which it is 100% saturated.

Simply, air has a given capacity to hold water vapor.  Warmer air has a greater capacity to hold water vapor than cooler air.  Consequently, the total amount of water vapor that the air can hold is primarily a function of its temperature.  As air is cooled, its capacity to hold water vapor is diminished.  When cooled to a temperature at which it can no longer hold the water vapor it contains, the air is said to be saturated and “condensation” (liquid water) will become visible on surfaces.   

What is Relative Humidity?

Relative humidity is a ratio of how much moisture vapor is in the air, compared to the air’s total moisture vapor capacity at a given temperature.  It is a measure of the moisture loading in the air. 

If the relative humidity is 50% at 73ºF, the air contains 50% of its water vapor capacity at that temperature.  If this same air is then cooled, the relative humidity of the air will increase.  This is because the air has less total capacity to hold that same quantity of water vapor when its temperature is reduced.  When the air is cooled to a temperature at which its relative humidity increases to 100%, it is saturated and condensation will form.        

What is Vapor Pressure?

·        Vapor pressure is the driving force that moves water vapor through a vapor retarder. 

·        The higher the temperature and humidity of the air… the greater the vapor pressure. 

·        High vapor pressure exists on the warm side of the vapor retarder while low vapor pressure exists on the cool side. 

·        Moisture vapor moves from areas of high vapor pressure to areas of low vapor pressure.

How are Dew Point Temperature, Condensation and Relative Humidity Related?

The dew point temperature is the temperature at which air cools to its saturation point and condensation begins to form.   Dew point temperatures can be determined, in Table 1, if the temperature and relative humidity of the air are known.