The Condensation Process
This occurs when warmer moist air comes in contact with cold surfaces such as framing members, windows and other thermally conductive accessories, or the colder region within the insulation itself (if moisture has penetrated the vapor retarder).
Warm air, having the ability to contain more moisture than cold air, loses that ability when it comes in contact with cooler surfaces or regions. When this happens, excessive moisture in the air is released in the form of condensation. If this moisture collects in the insulation the insulating value is decreased.
In dealing with condensation, air may be considered to be a mixture of two gases-dry air and water vapor. One thousand cubic feet of air at 75°F can hold up to 1.4 pints of water. At 45°F, it can hold only 0.5 pints.
Relative humidity is the percentage measurement of the amount of water vapor present in the air in relation to the amount it is capable of holding at that temperature.
Therefore, 50% relative humidity would mean that the air is carrying only one-half of the total amount of moisture that it could be holding at that particular temperature.
Cold, outside air is usually much drier than warm inside air. Therefore, you can lower the relative humidity by bringing in outside air to mix with and dilute the moist inside air. At 100% Relative Humidity, the air is “saturated.”
The temperature at which the air is saturated and can no longer hold additional moisture is called the dew point temperature. Whenever air temperature drops below its dew point, excess moisture will be released in the form of condensation.
Condensation problems are most likely to occur in climates where temperatures frequently dip-to 35°F or colder over an extended period of time.
Two things must be present for condensation to occur: warm moist air and cool surface temperatures below the dew point. The proper control of these two factors can minimize condensation. In metal buildings, we are concerned with two different areas or locations: visible condensation which occurs on exposed surfaces below dew point temperatures; and concealed condensation which occurs when moisture has passed through the vapor retarder and into interior roof and/or wall cavities and then condenses on a surface below dew point temperature.
To effectively control visible condensation, it is necessary to reduce the cold surface areas where condensation may occur. It is also important to minimize the air moisture content within a building through the use of properly designed ventilating systems.
Concealed condensation is the most difficult to deal with and can be the most damaging to any kind of structure. This type of condensation may be controlled in metal buildings by the proper ventilation. Additional condensation control can be accomplished by venting the cold cavities of the walls and roof.
Proper Ventilation of a Building
Proper Ventilation in a building can be an effective measure for controlling condensation. The end use of the building must be the controlling ventilation design parameter, especially above drop ceiling areas. Ventilation system design should be done at the initial building design stage. Use of a design professional is strongly recommended to assure the best possible system.