AIR-WATER VAPOR PROPERTIES
In order to solve excess moisture problems, you must first understand the properties of air-water vapor mixtures.
Air is a mixture of invisible gases–dry air and water vapor. Each exerts a separate pressure. The water vapor pressure controls moisture movement through walls, windows or ceilings of homes. The dry air pressure and water vapor pressure together will determine air movement through leaks and openings in the home. A higher pressure will always seek a lower pressure–somewhat like air escaping from a balloon.
The vapor pressure increases as the air’s moisture content increases. The amount of moisture that a given volume of air can hold depends on the temperature of that air; the warmer the air, the more moisture it can hold.
Air’s moisture content is normally given in terms of relative humidity. Relative humidity is the amount of moisture it could hold at that temperature. Therefore, 50 percent relative humidity means the air is holding half the amount of water it could hold at that temperature. This method of indicating moisture content in the air is misleading, because it doesn’t give a true picture of the actual amount of water vapor in the air. As an example, air at 35 degrees and 75 percent relative humidity contains about half as much moisture as air at 85 degrees and 25 percent relative humidity.
As a given quantity of air is cooled, the relative humidity of this air increases. If the air is cooled sufficiently, it will reach 100 percent relative humidity. The air is then said to be saturated. The temperature at which saturation is reached is known as the dew point temperature.
Condensation will begin to appear at this temperature. This is why air coming in contact with a colder surface, such as a glass of ice water or cold window, will deposit water droplets on this surface.