What Exactly Does 'Relative Humidity' Mean?

The term ‘relative humidity’ refers to how full of water the air is in relation to the maximum amount of water it can hold at a given temperature.  On a hot summer day, the air may be 84 degrees with 75% relative humidity entering into your crawl space vents.  Your crawl space may be 66 degrees, but the surface temperature of the walls and dirt floor is 62 degrees.  So why is this a problem?

For every one degree the air is cooled, the relative humidity goes up by 2.2%.  This is because cool air holds less water than warm air.  So looking at the example above, the difference between the outside air (84 degrees) and the air in the crawl space (62 degrees) is 22 degrees.  Multiply 22 degrees by 2.2% relative humidity, and the result is a 48.4% increase in the relative humidity in the air in your crawl space.

The 84-degree air started out with a 75% relative humidity, meaning it was 75% full of water.  Because it was cooled to 62 degrees, that 48.4% must be added to the relative humidity.  This means that air now has a 123.4% relative humidity.  The air cannot have a relative humidity of over 100%, so it starts releasing the moisture onto anything and everything it can—including the organic materials in your crawl space, resulting in rot and mold.