Crawl space Ventilator

In order to understand crawl space ventilation, we must go back to freshman year earth science class.

Relative humidity means how full of water the air is relative to the maximum amount of water it can hold at a given temperature. Look at what happens on a hot summer day. 84 degree air with 75% relative humidity entering crawl space vents. The crawl space is 66 degrees but the surface temperature of walls, dirt floor and floor joists is 62 degrees. When the current 84 degree air enters the cool crawl space and begin the drop in temperature, it can no longer hold the same amount of moisture. That moisture will begin to condense on the first surface it touches or fall to the ground creating puddles of water in the crawl space.

For every one degree we cool the air, the relative humidity goes up by 2.2% because cool air holds less water than warm air. So looking at our summertime situation, the difference between the outside air we let in at 84 degrees, and the crawl space at 62 degrees, is 22 degrees. 22 degrees multiplied by 2.2% is a 48.4% increase in relative humidity. Our 84 degree air started out with 75% relative humidity; in other words at 84 degrees it was 75 % full of water. We cooled it to 62 degrees so we have to add 48.4% to the relative humidity. So that's 123.4% relative humidity. But wait a minute; we can't have over 100% relative humidity. Why not? Because at 100% the air can not hold any more water and must give up its moisture.