Humidity is often the most common term used when evaluating moisture issues. Relative humidity is defined as the ratio of the actual amount of water vapor present in a volume of air at a given temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.
While relative humidity is an important measure, it can sometimes be misleading. The biggest problem with measuring humidity is that it is relative humidity (rH). Most people forget about the fact that it is relative to the temperature. The warmer the air, the more water vapor the air can hold.
So, as an example, if all other things are equal, which one do you think has more water vapor in the air?
- Air Mass A: Air with 45% relative humidity at a temperature of 95 degrees F.
- Air Mass B: Air with 90% relative humidity at a temperature of 70 degrees F.
The answer is Air Mass A. Even though the relative humidity is only 45%, the air is so much warmer that it is holding more water vapor. This is not obvious but very important in understanding proper ventilation.
This particular example applies well to crawlspace ventilation. In many summer climates, most crawlspaces remain cooler and around 70 degrees F while the outside air can be around 90 degrees F. This is where many people mistakenly think that the 45% rH air from Air Mass A would be “better” than the 90% rH from Air Mass B because the relative humidity is so much lower. However, remember that in this scenario, Air Mass B actually has drier air. It is not beneficial to ventilate with the outside air from Air Mass A as it would actually be bringing in wetter air from the outside. The hotter air with more moisture will cool off when it is in the crawlspace causing the relative humidity to increase higher than before.
Because Relative Humidity is not the sole measure that should be used for proper ventilation, the ATMOX Systems use Dew Point. Please read webpage on Dew Point to understand this critical difference.