We know that air enters the home through the lower levels, moves upwards through the home, and exits through the upper levels.
What happens if outside air is being pulled into the basement or crawl space through vents or other openings?
Basements and crawl spaces are naturally cooled by the earth around them which keeps a fairly constant temperature year-round. Any air that enters that space from outside will be cooled as well.
Imagine this: It’s an 80 °F (27 °C) day with 80% relative humidity.
The higher the relative humidity number is, the closer the air is to becoming “full” of water. Warm air holds the most water and as it cools and “shrinks”, it’s able to hold less water. The relative humidity number shows how much humidity is in the air, relative to how much it can hold. Air at 80% relative humidity is 80% “full” of water. When the humidity rises above 100% outside, it rains.
Picture that humid air moving into your crawl space or basement. This cool, underground space drops the temperature of the air to 68 °F (20 °C). Because of this, the air’s relative humidity goes up, even if no new water is added to the air. For every 1 °F the temperature drops, the relative humidity of the air will rise by 2.2%.
In this case, the relative humidity will rise by 26.4% (12 °F x 2.2%). Add the 80% humidity you already had, and you have 106.4% relative humidity. However, when the humidity reaches 100%, the air can hold no more water! So this extra humidity is dropped from the air as condensation, which is deposited on cool surfaces in the space, such as wood, metal, or concrete.