What's the Problem with Moisture?
Up to 50% of the air we breathe on the first floor of our home comes from the CRAWLSPACE! Have you looked in there?
The problem with a vented dirt or concrete crawlspace is that as warm air rises in your home, it brings up with it the air that was previously in your crawl space, including moisture and mold spores, as well as anything else that may be airborne down there.
As this air rises in your home, replacement air enters from the lowest part, your crawlspace. This replacement air is made up with unconditioned outside air that enters through vents and other leaks, and air that is actually sucked from the crawlspace floor, either dirt or concrete, which is typically filled with lots of moisture. This natural upward air movement is called the "stack effect" -- this is how chimneys work.
Consequently, whatever is in the air at the lowest point eventually flows up into the living sections.
Up to 50% of the air we breathe on the first floor of our home comes from the crawl space!
A dirt crawl space with vents is a never-ending source of moisture due to humidity and high dewpoints. Even if the dirt's surface seems dry, digging down a few inches reveals moist earth which is a constant source of evaporation which raises humidity levels in the crawlspace.
Why Is Crawl Space Moisture so Bad?
Because it's the moisture that leads to mold growth, musty odors, and expensive structural damage, not to mention that insects and critters love moist environments and energy costs are higher. Moisture ruins houses by providing a hospitable environment for mold, fungi, and insects that destroy wood framing.
Crawl space moisture, and the mold and mildew that thrive in this environment, affect not only the floor system directly above, but also the entire house. The principal cause of damage to organic materials in general, and wood in particular is water. Thankfully, this we can control.