A vented dirt-floor crawl space is a VERY BAD IDEA!

You have moisture in your crawl space, lots of it.
Where is it coming from?

1. Water leaks
We routinely find minor leaks when inspecting a crawl space. These are sometimes in supply lines, but most often from drainage lines. The homeowner is usually unaware because the ground absorbs the obvious wetness and then the moisture slowly moves back into the crawl space air.

Notice the water-soaked insulation in the photo at the right. There is so much moisture under this house that the insulation has become water-saturated. Eventually, this insulation will begin falling to the ground due to the weight of the water.

2. Ground water intrusion
Practically every home we inspect has rainwater management issues. If water is not diverted away from the foundation of your home, it can sit against the block, and will eventually seep into the crawl space. In severe situations, standing water collects throughout the crawlspace. This is a serious condition that is renewed with every rainfall.

We often see foundation walls like the photo to the right. At this home, rainwater collects in a large puddle against the outside of the foundation wall every time it rains. The water is passing through the blocks (foundation blocks are very porous), into the cavities, and falls to the bottom. The dark gray blocks in this photo are full of water. As the water seeps out of the lower blocks inside the crawl, it forms the standing water you see. Standing water in a crawl space is a very serious problem.

3. Condensation from outside air entering through your vents
The original reason for venting crawl spaces was to provide air movement through the crawl space so that it could "dry out".

The TRUTH:
Crawl space vents do little more than supply a constant source of additional moisture to your crawl space, particularly in the warmer months (especially here in central North Carolina). Opening your windows after having your carpets cleaned is a good idea, but your crawl space is a very different place...it is cooler. In this area, a typical crawl space stays between 65° and 75° in spring, summer, and fall.

* * * RELATIVE HUMIDITY * * *

The concept of "relative humidity" states that for every degree drop in temperature, relative humidity increases by about 2.5%.

Let's do an example:
If your crawlspace was 70° and 80% humidity, and the outside air was 90° and 90% humidity (typical for a mid-summer day in Greensboro or Winston Salem), then the outside air would cool 20° degrees upon entering a vent. This rapid cooling would increase the relative humidity by 50%. Add this to the 80% humidity already in the crawl, and we get 130%, which we know is not possible because water vapor condenses at 100%. When these very common conditions exist, it is literally raining in your crawl space!

This photo to the right is the A/C duct in close proximity to a crawl space vent on a hot summer day in Greensboro, North Carolina. This illustrates what we have just described, relative humidity, and what happens when the humidity goes to 100%.
It was raining under this customer's house (see photo at right).

Face it. With all this moisture present, for most of the year, mold and structure damage in your crawl space is all but inevitable.