Two of the worst things you want to see in a crawlspace, poor ventilation, AND wet conditions. The house I inspected yesterday, had three things going against it.
1. Poor ventilation. The house was built about 1 block too low, so most of the vents were not about the ground. Not only did they not install enough vents around the house, but where there were vents, they managed to effectively block the air flow by building little boxes around the vents. Of course the vent wells were full of leaves, mulch, grass clippings and who knows what else to help block the air flow.
2. The slope of the land around the house directed any water toward the foundation, instead of away from it. There were low spots in many areas, so water just collected next to the foundation walls. The gutters hadn't been cleaned in a decade or so, and the downspouts just dumped the waetr right next to the foundation, instead of flowing away from the house.
3. No vapor barrier on the ground in the crawlspace. Water vapor from the saturated earth under the house just condenses on the wood framing.
So the result was a bunch of fungus growth on almost all of the floor joists along the from of the house. Many of the joists were so rotted the wood was brittle. Termites had also found their way into the house and were enjoying the free buffet.
I was doing the inspection for a couple that are renting the house and thinking of buying it. Now faced with a pretty costly repair, the seller will either have to reduce the price of the house, do the needed repairs, or they will be looking for another house to buy (their comments).
Here are the photos of the fungus growth and the rotted wood.