1 - Moisture:
Moisture can seep into the foundation of a house and cause damage over time. If that’s not bad enough, a dark and moist environment like a crawlspace can offer an excellent breeding ground for mold. Most of the air in your house goes through your crawlspace at some point. That air can carry mold spores into your home and reduce the air quality of the entire house.

Keep an eye out for moisture in your crawlspace, especially after it rains. You can detect it in the air if you have a good nose. You can also touch the ground and the walls looking for wet spots.

 2 - Rodents, insects, and other pests:
Keep an eye out for critters and signs of their presence, such as animal feces, nests, and gaps in the crawlspace that could serve as entrance and exit points. Insects and rodents should be eliminated quickly, as unchecked activity can quickly grow into an infestation. A pest control professional can help you determine the extent of the problem and recommend the best mitigation strategies.

 3 - Wood rot:
Rot can be a very costly problem. This form of fungus grows inside the wood, weakening it as it spreads. The good news is that there are lots of chemical products on the market that can kill and remove rot from wood. The bad news is that these methods don’t work when rot damage is too extensive. Detecting the early signs of wood rot can save you the expense of having to replace huge chunks of a house’s foundation.

 There are two types of rot that may affect your crawlspace: wet rot and dry rot. Signs of wet rot include the wood becoming darker, feeling soft or spongy when pressed, and the appearance of cracks in the affected area, as well as localized fungus growth, which may resemble mushrooms.

 Dry rot will also cause wood to feel weak and spongy. This tends to be a more destructive variety of rot, but it’s also easier to spot due to the growth of the mycelium — a white substance that looks a bit like cotton or wool and clings to the affected wood. If you spot that, or any other sort of fungal growth or rot, get a professional involved.