Crawlspaces are notoriously damp and humid spaces. If you have dirt walls and floors in your crawlspace, moisture from the earth is entering directly into to the area. This can come in the form of humidity, when the air sucks the moisture from the earth directly into the air, or it can come in the form of water leaks, when water builds up in the earth and leaks into your dirt basement, where there's no barrier to stop it.
Even if your crawlspace has concrete walls, however, you can still get moisture in the area. If water is actually flooding your crawlspace, you will need a waterproofing system with a sump pump to keep it free of standing water.
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