One of the more common issues I encounter when performing a home inspection on homes with a crawlspace is elevated moisture in the floor joists. Many times this condition causes microbial growth on the floor joists, premature degradation of the joists, and failing insulation. There are multiple factors that can contribute to this condition. The vapor barrier could be missing or incomplete. This can allow moisture to evaporate from the soil. The grading or roof drainage could be putting water against the foundation and thus entering the crawlspace. This condition is one or the more common sources I observe. The final source of moisture is thru the crawlspace vents.
Many times the moisture content in the floor joists is higher in the summer time because hot and humid air is pulled into the cooler crawlspace where it condenses onto the insulation and floor joists. Many times during this time of the year, I’ll leave the crawlspace with my back wet from the dripping insulation. The moisture content of the floor joists during that time of year is often over 16% and is considered to be elevated. Building science and building practice are not necessarily in line with each other because vents are a code requirement. Here is a great article that explains what is, or may be going on, in the crawlspace.
https://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces