There is lots of controversy regarding the ventilation or the lack of ventilation in crawlspaces under homes. Locally here in Pensacola, FL and Gulf Shores, AL I have seen only a few crawlspaces that are not vented. Of those few homes I saw which are not vented, they were not updated with a completely sealed structure nor an HVAC system installed. The result was no mold (yet).
The pictures are from a crawlspace I inspected yesterday, which is typical for me to see this type of mold / fungus.
Building code says put a vapor barrier down on the ground and add some vents. It is very rare not to see some mold growing somewhere under these homes. I would like to see some sort of retro fit kit that is easy, quick and relatively inexpensive that would close in and condition crawlspaces that do exhibit mold conditions.
International Residential Code
408.2 Openings for under-floor ventilation. The minimum net area of ventilation openings shall not be less than 1 square foot (0.0929 m2) for each 150 square feet (100 m2) of underfloor space area. One such ventilating opening shall be within 3 feet (914 mm) of each corner of the building. Ventilation openings shall be covered for their height and width with any of the following materials provided that the least dimension of the covering shall not exceed 1/4 inch (6.4 mm):