Upon opening the crawlspace door, the author saw large, obvious patches of white wood rot and black mold.
The OSB ceiling looks considerably better at the end of the job, after the crew has scrubbed it with a borate solution.
Moisture condensing on the cold masonry columns on the perimeter of the crawlspace was wicking into the adjacent cripple wall framing and insulation batts.
In the worst parts of the crawlspace, wood and insulation facings were covered with slick black coatings of wet, slimy mold.
The entire crawlspace was equipped with just two small vents. One is shown here as viewed from outside the crawlspace under the exterior deck.
OSB and Tyvek between the crawlspace and the floor protected the floor structure from damage, saving the floor (and perhaps the house) from total ruin. The OSB was heavily attacked by fungus, but could still be cleaned up with brushes and a borate solution.
The sprayer allows him to saturate the underfloor OSB with a solution of Tim-bor borate, killing mold, rot organisms, and insects without toxic risk to people.