A.Paul Fisette responds: Mildew needs a local relative humidity of around 70% to grow. Even though the earth feels dry to the touch, it isn’t. That’s probably where the moisture is coming from. (I’m assuming there are no other obvious sources of moisture, such as a reverse grade outside, or gutter downspouts dumping water against the foundation.)
As a first measure, you should install a polyethylene ground cover throughout the crawlspace. There is quite a bit of research showing that ground cover eliminates up to 75% of the moisture that ends up in the air as humidity in a crawlspace. The mildew itself will probably not cause any great problem in the crawlspace, but this moisture...
to read the full article.