My sister's house located in Maine has a crawl space under her 3 bedroom ranch. The house is about 20 years old and has fiberglass batts installed between the floor joists with the vapor barrier up against the floor. Ground moisture is attacking the floor joists and carrying timbers, saturating the fiberglass and causing it to fall down.
Some "experts" have told her she should have a vapor barrier laid over the dirt coming up around the inside of the foundation and fastened to the sills. I disagree with this for a couple of reasons:
1) the lack of drainage and buildup of water in the spring
2) If anyone were to go into the crawlspace to work they would damage the vapor barrier as they crawled over it.
I am planning to install Tyvek underneath the floor joists, basicly wrapping the whole supporting wood structure in a tyvek envelope and placing containers of silica gel in those bays with the worst moisture accumulation.
I am concerned about the properties of tyvek that allow the passage of water vapor. My question is; Will the moisture from the space still pass through the Tyvek and continue to attack the floor joists, carrying timbers and fiberglass?