Even in dry climates, you'd be amazed how much moisture comes through the floor.
I think closed cell insulation might be ok, but when I think about using it I always ask the question: is moisture going to get trapped by it?Human habitat has moisture that could get trapped in, and possibly harm, the floor joists, but I hope someone else will weigh in on this, too.
My first thought, and it is cheaper than home foam insulation, is to:
- lay a vapor barrier in the crawlspace,
- run it up the foundation walls and seal it,
- then add positive ventilation in the crawlspace with a small fan built for this purpose,
- then insulate the floor with any insulation that is designed to fit between joists, such as cotton batts or formaldehyde-freefiberglass, or cellulose in a netted installation.
For more information:
Read Florian Speier's Q&A "We have an unheated crawlspace under our house that we would like to insulate."
Also, read Danny Kelly's Q&A "What kind of waterproofing can you use in crawlspaces to help with going green?"