Look at the Building Science Corporation web site for more information
on crawl spaces.
If the crawl space is often damp from surface drainage or from ground
with high water table, a sealed crawl space can be difficult and
expensive. If it the environment is right for it, a sealed crawl space
is a good idea. Many building codes make that arrangement illegal and
some require an engineer's report. As I remember it, Steven Winter
Associates is in your area and specializes in this sort of problem.

TB