Is it truly enclosed or is it vented. If it is enclosed then the only small project I can think of is to stick a separate dehumidifer in there (one that allows a garden hose to hook up to it) and drain the water outside. Check on the dehumidifer often to make sure everying is working.

Conditioned crawlspaces usually employ a 20 mil poly cover over the ground and walls, sealing the space. Then they run a dehumidifer in the space also. So I am not sure you are totally enclosed.

In most cases, the crawlspace would be vented to allow airflow and with proper insulation under the floor and the plumbing on the hot side of the insulation. Think of a crawlspace like an attic, unless you seal the entire attic you need ventilation and insulation. So maybe you need proper ventilation instead.

Hi - It is enclosed. Walls are concrete and foam insulated. Floor is cement with vapor barrier underneath. Ceiling is exposed with pink fiberglass insulation. Somehow though humidity must be getting in...it is not from first floor above since that is conditioned. And humidity creeping up now to 64%. There is a window that goes to the basement (as that window used to be exposed to outside, and is now the crawl space). I was thinking i could keep that window open (there is a screen) to the basement (which has a dehumidier). Not sure if that will suck enough air from the crawl space to exchange the dry air in the basement (from the dehumidifer) into the crawlspace. Any thoughts?

Yea, I would open that window and allow the space to breath during summer. If that doesnt help then stick another dehumidifer in there or maybe a floor fan on low

Are your rim joists insulated too? Lots of air exchange there.

Here is a tool that might help you:http://dpcalc.org