Madison Park

I don't know anything about the flex ducts.    I had not been under my house in a couple of months so I did go open the door based on your post today, and it was nice and dry.   Serach online and see what you find on the ducts.  There is probably something they can be wrapped with to insulate them better.

Sounds like a good plan to me.  Crawlspace vents as with all moisture related issues are really never a simple, one-rule works everywhere issue.  In reality, some of your cooled and dried air is falling into your crawlspace and lowering the temperature and humidity.  Keeping it in works at my house, by just leaving a couple vents open in the summer.  My floors are not insulated, my crawlspace is cool, but it's also dry.  

Let us know how your testing develops.  

It's my opinion that you shouldn't mess with the flex ducts, they cannot be easily wrapped with anything and will grow quite large if you do.  Nor is it likely to dramatically improve the condensation problems you have.  They are not rigid in the middle and laying things on them will restrict the airflow.  

You should also check for other sources of moisture such as a dryer vent duct that is loose or blocked or a leaky pipe while you're down there exploring your crawlspace!  

http://www.ehow.com/how_8561538_add-insulation-flex-duct.html

This was an e-how link so people do insulate the flex.  I think you would probably have to wait until completely dry.   I had mine done in Oct. after everything dried out.  I had a heating and air company out the first of July doing general airconditioner maintence. They gave me a great deal.  When I say great they did it for free.   It had to be dry or the tape would  never seal.     

I found one site where someone used bubble wrap on their flex ducts.   They were doing it to try and lower their air bills because the flex was in the attic.  

Anyway again good luck. 

I would of never thought to have mine wrapped had the county inspector not told me it would make a big difference in the rain forrest effect I was having under my house.