The original symptoms last summer was that all of the a/c ductwork in my crawlspace were sweating like nuts. The insulation on all the ducts was soaked and dripping water. The floor insulation was also soaked and hanging down everywhere. I also was having mold start to grow on the joists.
I already had a vapor barrier, over the usual 80-90% of the ground.
I've owned the house for about 1.5 years. About a year before I bought it, it had a geothermal heat pump installed.
Had a french drain and sump pump installed, as well as 2 large ventilation fans in the crawlspace vents, at the end of last summer. I also had new vapor barrier put down. That seemed to help at the time. But this summer, I found that the ducts were starting to sweat again.
So I sealed up all the vents, disabled the vent fans, and put in the dehumidifiers. Now, everything is dry. But the temperature is up under there, and of course, I'm jacking up my electric bill. In the short term, this is probably okay, though.
Since then, I had a patio installed, and the guys installing it discovered that the underground drain pipe for all the downspouts on one side of the house had been screwed up when the geothermal heatpump was installed. The pipe was completely blocked. I think this is probably where the moisture was coming from.
I probably also need to have the insulation on all the ductwork replaced, though I have yet to find somebody to do this. The company that installed the geothermal heatpump has just been jerking me around on this for a year. I am really pissed at them. It seems like no other companies are willing to work on it because it is a geothermal heatpump.
BTW, what are peoples opinions about having the existing mold cleaned off the floor joists once the moisture problems are fixed for good? The company that installed the french drain, sump pump, and vent fans also sprayed some sort of anti-mold stuff under there. We've never noticed any mold problems in the house itself (and my family would notice this, we all have bad allergies).