We have new construction over a large (5'tall x whole house) crawl space.It was very wet and moldy. I bleached all overhead engineered wood beams and removed mold & mildew. I layed down heavy plastic over all dirt floor area and sealed seams with duct tape. There's 8 vents to outside in concrete walls for 1800 sq. ft. I have an ordinary oscillating fan going 24/7. The plastic is still accumulating moisture on top. Should I put in a Humidex air exchanger or a strong dehumidifier? Will the dehumidifier freeze up because it's so cool under there? Will the heavy dampness leave with the air exchanger? Should I tape the plastic to the concrete walls? Right now it's just laying along the edges. The concrete walls are damp- should I paint them with waterproof paint? I don't want mold down there again. I'll do whatever it takes to prevent all that bleaching- it was a nightmare.

 

Aunt Gayle,
I live on the Albemarle Sound in coastal NC with very high ground and air moisture.
Our home is over a 3'-clearance whole house crawlspace, and we used to have the same problem...excessive moisture, dripping AC ductwork, dripping water pipes, mold on the wood floor joists, etc.
There is so much moisture in the air, that I elected not to put down a vapor barrier for the ground moisture.
I contacted W.W. Grainger Company's tech support, gave them the cubic footage of the crawlspace area (height X width X length), and they gave me a cfm exhaust fan recommendation.
I went to the next higher exhaust fan (more than double their cfm recommendation). We have a 32' high A-frame with a 28' X 44' crawlspace.
It is a 10" louvered exhaust fan with 580 cfm (as used in dairy barns, etc.). I built a pressure-treated plywood and p-t 2X4 crawlspace door just to mount the fan in, and put screen and hardware cloth on the outside to keep unwanted guests out.
To give the door stability and to allow room for the louvers to open/close inside the hardware cloth/screen barrier, I put two layers of 2X4s on the inside and one layer on the outside of the door around the fan opening.
You can put the fan on a humidistat or a thermostat or do as I did, allow it to run 24/7 except when I close all crawlspace vents for the freezing winter.
It has a very small motor (which I wired just to plug in under the house) with slack in the wire to open/close the door for access. It uses very little electricity.
It stays bone dry under our house, even during periods of heavy rains.
Ventilation and constant air circulation is the key.
Hope this helps you.
Good Luck!