Why are your air ducts sweating in the crawlspace? Why is that a problem? When ducts start sweating, they create a vicious cycle.
If you can remember back to your Eighth Grade Science Class, you would know that warm air can hold more water than cold air. The point at which the cool air can hold no more water is called dew point. Condensation happens at dew point. Click here for more information than you want to know about how the psychrometric chart can be used to determine when ducts will sweat.
Here is a much easier to use calculator to determine when dew point is reached and condensation is produced. Here is the problem: when your AC is working properly, it is blowing about 20°F cooler air through your supply air ducts than what it is sucking in through the return filter. So, if you have your thermostat set at 72°, then the temperature of the air going through your air ducts is about 52°. The insulation on your ductwork is a maximum of an inch and a half to two inches, with and R value of 4.2 to 6. When it gets wet from condensation it looses even more R-value. The bottom line is, the temperature of the air in the duct is close to the temperature of the duct insulation surface. So use the dew point calculator to find out what conditions it takes to create condensation on your ductwork, understanding very little insulation from the ductwork is effective. In Nashville in the summer we will frequently experience temperatures of 80° and relative humidity of 80Rh, which produces a dewpoint of 74°. It would take a duct insulation value of at least R12 to prohibit condensation on the ducts, not possible without a custom, cost prohibitive space blanket insulation. The best way to prevent duct sweating is to lower the dewpoint by lowering the humidity.
How do you lower the relative humidity in the crawlspace? The less expensive way is to install a Crawlspace Conditioner. It draws in drier air across a passive dehumidifier to exhaust the moist air. It sometimes takes some experimenting to dial in the right cycle of cfm and air exchanges, along with the economy or Extreme vapor barrier to get the desired crawlspace environmental performance.
A more reliable way is to close the crawlspace, instal a dehumidifier and possibly fans for a larger area and create a semi-interior room in your crawlspace. If you were to create a conditioned crawlspace by Crawlspace Makeovers, the most extreme conditions your crawlspace would experience would be a temp of 70° and and Rh of 60%, which would create a dewpoint of 55°… no sweating ducts!