HVAC Ducts Sweat in Summer

Dillan Edmonds asks,

Hey, I live in southern Kentucky. During the summer months I have condensation dripping off the HVAC in the crawl space. What can I do about that? The crawl space is concrete block with vents on all sides of house and has black plastic on the ground.

Our Answer

The condensation is the result of warm moist air in the crawlspace striking the cold air conditioning duct. As the air is chilled, it releases moisture resulting in the dripping you are reporting. It’s like what happens when you bring an ice cold glass outside in the summer, the outside gets wet because the warm moist summer air is being chilled, releasing that moisture.
Two things can help:
 It sound’s like you already have a crawl space vapor barrier, which is good. You can further reduce the amount of moisture in the crawlspace by improving the grading and drainage at the foundation perimeter, as explained in this article about <a href="https://www.moneypit.com/flooding-dampness/basement-waterproofing-tips-0/" target="_blank" rel="noopener">preventing a wet basement</a>. The approach is exactly the same when you need to reduce the moisture levels in a crawl space. 
Second, you can <a href="http://www.homedepot.com/p/60-sq-ft-R-6-Insulated-Duct-Wrap-INSWRP60/100152008" target="_blank" rel="noopener">insulate the ducts</a>. Once insulated, the warm air will no longer be able to contact the cold duct surface. No contact, no sweating!