I always enjoy reading Allison Bailes' Energy Vanguard blog. It's full of practical advice and great examples of home performance issues. Here's an excerpt from his latest blog:
Back in the summer of 2005, I got a call to figure out a moisture problem in a new house. The homeowners took me into the master suite, where the first thing to catch my eye was the towel folded neatly on the floor. It hadn't just fallen out of the laundry basket; it was there to catch the water dripping from the air conditioner supply vent in the ceiling directly above. Another one was on the other side of the bed, and those two vents were dripping steadily. It didn't take me long to find the problem.
Air conditioner vents get cold. Water vapor loves cold surfaces. Sometimes it leads to mold, as you can see in the photo above. Other times it leads to the paint coming off and rust forming on the vent (photo at bottom). If you're lucky, it's just an occasional nuisance that doesn't lead to long-term damage to the supply register, indoor air quality issues, or the need for towels on the floor. And if you're really lucky, you wonder why I'm even writing about this because you've never seen it.
One cause, multiple reasons
There are a lot of reasons why this happens, but there's only one cause. Let's start with the cause. Condensation occurs only when humid air is in contact with a cold surface. If your air conditioner supply vents are sweating, you have a cold surface (the vent) and near it, you have air with water vapor in it. The threshold for condensation to occur is that the temperature of the cold surface has to be below the dew point of the water vapor in the nearby air.
Continue reading on Energy Vanguard: Why Do Air Conditioning Vents Sweat?