The short answer is condensation, but you may have already guessed that. So why is condensation forming on your ducts and what are the hazards aligned with this issue? Well, we will talk about that and also what you can do to prevent condensation from forming on your HVAC system in the first place.
First let's talk about why you have condensation on your ducts, also known as duct sweating. In the warmer months, your HVAC system is working the hardest to try and keep your home cool and comfortable. So, when you have that cold air blowing through your ducts, if the air around the ducts is warmer, there is a good chance that condensation will form. Just like if you were enjoying a glass of iced tea outside on a hot summer day, there would be condensation forming on your glass. Now there is quite the difference between a little condensation and having water droplets all over and dripping from your ducts. Condensation is more likely to happen, and at a higher rate, the more humidity that is in the air.
Air ducts are usually in the attic or the crawl space and there are dangers to duct sweating in both areas. If you have excess condensation dripping from your ducts in the attic it can lead to drywall or wood damage. Crawl space hazards on the other hand can lead to wood rot, which can cause sagging floors. Probably the most considerable hazard is that that excess moisture can generate the perfect environment for mold to grow in either area.
Now for the important stuff, how do you fix the condensation problem? Well luckily there are only two ways to fix your condensation issues, so you should be able to find the right path to an improved system.
The first is to wrap your ductwork in fiberglass insulation. This will form a barrier between the cold ducts and the warm air within your attic or crawl space. This process works well if you live in a dryer climate. But if your problem is a high level of humidity causing the duct sweating, you'll want to keep reading.
If you have a high level of humidity within your crawl space or attic, I'm talking about anything over 50-55%, then you will want to remove the humidity within the area. This can be done in an attic by installing attic fans or making sure the ones you have are working properly. The crawl space on the other hand can get a little more complicated and costly. Leaving vents open in the summer can allow that outside humid air to enter the area, but closing them without a way to circulate the air can lead to stagnant humid air. Crawl spaces also have a dirt floor that can get saturated and bring further moisture into the area. Talk about a moisture problem.
So without starting a whole new topic, here is a basic idea of what a crawl space may need to remove the moisture. Close and seal the vents, also try to seal any other areas that may allow outside moisture to enter the crawl space. Install a moisture barrier, also known as a vapor barrier. You can go with just a basic vapor barrier, but if you want to do it right I would suggest encapsulating the crawl space. Lastly, you'll want to install a dehumidifier. I strongly suggest you not to go cheap on your dehumidifier, make sure you purchase one that is rated for the size of the area you have. If you buy a dehumidifier too small for the area, you may end up paying more in the long run when it runs all day long trying to keep up with the load.
Condensation on your ductwork is not your amigo, and in fact can lead to serious problems within your attic or crawlspace if not properly dealt with. If you are seeing condensation on your ductwork, hopefully this has given you a basic understanding of your options to mitigate your moisture problems. And as always, if you're unsure about how to deal with your condensation issues, reach out to a local professional who can steer you in the right direction.