Advanced Air

Why Are There Water Stains on My Ceiling Around the AC Vents?

April 07, 2015

Have you noticed water stains around your air conditioning vents? It’s a common problem in Florida.

It’s usually caused by condensation forming on your ducts and leaking onto your ceiling.

Don’t ignore this problem! Water damage is expensive to fix and moist conditions like this encourage mold growth.

Let’s look at why this happens and how you can fix it.

Why condensation forms on AC vents

In a nutshell: cold ducts + warm, moist air = condensation

If you set a cold glass of water outside on a warm Fort Myers afternoon, the glass will soon be covered in water drops. That’s condensation.

The same thing can happen with your AC system and vents. Basically, when the cold air from your air conditioner mixes with the warm, moist air in your attic, condensation happens. And then it drips down onto your ceiling creating nasty water marks.

There are 3 common culprits of this problem:

  • Not enough insulation on the ducts themselves

  • Poorly insulated vent ‘boot’

  • An air leak


Let’s look at each of these in a little more detail and how you can fix them.

3 causes of condensation on your AC vents (and how to fix them)

Not enough insulation on the ducts themselves

Air traveling through your ductwork can cool the ducts to the point where they act like the cold glass on a warm day. To prevent this, the air ducts should be well insulated.

The ducts in this new build currently have no insulation. Image of AC vent reposted from NACHI.org

The fix: Insulate your ducts. If none of the ducts in your home are very well insulated, it’s probably time to take care of this. It’ll save you a lot of money on your air conditioning bills.

Poorly insulated vent ‘boot’

No, not these boots.

If you take off your AC vent and look up, you’ll see sheet metal. That’s the boot. The boot is the connection between your ducts and the vent.

Here’s a picture of one in a crawlspace. The boot is the metal part where the duct ends. In an attic, it would hopefully be surrounded by insulation.

Image of duct boot from Energy Vanguard

However, many homes have uninsulated boots. This is basically the same problem as uninsulated/poorly insulated ducts; the boot gets cold, causing water to condense on it and drip onto your ceiling.

The fix: You can sometimes get away with uninsulated boots if the attic insulation is covering them. However, it’s still best to actually insulate the boot.

An air leak

If the problem isn’t insulation on the ducts or boot, the next most likely problem is an air leak. Usually, the leak happens where the vent boot attaches to the duct. If not properly sealed, the cold air leaks out, meets up with the warm attic air and creates condensation.

You could also have an air leak around the vent itself (between the vent and drywall).

The fix: Find and seal the leak. If the problem vent is easy to access from your attic, you may be able to check for and fix the problem yourself. If you can’t find the leak, call a professional air conditioning company.

Painting over the ceiling stains

Once the problem has been solved, you’ll want to get rid of that ugly stain. We highly recommend a couple coats of KILZ primer on the stains followed by your choice of paint and color.

Painters swear by the KILZ primer brand because it’s great at keeping stains from showing through the paint.

Need help with your home comfort problem? Advanced Air is your Fort Myers expert. Contact us for more information.