Water in Air Ducts: Causes, Prevention and Removal

When most homeowners think of problems with air ducts, dirt and damage are typically the first two things to come to mind. While you can avisible damage on exposed ductwork or the outside of vents, moisture inside air ducts can be a hidden health threat.

Excess condensation can give mold a foothold in your vents and is both challenging and expensive to remove. In this guide, we’re going to talk about how water can find its way into air ducts, and what you can do to prevent it. We’ve also put together a list of quick tips to keep your ducts clear of condensation.

Why is there Water in my Ductwork?

The first step to fixing the issue of water in an air duct is to figure out where it came from. Unfortunately, that can be a tough task if you have a large home or ductwork in a crawl space. That means you’ll want to set some time aside and put on some clothing you don’t mind getting dirty in case you have to crawl under your house or into a small attic space.


The biggest reason for water to enter air ducts is because of condensation. If you’ve ever seen water dripping from an un-insulated trunk duct in the summer with the AC on, you’ve experienced condensation with your heating and air unit.

Whenever a duct is run through a room that isn’t heated or cooled, the result can be low duct temperature. That means there is a chance for condensation to form inside or outside of the air ducts that pass through rooms without AC in the summer. The same goes for air ducts in unheated rooms as they can experience condensation during the colder months

Excess Humidity in Attics and Crawl Spaces

If your ductwork is installed in a hot attic or crawlspace without proper ventilation, things can get humid in those areas during the year. High humidity can cause condensation inside or outside ductwork, which will lead to issues inside those areas.

If that space is used for storage, things can become ruined, and the indoor air quality in your house will certainly be affected. Both of these areas aren’t “conditioned” like a living space, which makes them ideal for issues like water in ductwork as well as a breeding ground for mold.

HVAC Installation or Service

Every year, millions of Americans have their HVAC systems serviced. While we always recommend using a certified professional, some are a bit more qualified than others. If you’ve had a new central heating and air system installed in your home or a recent repair… that could be the source of the problem.

An improper HVAC installation can cause a variety of problems inside and outside of your home. Unbalanced units to ones with poor drainage will cause issues but so will an oversized HVAC system. While a unit that is too small won’t be able to keep you comfortable, one that’s too large can lead to water in the ductwork from short cycling.

Have you had your unit serviced recently? If so, was it handled by a skilled professional or a handyman from Home Advisor? Something as small as a poorly sealed piece of ductwork can lead to moisture inside of your air ducts. That’s why we always advise using a certified professional to work on your HVAC system or a qualified contractor from our Quote Tool.

Problems caused by water in Air Ducts

While you may think a bit of evaporated water in an air duct isn’t a big deal, it can turn into one quickly when it’s a regular occurrence. That’s mainly due to mold, something that can find a foothold in the unlikeliest of places given the right conditions.

Mold loves dark, damp places that are dusty. Air ducts certainly fall into that category, along with several parts inside of an HVAC system. When you have water in your ducts, mold can grow and every time you turn on your unit, spores will blow throughout your home.

Health issues caused by mold in air ducts can range from itchy eyes and headaches to asthma attacks. People with underlying or respiratory issues are more at risk, but mold has a drastic impact on your indoor air quality regardless. When left unchecked, excess condensation will break down insulation around ductwork over time and can even damage drywall or other materials in the area.

Reducing Condensation and Moisture

Now that you know how serious water in an air vent can be, it’s time to talk about the best ways to get water out of AC ducts and ensure it doesn’t return. The first thing you should do is find the underlying reason for the water as we’ve discussed in this guide.

If you don’t see any reason as to why there is water in your system, then you’ll want to call in a technician to have a closer look. Otherwise, keep these tips in mind if you need to reduce humidity and remove moisture from an area in your house.

Check and Seal Leaks

There are several areas along the ductwork in a home that are prone to leaks, and some will be easier to check than others. If your air ducts are visible, give them a good inspection with the system running and look for a sign of a leak. It could be something simple to fix that only requires aluminum HVAC tape or a more severe problem.

A hole or damage to your ductwork can let moisture creep inside under the right conditions, but other types of leaks will also affect your air vents. Have a leaky seal around your toilet from an old wax ring? Well, that can cause a slow leak that can have a big impact in your basement over time. Gaps around improperly sealed vents like the one below are also an issue.

Unsealed Vent

The same goes for any dripping fixtures in a basement or crawl space. As for the attic, a leaky roof can have a direct effect on ductwork in that location as well. While you may still need to add one of the other fixes from our list below, finding and fixing any leaking pipes in areas with ductwork should be at the top of your list.


Depending on the age of your home, and the type of ductwork used, you may not have insulation around the ducts. Properly insulated ductwork can prevent condensation while also adding a level of protection to your air vents. A hole in or tear in thick insulation is much easier to deal with than punctured sheet metal.

If your ducts are not insulated, there are a couple of options at your disposal depending on the issue at hand. Ductwork insulation comes in several forms, although the classic fiberglass duct wrap is the most recognizable. This type of insulation works well on both rectangular and round air vents and usually has a foil facing like this durable wrap from Master Flow.

If there is a short run that can easily be replaced, flexible ducts covered in R6 and R8 fiberglass insulation are an option. Duct liners and duct board aren’t an option unless you’re replacing large sections of ductwork in your house, but standard isolation could be. If these bare ducts run through an uninsulated space like an attic, adding insulation to the room itself can keep condensation at bay.


The easiest way to bring the moisture level down in a room and prevent condensation is to use a dehumidifier – even if it may not be the cheapest solution. These machines are best suited for homes in damp locations or with below-grade basements which can stay camp and cool throughout the year.

The purpose of a dehumidifier is to remove excess moisture from the air and bring an area down to acceptable levels. It’s widely agreed that it should be between 30-50% humidity indoors, and there are hundreds of dehumidifiers that can easily do that. It’s important to choose the right size dehumidifier, however.

As useful as these devices are, you will have to deal with the moisture it pulls from the air. It’s stored in a removable bucket that can be dumped when full or can be pumped to a drain if the unit has a built-in pump. You can learn more about dehumidifiers on our site, and we have even compiled a list of machines that are ideal for damp crawlspaces.

Unused Ducts and Drains

This won’t affect every homeowner but can be a significant reason behind moisture in air vents. If you have areas in your home that are seldom used but have ductwork, sealing them off can help. That includes areas like garages and attics, and it’s something many homeowners do every year. Unfortunately, the old-fashioned way isn’t necessarily the best one.

Closing the damper on any air vent in your home can help prevent excess moisture in certain areas, but those dampers are far from air-tight. The only way to ensure those ducts are sealed is to tape them off or seal them completely. There are dozens of methods for this from mastic to cardboard, but we recommend using something temporary and easy to remove if necessary.

While blocking off some vents keeps moisture down, you need to check the drain line on your system to ensure moisture gets out. The condensate drain line on most central heating and air systems looks similar to what you see in the photo below. The pipe or line coming from this drain needs to be clear and free of debris so that condensation can flow freely from the unit.

Drain Pipe
Drain Pipe


Out of the myriad of issues that can plague homeowners with central HVAC systems, finding the cause of condensation in ducting is something most people can handle themselves. The important thing is to address the cause of the water to begin with or you’re essentially putting a Band-Aid on a larger issue. We also recommend reading up on mold in air ducts if you’ve had water in them and want to ensure great air quality indoors.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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