Mold In Air Ducts: The Causes & Solutions Homeowners Need to Know About

Kenneth Wilson

Are you concerned that there might be mold in your air vents? This can be a serious problem if you don’t act on the issue immediately as mold in your air vents poses some health hazard. To help you with this, we've listed in this article the actions that you can take to remedy this problem.

To remove mold in your air ducts, it’s recommended to hire a professional. There are thousands of mold species out there, and although not all mold are toxic, there's just no simple way of knowing if the one in your air ducts is or not. So it is the safest option to get an expert to inspect your air duct system.

According to the Environmental Protection Agency, having a professional clean out your vents costs around $450 to $1,000, depending on the extensiveness of the job. This expense is definitely worth it if it means potentially saving yourself and people in your household from further exposure to health risks.

Although we believe that hiring a professional is the safest way to deal with the problem. We've also listed below ways to deal with it if you choose to do it yourself. So let's get right into it.

Signs of Mold in Your Air Ducts

Whether you are wondering if your air ducts have mold infestation in them or you have a strong suspicion that it does, there are tell-tale signs that can confirm whether you have a problem or not. So without further adieu, here are the signs to look out for: 

Visible mold 

mold in air duct

The most obvious sign that will confirm if you indeed have a problem is if you are able to see the mold physically. Having so, you may want to inspect deeper into your air duct as the mold may have spread severely. You’ll want to look in the air ducts, drip pans, pretty much anything that it connects to.

Note that, even the most minor case of mold can develop into a complete system issue. Mold spreads fast if you provide the ideal growing conditions. So, in the fortunate case that the mold problem is minor, you need to act quickly on it to prevent a more serious issue.


Another prominent sign that you have mold in your air ducts is that you'll notice the wet, rotten, or stale odor when you’re using air conditioning or heating (if they share the same ducts). More often than not, you’ll smell mold way before you see it. 

To confirm if indeed you have mold, you'll need to be more sensitive to the odor. Check if the smell disappears when the air ducts aren’t pushing any air out. If there's a difference, then you can confirm that there’s some type of mold or mildew building up in the system.

Health Symptoms

One of the more serious signs that your air vents have mold is if people in your household are experiencing the following symptoms: 

  • Headache
  • Nausea
  • Irritation of the nose, throat or eyes

Mold in your air ducts can make people ill. So as mentioned, if anyone's starting to experience the symptoms when the AC or heating is on, you need to get this fixed ASAP.

Reasons Why Mold Develop in Air Ducts

Before we talk about how to prevent and treat mold growth in your air vents, you should know why there’s mold in the first place. From knowing this, you can counteract the issue more properly and lower the chances of mold occurring. Here are some possible reasons why mold grows in air ducts: 

Unit size 

When your AC was installed, you or the previous owners may have fitted an oversized unit for the property. Typically, larger units can cool spaces too fast, and because of the way they are programmed, they’ll turn off before dehumidifying the air. 

As you can imagine, this creates a lot of moisture, and this causes breeding grounds for mold and mildew. To fix this, consider decreasing the size of your AC unit as this will result with the continuous build-up of mold.  


Another leading cause of mold developing in your air ducts is the quick temperature change. As a standard practice, when using AC on a sweltering day, you should slowly bring the temperature down. This is because when freezing air is blasted into a hot environment, it creates condensation. 

Due to this moisture build-up, it’ll be the best environment for mold if it doesn’t have enough time to dry. Usually, condensation will only occur if you try to fill a room with air 20 degrees lower or higher than the original room’s temperature. This is very easy to avoid. The only thing you need to do is to gradually change the temperature rather than heating it on extremely low or high heat.   


air vent leaking

Another reason mold could develop in air ducts is leaking. A leak can invite warm air into the system. If you’re pumping cold air into a room, this temperature differential can have a similar effect as the other causes mentioned. Because both hot and cold air is in the system, condensation can arise. Again, if this does occur, moisture will quickly build up. 

The same rules follow. If moisture builds up and doesn’t have the conditions to dry, then mold or mildew can occur. 

Temperature differentiation is the main reason for mold growth. If cold air meets with hot air, it’ll cause condensation, which then creates the perfect environment for mold to thrive.

Moreover, condensation in your duct walls and even in the interior of your home not only causes mold infestation in your air ducts but also further damage to your property. If condensation appears indoors, mold can grow exponentially.

Remedies to Mold Infestation in Air Ducts

Now that you know what causes mold, it's now time to know how to remedy it. Here are the best ways to do it.

Hire A Professional 

The most ideal way to clean mold out of your air vents is to hire a professional. I recommend this because is the safest way to treat the problem especially if the mold infestation is severe. Mold can be toxic, and it can cause some severe issues if not taken care of correctly. 

Most scrubbing methods may look like an effective way to get rid off the mold but remember, mold is a microorganism and it needs specific chemical treatment to fully kill it. You definitely have the option to do it yourself but surely professionals can remove and prevent it from coming back way more efficiently.

Another good reason to hire a professional is that you can accidentally damage your AC system trying to clean the mold. This can potentially cause you more money. Again, professionals know the ins and outs of an AC system and have taken the appropriate training to ensure they don’t damage anything while performing the cleaning. 

Lastly, we understand that this may not be an option for all AC owners. If that’s the case, you may want to follow the DIY instruction below. Take note that doing so has some potential risks to your health and to your home so make sure to take all the precautionary measures.

Do It Yourself

If you believe you can tackle the job in a safe manner, and the mold infestation isn’t too severe, then proceed to follow the guide below. Here’s how to successfully and safely clean your air ducts yourself:

Step 1

  First, you need to purchase and gather all the necessary materials for the cleaning process. You’ll want to purchase a mold removal solution and mold inhibitor. Make sure that the products are EPA-approved for the best and safest results. 

Step 2

  Before commencing the cleaning, be sure to wear protective clothing because mold can be toxic. In addition to that, the solutions used to remove mold are extremely potent and can be harmful if it came contact with your skin.

Having so, the EPA recommends wearing rubber gloves, coveralls, protective eyewear, and an N95 mask before undergoing the work.

Step 3

  Now you have the tools and safety gears. It is time to turn off your AC or heating system.

Step 4

  Take your air vent off and deep clean both the vent and the opening (further instructions on this below). Then, head over to your disposable filters and make sure to change them. 

Step 5

  For this next step, you’ll need a fogger machine (sometimes, you can rent them from your local hardware store). Along with the fogger, you will need a mold controller chemical.

Use the fogger to mist the mold control solution into your air ducts. This process will essentially eliminate mold in areas that are not reachable. 

NOTE – If you use a fogger method, be sure to follow the manufacturer’s instructions. Also, be aware that you’ll probably need to cover all air outlets of your system when performing this task. 

Step 7

  After misting your system with a mold controller, you’ll want to apply an even application of mold inhibitor. This solution will help reduce the possibility of mold growth reoccurring. 

Step 8

  Once completed, reinstall the air vent and monitor your HVAC system in case of new mold growth. If mold keeps returning, it’s best to seek professional help. 

Laundry Detergent for Mold Remediation

A hugely popular method of cleaning air vents is laundry detergent. This is because it’s simple and easily accessible. Granted that this may not be as effective as when mold is professionally treated, this may do for mild cases of mold infestation. Here are the steps on how to use it:

laundry detergent to remove mold

1. Before you commence with the cleaning, turn off the AC or heating. 

2. Afterward, you’ll want to remove the vent grill and place it in a bucket with some laundry detergent. When doing this, make sure the grill is completely submerged. 

3. While your grill is soaking, grab a vacuum with a long stem and begin to hoover out any dust or small debris from the air vents. 

4. Wait around 15 minutes, and then take out the grill from the bucket. 

5. You’ll then want to spray and wipe down the inside of your air vents with warm and soapy water. 

6. Once dry, you can then apply some disinfectant spray inside your vents, vent grill, and surrounding areas. After you’ve wiped the mold, you’ll want to use this to reduce the chances of it reoccurring.

7. Before you turn your AC or heating on again, be sure you completely dry everything. 

8. Then, bin everything you’ve used to clean the air ducts and grill.  

After completing the above, always make sure you monitor your air ducts. If you spot the signs of mold growth (mentioned above), we recommend you seek professional help. 

Ways to Prevent Mold Growth in Air Ducts

After you’ve cleaned your air ducts, I can imagine you want to prevent mold from appearing for the foreseeable future. This is understandable because cleaning or getting your ducts cleaned can be rather frustrating.

Luckily preventing mold growth in your air ducts is much easier than you think. To do this, check out the methods below:

Use Inhibitor Products

You should consider purchasing EPA-registered inhibitor products, as these can prevent mold regrowth in your air ducts. Before applying, be sure to follow the manufacturer’s guidelines to ensure you’re using it correctly. 

Clean Regularly

Without a doubt, cleaning regularly is a great way to reduce the chances of developing duct mold. Moreover, because drip pans collect non-needed water, mold can multiply here. To prevent this, you should clean your drip pans regularly. 


Moisture is the leading cause of mold growth. Due to this, consider purchasing an excellent dehumidifier to prevent moisture from building up. 

Inspect Religiously

You can either hire a professional for this or become knowledgeable yourself. However, you’ll want to check for leaks, mold growth, and any other faults with your system that could be causing a mixture of air. 

Replace Filters

Never overuse HVAC filters. This can be a significant issue as to why there was mold growth inside your air ducts. 

Install UV lights or Ionization Air Purifiers

Consider upgrading your system with UV lights and ionization air purifiers, such as coils or fan units. 

Final Thoughts

As you can see, the reason for mold growth inside your air ducts is because of the mixture of hot and cold air. This creates moisture, which turns your air ducts into a breeding ground for mold. However, after reading, you should now know how to clean your mold-ridden air ducts and understand various prevention methods to reduce the chances of this occurring.

Kenneth Wilson
July 26, 2021

Kenneth Wilson

Retired contractor. Currently residing in Southwest Florida. Now in semi-retirement, I write and manage this blog focused on helping home owners make savvy decisions when it comes to finding contractors and getting their projects done. I also operate remodeling design service for homeowners.

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