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Condensation on Ductwork – What Causes It and How to Fix It.

Condensation on Ductwork – What Causes It and How to Fix It.

It might not sound like it, but condensation on ductwork can be a serious problem, causing all sorts of secondary problems like leaking ceilings and mold infestation.  You might not think that this is an issue that people in Santa Clarita have to deal with, but most people with condensation on their air ducts don’t even know about it.  To be perfectly honest, condensation on ac ducts is more of a problem in the south where humidity levels are generally higher, but it can happen anywhere in the country, even in California.  In this article, we will address how this condensation forms on your ducts, why this can be a problem, the two causes of condensation on ductwork, and what you can do to fix it.

How Does Condensation Form on Ductwork?

condensation on ductwork - sweating ductsIf you are already familiar with what causes condensation, then skip to the next section.  If not, take a few minutes to review some of these basic principles.  It will help you when we start to troubleshoot a condensation on ductwork problem.


In order to understand why you have condensation on ductwork, you first need to understand how.  Before we dive into the HVAC aspect of this article, we will need to have a brief review session of high school chemistry.  Whether or not you realize it, there is water all around us.  Water is even in the air we breath in the form of water vapor.  In fact, that is what humidity is; it is a measure of how much moisture is in the air.  However, air can only hold a limited amount of water.  When the air is holding as much water as it can possibly hold, the humidity level is said to be 100%.  Likewise, when the air is only holding half of the water that it can hold, then the humidity level is 50%.

Dew Point

Now that we know what humidity is, we can talk about the dew point.  We know that the air can only hold a limited amount of water, and that when it reaches this limit it has 100% humidity.  But the amount of water that air can hold is not a constant, and it varies greatly depending on temperature and pressure.  As the temperature of air goes down, the amount of water it can hold also goes down, causing the relative humidity level to rise.  The atmospheric temperature at which the air is saturated with water is called the dew point.  Once the air temperature drops below this dew point, dew will begin to form.  The process by which water begins to condense from a gas into liquid form is called condensation.


Condensation forms when the water vapor in the air drops out of suspension and condenses into a liquid form.  The colder the air, the more likely it is that water will condense.  Condensation forms when warm, humid air comes into contact with a colder surface like your air conditioner’s ductwork.  This colder surface then chills the surrounding air to a temperature below it’s dew point which causes the water vapor to condense into a liquid, just like on the outside of a glass of iced teas sitting outside on a hot day.  More information on condensation can be found in this old-fashioned school video: Condensation and Evaporation.

Why is Condensation on Ductwork a Problem?mold from condensation on ductwork

Who cares?  So water is condensing on your AC ducts.  Why is this a problem?

When water condenses and drips into your attic or home, it can cause a some serious secondary problems: First, water dripping off of your ducts and onto your insulation can cause the insulation to compress, which decreases it’s R-value, or ability to insulate (for more information on R-value, what type of insulation and how much of it to use in your attic, try: What Type of Insulation Should I Use in My Attic and How Much?).  Second, when water condenses and drips off of your ductwork it can cause your ceiling to leak, which can rot drywall and cause your ceiling to collapse over time, not to mention it looks horrible.  Finally and most importantly, condensation on ductwork can then drip off of your ducts and feed mold growth.  Mold can be a health issue and needs a constant supply of water to thrive.  For more information on mold and how to prevent it, try: How Do You Prevent Mold?

The Two Causes of Condensation on Ductwork

As we saw in the ‘how does condensation form on ductwork’ section, we get condensation when warm, moist air comes into contact with a cool surface.  Well what is cooler that your air conditioning ducts?  As such, the only way that water can condense on your ductwork is if one of two conditions exists: First, if you don’t have adequate insulation on your ductwork, and second, if there is excessive moisture in the air.

Effect of Inadequate Insulation on Ductwork

Insulation works because it is very inefficient at transferring heat.  Think about it, if you touch the metal side of a frying pan on the stove, you get burned right?  This is because metal is very good at transferring heat and also why it is a horrible insulator.  Likewise, if you grab the plastic or wooden handle of a frying pan on the stove, then it will not burn you because these materials are bad at transferring heat.  That is why they are such good insulators.

If you have the proper amount of insulation on your ductwork, then condensation will not form because the warm, moist air will never come into direct contact with your cold ducts.  Instead, it will hit your warm insulation and move on.  However, if insufficient insulation exists then this warm air can still get to your ductwork, causing condensation on ductwork and all of the problems addressed in the last section.

damage from condensation on ductwork - sweaty ductsEffect of Too Much Moisture in the Air

Just like when you have too little insulation, if you have an excessive amount of humidity near your ductwork, either in your attic or crawlspaces, then you increase the likelihood of condensation forming.  Think about it, condensation on ductwork probably isn’t a big problem in Phoenix, Arizona, but why?  Because the average humidity level is 10-20% in Phoenix.  However, an attic in Pensacola, Florida is likely to have a humidity level closer to 100% and the risk of condensation on ductwork would then go up quite a bit.

Prevention and Treatment of Condensation on Ductwork

Fortunately, unlike some of the other HVAC problems we’ve addressed like, why is my heater blowing cold air or why is my furnace not blowing air, condensation on your AC ducts is a very easy problem to fix.  Like we addressed in the previous section, there are really only two causes of condensation on ductwork: inadequate insulation or too much moisture in the air.  The solution for each of these is relatively easy and cheap, and if this is a problem you’re facing then I’d just recommend you do both.  The easiest way to accomplish this is to have your ductwork completely replaces with modern, insulated flex-ducting that has a vapor barrier build in.  If you are a do-it-yourselfer or are on a modest budget, here are some alternatives:

Solution for Inadequate Insulation on Ductwork

If you don’t know where I’m going to go with this then you probably aren’t even reading the right article.  Add insulation!  Bare sheet metal is the worst case scenario, but any uninsulated ductwork needs to be addressed.  Buy insulation in batts or blankets and begin snugly but not tightly wrapping your ductwork.  If your wrap it too tightly or compress it with duct tape then you will reduce it’s R-value, defeating the purpose.  Again, reading What Type of Insulation Should I Use is a good place to start for information on R-values and different types of insulation.  The insulation you use to wrap your ducts should leave no gaps and be at least two inches thick to work effectively at reducing or eliminating condensation on ductwork.

Also make sure to use HVAC tape and seal all of the joints between duct segments.  No air should be able to penetrate the ductwork and the joints are a natural weak point.

Moisture in the Air – Wrap Your Ductworkcondensation on ductwork crawlspace

The second problem that needs to be addressed it the natural moisture in the air.  The most common cause for this is inadequate ventilation in your attic.  Your attic has vents for a reason!  If there is inadequate ventilation in your attic, then contract your local roofing contractor for information on adding roof vents.  Otherwise, moisture will build up and you can face possible mold problems in your attic.

The next step is to ensure that you add a vapor barrier to your ductwork.  Most modern ductwork comes standard with insulation and a vapor barrier, so my recommendation would be to just replace your ductwork.  However, special attention should be given if your ducts do not.  Add a vapor barrier by buying vapor barrier wrap available at Lowe’s and other hardware stores and following the directions included.  It will consist of covering every area of your ductwork with at least one layer of wrap.  If there is a section of ductwork in your basement or a crawlspace that is lying on the ground, try to suspend it off of the ground and also add a layer of vapor barrier wrap on the ground to decrease moisture absorption into the air.

If you live in a particularly humid area of the country, then consideration should be taken into purchasing a dehumidifier for your attic or crawl space.  They are relatively affordable and usually run about $250.  They have a collection reservoir and you can either empty their water bucket every few days or add a drain to them that runs outside.  Either way, removing humidity from the air in your attic might be the most cost-effective way to handle your problem.

Final Thoughts on Condensation on Ductwork

If you still haven’t figured it out, something seems pricy and you want a second opinion, or something your contractor is telling you just doesn’t seem right, save yourself some money and consider using one of our most experienced air conditioning technicians to guide you through ASM’s online Air Conditioning Consultation Service.  It will save you money in the long run.

At the end of the day there are really only two causes for condensation on ductwork: improperly insulated ductwork and excessive moisture in the air.  Both of these are easily treatable.  Remember that condensation can cause all sorts of problems, from leaks to mold infestation so take the time to address this problem as soon as you can.  When the time comes or if you have the money, the best thing that you can do to solve this problem is to just have your air ducts replaced with modern ductwork that includes insulation and a vapor barrier built-in.  If you don’t live in the Santa Clarita or Greater Los Angeles area, or are still in the research stage then I’ve included a guide on how to find a reputable contractor in your area: How to Choose a Contractor.  I hope this article has helped solve your sweaty duct problem.  For more information on other related air conditioning and heating topics, visit our ASM Air Conditioning Blog.

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