Why is My Ductwork Sweating

Why Does Ductwork Sweat?

I recently received a great question on one of our crawl space encapsulation posts:

Hi! I am in the process of encapsulating my crawl space and when checking the insulation around the duct work, it is completely soaked. Do I need to replace the insulation over the duct work or can I remove all duct work and leave the ducts non-insulated?

If you’d like to read the article:

Crawl Space Encapsulation Questions


It is a great question that many homeowners ask dealing with improving the indoor air quality of the home. Sweating ductwork can affect the home by causing humidity to increase and saturate whatever space the ducts are located. Not to mention humid air being passed to all parts of the home via the ducts.

The Ductwork Sweating Problem

My answer to the homeowner was to address the root cause of the problem. I feel many people only look at fixing the symptom of sweating ducts. She asked if she should remove insulation, which seems logical, but that will not stop the ducts from sweating.

The ducts are sweating because of high humidity in most cases. Leaky ductwork will make the problem even worse. The more cold are escapes the ducts and hits the warm air in the crawl space, the more the ductwork sweats.

image of ductwork sweating

Should I Remove Duct Insulation

The only reason to remove the insulation is if it is moldy or really damaged, but removal should not be permanent. Removing the insulation will expose the cold ducts even more; fixing the humidity problem will solve the issue.

If you remove the insulation, replace it after you air seal the ductwork. This will make sure the cool or warm air produced by your HVAC system is taken to the areas intended. In the winter, ductwork without insulation

Will a Dehumidifier Stop Sweating Ductwork

The number one question we receive on our YouTube Channel and comments is “Do I need a dehumidifier?” If you are a DIYer or looking for someone to fix the humidity problem in your attic, basement, crawl space, or whole home, the dehumidifier is the best solution.

Contact Crawl Space Ninja to Solve Your Moisture Problem!

Do you need help fixing a humidity problem or ductwork sweating in your home? Please click here to contact us. Also make sure you check out the video below and please leave us a comment on what you have done to improve your home’s energy efficiency.

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12 thoughts on “Why is My Ductwork Sweating”

  1. Great channel! I have a newer home and am considering installing a dehumidifier in the crawl space. ( seems humid, duct work causes condensation which drops onto the crawlspace floor which is covered in plastic). My question is: Do I need to encapsulate for the dehumidifier to work properly? I was going to seal the vents ( saw your video on that ).

    1. Great question George. I think installing a dehumidifier and sealing the vents is a great step in correcting the condensation problem you face. Just keep in mind that the better the ground is covered and the less hot humid air infiltrates the crawl space, the less the dehumidifier will run. If you install a small dehumidifier with very little power, it may not keep up. The 1820 by Aprilaire is great but if your ground is exposed and the wood moisture level is high, it may not keep up either if your crawl space is fairly large. This would mean you should step up to 1850 or 1870 to make up for the encapsulation not being installed. I hope that helps. Here is a link to our dehumidifiers if you want to read more about their specs: https://diy.crawlspaceninja.com/product-category/dehumidifiers/

  2. Matthew Lariviere

    I recently hired a mason to brick up the vent holes and “anywhere you can see light coming in.” It worked awesome. RH holding 35-40% only 12 hours later. Home was built in 74. Now I notice a dirty sock smell coming from the vents when the AC comes on and I think its the old duct insulation probably soaked in mold from 40 years of humidity. My HVAC unit is 2 years old, MERV-11 at the unit, changed every 3-6 months.

    I currently have a 70 pint basement dehu hacked under there with a pump with plans to swap it out for the 1830. Already a dedicated circuit from the old HVAC humidifier.

    Do I have the right idea to finish it off by getting a powder coated steel insulated crawl door from grainger, remove the duct insulation permanently and mastic paste all the duct joints to finish the job?

    That would leave me at hiring someone to pull the old vapor barrier out and rewrapping 1200sqft. Ive been quoted anywhere from 6-12k for just a rewrap.. doesnt sound right.

    Is my plan a good plan and whats a reasonable cost to just wrap floor, walls and pillars?

    To top it off I put a smartthings multisensor in there to keep tabs on light levels, motion (rodents), and RH. Very effective.

    Burlington, NC

    1. Hi Matt, properly installed vapor barriers for walls, pillars and floor can cost more than most expect due to the labor element of doing it correctly and the company standing behind the warranty. Be sure to get several estimates and compare company reputation would be my suggestion. I like the removing duct insulation and mastic but I would also reinsulate if your local code requires it. If it was my house, I would anyway to create a thermal break between ducted air temp and crawl space temp even if not required by code. Hope that helps. Thank you.

    1. Hi Tanner, we see that quite a bit. Normally it is due to cold metal duct boots sweating and leaching water into the subfloor. Do you have a dehumidifier installed under the home? If not, that may help.

  3. FL home on a concrete slab built in 1970. Air handler is in a very small utility closet just big enough for the unit. I noticed paint/cracking peeling on the ceiling where the ducting runs in the attic. It’s much more apparent closer to the air handler and tappers off at the furthest vents. The drywall is really wet around the vent closest to the air handler. The utility closest is musty and I can see water stains on the walls.

    I can only access half of the house through the attic. I know it’s crazy but, I have no way of getting above the air handler in the attic to actually see what’s going on. I’m assume it’s sweating pretty bad. It’s been extremely humid and good amount of rain. I ruled out a water leak just because the paint seems to follow all the ducting.

    It looks like my soffit vents are covered with insulation but I can’t get to them. I have two gable vents but they are on the same side of the house so no flow. Would improving air flow in the attic with two more gable vents opposite of each other, gable vent fans and a whole house dehumidifier solve this issue?

    Wish you were closer so I could have you all come out!

    1. Daniel, in your heat and humidity, I would get the humidity controlled in the attic. Not sure ventilating it will solve anything as more heat and humidity may enter. There is a new trend called attic encapsulation that I am a fan of, it seals the attic similar to how we seal crawl spaces from the outside environment. Then install a dehumidifier to control attic humidity. Sounds like your attic is already sealed. My concern is if you vent, with your AC in the attic, the condensation may get worse. Also controlling attic humidity may not control living space humidity. I’d recommend researching attic encapsulation and check local contractors and codes for their opinion. Here is a video we did for a whole home dehumidifier install without using HVAC ducts, this would be great for controlling living space humidity. Hope that helps. https://youtu.be/DSO0V42ofoY

  4. We continue to have condensation building up between our main floor and basement, resulting in drywall damage in the basement area. We have sealed around the exterior of the home, had the home checked for foundation issues (there were none) and installed a return vent in the room to help with circulation – we even installed a pergola to provide some shade for the area in question.

    What would you suggest we do so that this does not continue to be an issue and something we continually have to address?

  5. Michael, we live in the South, very humid (Macon, GA). Bought a 1960’s Ranch Home for our daughter. Just installed a sump pump to take water out, we have a vapor barrier on ground, but the HVAC Air Ducts are sweating….a lot! if we were to install a dehumidifier in our crawl space, does the whole crawl space need to be encapsulated or will the dehumidifier be okay without the crawl space being completely enclosed, with just the vapor barrier down? We are getting quotes this week from 2-3 HVAC Companies to give us an opinion on air ducts sweating, but wondering about a dehumidifier. Thanks for your help!

    1. If crawl space is not encapsulated then you may want to step up the dehumidifier size to make sure it is pulling enough moisture to eventually rest. You cannot oversize a dehumidifier. If you can at least have ground plastic, that would be great. The sweating should stop once humidity/dew point are controlled. If you need help sizing, here is a video: https://youtu.be/0vwAOiiWFyw?feature=shared Hope that helps.

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