I'm getting a lot of condensate on my HVAC ducts that run though my first floor (and HVAC blower) especially in July / August when humidity levels are outrageous (in NJ). The 1st floor space space is rather frigid as one face of it is below grade so it's hard to convince the occupants to open the registers to dry it out (at least not without opening the windows and letting the humid air back in). However even on days when humidity is lower, I can occasionally notice a small amount of condensate build up which has clearly led to mold growth under the previous owners care. Further compounding things is the natural tendency for the cold air to make its way back down to the ground floor via the small gaps near return / air filter area and in the registers themselves.

I know most people say to just insulate it, but the previous owner did this in a few places (with neoprene peel-n-stick in some places, and standard batt in others) and it looks terrible, and it appears with standard batt insulation just provides another surface for mold to grow on.

Is there any sort of water repelling coating that that I could use to prevent the condensate from building up in the first place the way RainX works on car windows?

Is there some other trick I should be using to reduce humidity or condensate build up on the duct work?

  • 1
    @dandavis, I'm not sure getting the condensate to drip more quickly is going to solve the overall dampness problem -- it's just likely to transplant the location where the mold grows.
    – virtualxtc
    Commented Aug 6, 2018 at 20:27
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    rainX is paraffin; a traditional waterproofing that's been used for centuries on all manner of materials. I for one find it useful on toilet seats to avoid surprises. reduces shower and sink maintenance as well.
    – dandavis
    Commented Aug 6, 2018 at 20:42
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    You don't have a condensation problem. You have a humidity level problem. Don't treat the symptom--run a dehumidifier. Even if you insulate your ducts, your humidity level is so high that you're sure to have other issues.
    – isherwood
    Commented Aug 6, 2018 at 20:51
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    You might try increasing airflow by running the fan at a higher speed to reduce the temperature differential. I'm in Minnesota, where we regularly see temps in the upper 90s with dew points in the 70s. The issue you describe isn't a problem for most of us and shouldn't be for you.
    – isherwood
    Commented Aug 6, 2018 at 20:59
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    Yep, we're all talking about dew points in the low to mid 70s. There's nothing special about Florida humidity. The physics of it is the same.
    – isherwood
    Commented Aug 6, 2018 at 21:11

2 Answers 2


I call it “The Coke bottle” effect. When you take a coke bottle out of refrigerator, it sweats on the WARM side.

The same thing happens when you start pumping cold air through your metal ducts in a warm atmosphere. (And vice versa for warm air.)

Paint doesn’t isolate the metal duct surface so you’ll have condensation.

I’d look to “duct insulation” which is made specifically for ducts. Some are self-sealing and have a peel and stick adhesive so it would be easy to apply.

  • 1
    I agree there is a thin neoprene self adhesive sheet that is amazing at eliminating this problem but it is over 5x the cost of fiberglass duct wrap. The trick is keeping the warm moist air away from the duct, if sealed up with insulation there won't be any moisture to grow mold.
    – Ed Beal
    Commented Aug 6, 2018 at 14:53
  • I've been researching this all night as I'm in the middle of a reno which is giving me access to the top side of the ducts that is too close to the ceiling to normally paint. Someone in a forum recommended spray foam over the peel-n-stick stuff as it will completely seal and provides a better R-value, however, I can't figure out how I'd apply it evenly, or get it to form into a shape.
    – virtualxtc
    Commented Aug 6, 2018 at 20:31

My contractor started installing a soffit before I was ready, so I ended up using expanding foam as a way to insulate isolate my ducts and shaved it to size before it was fully enclosed. It worked, but I'd advise against using regular spray foam (like I did) as it will expand and cause your ducting to shift out of place.

If you go this route, choose the window foam variety which is designed to be "minimal-expanding" as to avoid warping and shifting your ducts (both polyurethane and the latex varieties should work).

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Alternatively, you might try a 'no sweat' kind of product I found while previously researching this, but why this answer is currently ranked at -1 as I couldn't find any studies or reviews of how well they work:

Anti-Sweat Spray Insulation 18 Oz. Aerosol Can

This Franklin Machine Products 143-1128 No Sweat Cold Pipe Insulation Spray

enter image description here

  • One thing to keep in mind. Whatever "coating" or treatment you use for this is going to be outgassing or otherwise getting into the air stream and people will be breathing it continuously. Remember, people used to line air ducts with asbestos and that lead to a lot of sickness and death (a friend of mine died from that after working in a state office building for 20+ years). You may find that these treatments will not be qualified for that kind of application. Read the MSDS (Material Safety Data Sheets) on it before you buy anything.
    – JRaef
    Commented Aug 6, 2018 at 22:08
  • @J.Raefield good point, however, according to the manufacturer of the last product, it's supposedly 'food grade' safe
    – virtualxtc
    Commented Aug 6, 2018 at 22:17

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