My air return system is in a small closet in my garage. When I run the A/C when it's hot and humid out, the ducts sweat and drip a lot of water onto the wood platform that the HVAC and water heater sit on. The homeowner before hand must have not cared and never did anything about it.

I on the other hand don't like seeing all the water laying around, and I don't want it to eventually rot the wood to death.

What is the best way to insulate sheet air ducts?

Here are some pictures of my fiasco too.

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    Apart from being unsightly and potential rot, this is money going out of your pocket and into the electric company's pockets. A nontrivial amount of money - ducts in unconditioned space should be HEAVILY insulated. On the unsightly rot front, I'm unclear if your condensate drain pipes are also dripping right on the platform... – Ecnerwal Jun 15 '17 at 0:38
  • And close the damper on the humidifier which appears to be open.(top picture) Close summer, open winter. – d.george Jun 15 '17 at 10:26

(links are to Google image search)

With foil and fiberglass duct wrap you have to use tape, and be able to get all the way around or it won't stay there (the tape has to lap over its other end or it will not hold).

I'd recommend self-adhesive foil and foam duct wrap.

You might want to do the easy-to-reach and the majority of it with fiberglass, and use the self adhesive where you have to, to keep the cost down.

Surface prep is to wipe with alcohol - do not use mineral spirits. And of course; to be dry.

  • I'm going to go with your option of wrapping it in self-adhesive wrap. Thanks for your input! – DrZoo Jun 23 '17 at 18:36
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    I was looking for a different question I asked and saw this one. Update, a year later. The self-adhesive foil and foam duct wrap had amazing results! My wrap job was a bit sloppy in some areas because of space issues. Nonetheless, there is no more condensation, and no more wet boards/floor. This was an excellent, and inexpensive fix to the problem. – DrZoo Jun 22 '18 at 20:42

You are experiencing what I call the , "coke bottle effect". When you take a coke bottle out of the refrigerator, it meets that warm room air and starts sweating (poof...condensation). For your situation, it's when the cool air hits the warm metal duct.

To fix this, you can 1) insulate the outside of the duct (so it doesn't warm to room temperature), 2) insulate the inside of the duct so the cool air never meets the warm metal duct, or 3) both...

1) Insulating the exterior of the duct does not guarantee you'll keep the duct isolated from the warm room air...in fact, the duct will probably rise in temperature as the closet rises in temperature.

2) Insulating the interior of the duct (yep, they make ducts like that,) will "protect" the cool air from meeting the metal duct. This is better, but not perfect, depending on the thickness of the insulation, temperature difference, etc.

3) To do both, I'd just change the metal duct to a prefab insulated duct without metal. This will work best in your case.

Or, you could install a metal "condensation drip pan" on the floor of your closet. We use to provide these under air handlers before we learned to install a condensation drain to the exterior. The principle of the "condensation drip pan" is that the moisture will evaporate before the pan overflows. I don't recommend this (and you'll need to careful to fit it around the duct penetrating in your floor), but it is an alternative.

  • It may be semantics, but your explanation of what's happening is not quite right. The cold air in the system is cooling the ducting. When the warm air comes into contact with the cold duct, it cools down and gives up some of the water its holding. You can prevent the condensation by preventing the duct from cooling down (interior insulation), or keeping the warm air away from the cold duct (exterior insulation). – Tester101 Jun 15 '17 at 11:03
  • @Tester101 Hmmm...Yes, I agree, but doesn't the same thing happen in reverse when, as he says, "...When I run the A/C..."? Isn't this cold air hitting warm metal duct? – Lee Sam Jun 15 '17 at 16:01
  • Yes, the cold air does hit the warm metal duct, but how does that matter? The condensation is caused by the air around the cold duct losing heat, which causes it to also lose water vapor. – Tester101 Jun 15 '17 at 16:14
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    @DrZoo since the ducts already exist, then insulating the outside is easiest. – Tester101 Jun 16 '17 at 2:41
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    @LeeSam Ah, but does "the coke bottle effect" occur in the Atacama Desert? – Tester101 Jun 16 '17 at 13:35

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