# Condensation on HVAC ductwork **URL:** \ **Category:** Factual Questions\ **Created:** [August 3, 2017, 9:12pm UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718 "2017-08-03T21:12:30Z")\ **Posts on this page:** 18\ **Page:** 3
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [August 6, 2017, 2:02am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/41 "2017-08-06T02:02:02Z")
If you don’t get all the edges taped down tightly what you’ll be creating is like a diaper worn with impermeable foil outer pants. The moist air will easily get in through the gaps in the tape job. Then water condense on the extra cold metal. The water will then run off and permeate the batting. But the liquid will be held in by the foil. So now you’ve got a section of permanently soaked batting. Which becomes a ginormous mold colony pretty quickly. You don’t want to build a condensate trap. Instead you want to ensure no humid air can get in contact with the cold metal. That’s a tall order for a retrofit job on a small section of duct working inside a cramped hole in the wall. ---
**Author:** ![Claude\_Remains](https://avatars.discourse-cdn.com/v4/letter/c/898d66/32.png) [@Claude\_Remains](https://boards.straightdope.com/u/Claude_Remains)\ **Post date:** [August 6, 2017, 6:20am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/42 "2017-08-06T06:20:36Z")
> [@iiandyiiii](#): > > We own a first floor condo (DC area). About a week ago we came back from a weekend trip and there was a small wet spot on the ceiling against the wall in the living room. We knew this was under some of the HVAC ductwork, since the vents were nearby. We had a handyman cut out a section of the drywall on the ceiling to look inside. A short segment of the HVAC ductwork was sweating, presumably because of humid air in the ceiling-space and the cold metal of the HVAC ductwork. > > I called an HVAC specialist, who came out and looked at our system (a relatively new heat pump HVAC system). He said this is a common problem in the summer. He noticed that some of the vents in various rooms were closed, and he said that we kept the system running a little too cold (we often have it at 70, and he says it normally shouldn’t go below 72 to avoid this). He recommended opening all the vents and keeping it around 72. He recommended running a fan directed at the open hole in the drywall to dry it out. He said the condensation was probably caused by an excess of cold air in this particular section of the ductwork due to the unequal flow since some of the vents are shut, and since the setting was colder than normal (and hotter weather than normal). > > We did all this, and ran the fan overnight. Voila, the condensation dried up – this morning, the HVAC ductwork was dry. Before I left for work I turned the fan off to see if the condensation would come back without the fan running (obviously we’ll have to close up the drywall at some point, and thus a fan wouldn’t work). > > Unfortunately when I came back from work today the condensation was back and dripping, albeit seemingly slower than before. The HVAC guy did say that if this didn’t work, we should go to Home Depot and buy some foam insulation, cut it into the right size, and put it under the HVAC ductwork to try and insulate it. I’ll plan to do that this weekend, while running the fan in the open space to keep from dripping on my floor (though I have buckets down). > > Is there anything else I’m not considering? I’m worried that I’m going to stuff the insulation in and it does nothing. Not a huge deal, but should I get a second opinion before going forward with that? Any other ideas? As an HVAC tech I would have zipped up the ductwork for you that day and left you happy. Did this “specialist” check the float switch on your drain pan and the drain line itself by any chance? Who Goes on a call only to tell a customer to go to Home Depot on a wing and a prayer? ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [August 6, 2017, 12:32pm UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/43 "2017-08-06T12:32:19Z")
> [@LSLGuy](#): > > If you don’t get all the edges taped down tightly what you’ll be creating is like a diaper worn with impermeable foil outer pants. > > The moist air will easily get in through the gaps in the tape job. Then water condense on the extra cold metal. The water will then run off and permeate the batting. But the liquid will be held in by the foil. So now you’ve got a section of permanently soaked batting. Which becomes a ginormous mold colony pretty quickly. > > You don’t want to build a condensate trap. Instead you want to ensure no humid air can get in contact with the cold metal. That’s a tall order for a retrofit job on a small section of duct working inside a cramped hole in the wall. This makes sense, but I’m not quite giving up yet. Each time I tape up the seams or add another layer, I think I’m sealing up some of these gaps and there’s less condensation when I leave it for a time. I know it’s likely that there will always be a few gaps, but isn’t it possible that if there’s just a tiny bit of bare metal that can be reached, the condensation will be low enough that I’ll never have to worry about it? Hopefully, anyway. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [August 6, 2017, 2:03pm UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/44 "2017-08-06T14:03:05Z")
Note that your dew points have crashed today … set the thermostat to 58ºF to continue your experiment … tomorrow the humidity will be back up but the forecast says dew points go back down in the low to mid 60’s the rest of the week … Perhaps you could lower the thermostat in two degree steps, I’m curious as to what point your wife puts her hands on her hips, taps her foot and gives you that “other” look … all in the name of science mind you … ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [August 6, 2017, 2:21pm UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/45 "2017-08-06T14:21:24Z")
> [@watchwolf49](#): > > Note that your dew points have crashed today … set the thermostat to 58ºF to continue your experiment … tomorrow the humidity will be back up but the forecast says dew points go back down in the low to mid 60’s the rest of the week … > > Perhaps you could lower the thermostat in two degree steps, I’m curious as to what point your wife puts her hands on her hips, taps her foot and gives you that “other” look … all in the name of science mind you … Yeah, I’m going to try and keep the experiment going at least until another high-humidity day, if my wife can put up with the hole in the ceiling for that long! ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [August 6, 2017, 4:13pm UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/46 "2017-08-06T16:13:21Z")
Condensation at the seams makes me wonder if the duct is leaking cold air. Is this problem at a joint in the ductwork? Can you feel cold air blowing in the space? ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [August 6, 2017, 10:00pm UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/47 "2017-08-06T22:00:08Z")
> [@Dag\_Otto](#): > > Condensation at the seams makes me wonder if the duct is leaking cold air. Is this problem at a joint in the ductwork? Can you feel cold air blowing in the space? I didn’t check this before I wrapped the ductwork – it’s possible, but I can’t detect anything, possibly because it’s all wrapped. ---
**Author:** ![Barbarian](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Barbarian](https://boards.straightdope.com/u/Barbarian)\ **Post date:** [August 8, 2017, 1:14am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/48 "2017-08-08T01:14:53Z")
What kind of tape are you using? Ironically, duct tape is pretty useless in this situation. I recommend aluminum tape – which is a bitch to work with, but once attached, is going to last for decades. ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [August 8, 2017, 9:02am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/49 "2017-08-08T09:02:13Z")
Foil tape. Latest update: there’s still dripping. Not a lot - maybe a tablespoon per day, but that’s probably too much. I don’t think I can get all the gaps and seams with the tape, so I’m considering tearing down the stuff I added and using the spray insulation which arrives in the mail tomorrow. ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [August 12, 2017, 3:04am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/50 "2017-08-12T03:04:55Z")
Probably not 1% of houses insulate the duct in the house. (not that they’re a bad idea) But I would want to know why. Towards that end I would want to know: The size of the duct work The Model number on the A/C unit outside. The humidity level in the basement. The return air temperature. The supply air temperature. Insulting may address the symptom and not the cause. ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [August 12, 2017, 3:22am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/51 "2017-08-12T03:22:43Z")
This morning I tore down all the soggy foam insulation wrapping and used the spray insulation (billed as “Anti-Sweat Cold Pipe Insulation Spray”), spraying several coats over all the exposed duct I could reach. There’s one area that I couldn’t reach with the spray can – parallel to the wall and against the back wall, with only a half-inch or so between the duct and the wall. Probably about 8 inches by 4 inches or so of bare metal. After blowing a fan to dry the spray insulation coats, I turned off the fan and left it for the rest of the day. Luckily, today was pretty humid with a high dew point, so it’s a pretty good example day, I think – tomorrow will be even more so, I believe. Results: the spray insulation actually WORKS GREAT! Thanks to whomever suggested it. There wasn’t a single drop of condensation on any parts of the duct that I sprayed. The only condensation was on that single bare patch that I couldn’t reach with the spray can. It looks like it’s a grand total of about a half-tablespoon of water for the whole day from condensation on that patch, based on what was caught in the bucket underneath (not even enough water to pool at the bottom – just a few splatters). I’m not sure what to do about it. My fingers can sort of reach that part, and I could get some more adhesive foam insulation like the stuff I used before, but I’m not sure if that would improve things or just mean more soggy foam insulation. I’m very, very pleased with the spray insulation – it worked as billed, and there’s been no sweating at all over those areas sprayed, including parts of the duct that had heavy condensation earlier. I’m wondering if I’ve done enough and the little condensation left isn’t enough to actually cause damage to the drywall (once the drywall is replaced – still open for now). ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [August 12, 2017, 3:24am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/52 "2017-08-12T03:24:05Z")
> [@raindog](#): > > Probably not 1% of houses insulate the duct in the house. (not that they’re a bad idea) > > But I would want to know why. Towards that end I would want to know: > > The size of the duct work > The Model number on the A/C unit outside. > The humidity level in the basement. > The return air temperature. > The supply air temperature. > > Insulting may address the symptom and not the cause. It’s a 1st floor condo, so no basement – the unit itself is up on the roof of the building (5 stories). As for the other stuff, I might be able to answer some of it with a bit of effort – I’ll get back to you. Thanks! ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [August 12, 2017, 3:29am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/53 "2017-08-12T03:29:23Z")
The insulation spray is actually white and as it is now the duct looks like it’s been frosted by ice – kind of like things left in the freezer too long. ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [August 12, 2017, 3:38am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/54 "2017-08-12T03:38:50Z")
As KPIs (key performance indexes) I’d want to see: Supply air at 55-59 degrees (air at the registers) Return air 75-78 degrees (the air going to the furnace) So…a ‘delta’ or temperature difference of 16-22 degrees. In other words a perfect world has 75 degrees _ **going to** _ the furnace and 55 degrees _ **coming out** _; a 20 degree drop. If the _air in the supply duct is too cold_, or the _ambient relative humidity outside the duct is too high_, the duct will sweat. I’d also want to see if the unit is oversized relative to the duct size. I’d also check the fan speed. (the fan should be at the highest speed) ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [August 12, 2017, 3:49am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/55 "2017-08-12T03:49:54Z")
In other, other words anything that would inhibit air flow will create high temperature difference; a difference grater than 22 degrees. (and temperatures colder than 55 degrees) That could be An overcharged unit A dirty filter A dirty evaporator coil Restricted return air registers or too many closed supply registers A poor envelope: walls,ceilings, windows allowing too much outside ambient humidity Fan speed set to low An oversized unit (least likely but I see it) ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [August 12, 2017, 5:10am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/56 "2017-08-12T05:10:58Z")
> [@raindog](#): > > Insulting may address the symptom and not the cause. Reported. I don’t think we need to resort to this in GQ. 😃 ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [August 12, 2017, 11:50am UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/57 "2017-08-12T11:50:14Z")
> [@Dag\_Otto](#): > > Reported. I don’t think we need to resort to this in GQ > > 😃 Eagle eyed Dag Otto! 🙂 ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [August 12, 2017, 11:30pm UTC](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718/58 "2017-08-12T23:30:07Z")
If you knew how many times I make that mistake myself you would laugh. Sounds like the spray on insulation is a closed cell insulation. Closed cell insulation is pretty good at preventing moisture migration, but I’ve only really have seen it used for pipes. [Previous page](https://boards.straightdope.com/t/condensation-on-hvac-ductwork/792718.md?page=2)