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?
I’m going to agree with your HVAC guy (and **Mind’s Eye, Watering **) … as much of your duct work as possible should be insulated … the water is coming from the humid air resting against the cold metal and condensing on the metal … even a 1 inch layer of insulation with slow this down by a good margin, and what little water does condense will be absorbed by the fiberglass and not stain your new drywall …
In your specific situation, the insulation on the duct work may not save very much energy … but if we look at the general case then this insulation could save a meaningful amount of money … and the less fossil fuels we burn the less risk to the solar system we’ll cause …
There’s no insulation on the bottom of the ductwork – there’s some on the side that’s almost against the wall. If there should be, then that’s presumably a problem with the original construction, though that was in the 80s, so it didn’t appear to cause a problem until now.
I had that, it was a older house where the uninsulated heating ducts were converted for use for a/c, and they did sweat. It didn’t really cause a problem due to where they were (exposed), but I did wrap insulation around a section that would usually drip in the laundry room and that solved the problem.
Water vapor in the air will condense on a cold surface. Duct insulation, usually fiberglass, will prevent heat loss. However water vapor, being vapor, will be able to permeate the fiberglass to the cold duct surface and then condense. Only this time the fiberglass insulation will be there to soak it up. Wet insulation does two things - it doesn’t insulate worth a damn, and it allows a place for mold to grow.
What to do? Install a vapor barrier on the outside of the insulation. This is usually foil, which is impermeable to water vapor. With the barrier, water vapor cannot pass through the insulation and reach the duct.
The vapor barrier needs to be complete though - holes in it will allow vapor to get through, which is why all holes in the vapor barrier are sealed, usually with silver foil tape (Should be UL 181 listed).
I’m planning to get some of that insulation spray, plus some foil-wrapped insulation to install, and then I’ll “test” it all weekend with the hole still open to see if there’s more condensation. I really appreciate everyone’s input – thanks so much!
I would suggest skipping the insulation spray. It’s a mess to apply and will be an issue to remove if you do get a mold problem. The insulation wrapped around the duct should be sufficient.
Also, foil-backed insulation is probably not required. You can get paper-backed insulation that provides a decent vapor barrier. (The manufacturer applies some substance to the paper that makes it pretty impermeable.) I’ve had ducts insulated with this in NC, where they were exposed to very high humidity, and experienced no problems whatsoever over a 20-year period. Either way, don’t cheap out.
But is IS necessary to seal all the seams with good quality tape.
From dealing with this in the past, I’ve learned that moisture buildup on duct work/vents could be caused by the following:
[ul]
[li]Thermostat temperature set too low-- I have to keep mine a 74 degrees or higher.[/li][li]Uninsulated ductwork[/li][li]An oversized condenser for your square footage that’s short cycling and cooling the house too fast and before it can remove humidity[/li][li]Too many blocked or closed vents[/li][li]Poorly insulated windows allowing moisture into the home[/li][li]The evap coil being dirty[/li][li]The fan speed on the control board running too fast* (probably the least likely scenario)*[/li][/ul]
The temperature could be part of it – right now we have it at 72, and there seems to be a little bit of condensation (earlier, at 70, it was a significant amount). 72 already feels too hot for me and I really hope I don’t have to turn it higher.
Uninsulated ductwork is almost certainly a part of it, but it’s funny that it’s just a tiny part of the ductwork that is sweating (about 2-3 feet). In order to insulate all of the ductwork in the condo I’d have to tear out most of the ceiling and probably spend thousands of dollars. I’m hoping I can fix this problem just by insulating this one problematic section that’s already open with a hole in the drywall.
There were a couple of closed vents, and we opened them. This might have slightly lessened, but not eliminated, the problem.
Not sure about the windows. There’s a big sliding glass door to the patio very close to the problem area.
Don’t know about the evaporation coil or fan speed. Not even sure how to check that.
If the system is being maintained properly and the filter is changed regularly, then the coil is probably clean. The fan setting would depend on the system and generally is tinkered with. I just wanted to throw them out there as slight possibilities, since it’s a frustrating situation to deal with.
My duct work was only sweating on the west side of the house for some reason, I’m not really sure why. They still sweat when it’s really hot, but seem to dry out over night now rather than staying wet for weeks and causing mold growth.
Thanks for the input. I ordered the spray, since it’s not available locally, but maybe I jumped the gun based on a couple of recommendations above. I also plan to go buy some wrapped insulation and install it this weekend. Assuming I make sure it’s fully wrapped, does it need to be attached flush against the metal of the HVAC duct, or can it just sort of “rest” against it? Or should it rest on the top of the drywall (under the ductwork) to insulate the few inches of air-space between the ductwork and the drywall of the ceiling?