Unable to seal AC/Heater air duct located on roof

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Old 10-25-10, 10:53 PM
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Question Unable to seal AC/Heater air duct located on roof

Hello everyone,

I recently bought my first home and have inherited a stubborn water leak that seeps through the air duct and into my son's bedroom. The house has an old AC/Heating unit that is located on top of the roof. It has two air ducts that connect from the unit into the house providing heat and AC.

What I've done so far:

Hired a contractor to replace shingles in the area, (did not know duct was the problem at the time) We later discovered it was not the roof but one of the air ducts.

Next, I peeled off the old tape around the duct, bought air duct sealer and applied with a paint brush all around the joints and connections, then I bought a specialized water proofing tape called eternabond and applied it to the base and around the joints. Lastly I applied Henry's mastic to the base area. At first it seemed to work, but after several water tests the water came through.

I do not know what else to do or try. A buddy suggested I use duct tape and then apply metallic paint over the duct tape to weatherproof it. Another idea is to find some kind of membrane and apply it to the duct, apply duct tape, then seal it with some kind of mastic. The last resort is to find a specialist and hire him to do the job. Any other suggestions would be appreciated.

I will be posting pictures of the unit and what I've done so far.
 
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Old 10-25-10, 11:09 PM
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Old 10-25-10, 11:16 PM
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Old 10-26-10, 05:49 AM
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Did you use a waterproof/weatherproof mastic? I assume you are using the air conditioner and not the furnace. It is happening only when it rains?

I would work hard to figure out where the water is entering? You've done a nice job with the mastic and duct tape so it's hard to believe the water is coming in through the areas you treated.

Could the water be from condensation on the outside of the cold ductwork? I'm wondering if water is condensing on the outside of the cold, uninsulated ductwork in your attic. Then the water runs down the duct and into your son's bedroom.
 
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Old 10-26-10, 06:57 AM
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I recently had to do some drywall repair on just such a leak, Pilot Dane. The cold A/C duct was condensing in an area just above the drywall where an untaped seam was letting cold A/C air out into the hot, humid attic. It was resulting in a constant drip of water.

If this leak isn't related to rain, and it happens only when the A/C is running, that's a definite possibility.
 
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Old 10-26-10, 07:00 AM
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Hi, yes the mastic is weatherproof and the unit is an an AC unit and heater. This problem only happens when it rains. When we run the water test, we leave the water running for and hour and half and that's usually enough time for the water to begin seeping through the dry wall.
 
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Old 10-26-10, 07:21 AM
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Can you get up into the attic? Hopefully you can do your water test while your attic and better see where the water is coming from. You seem to have been thorough with the mastic and tape so I imagine you'd spot a possible leak site in the duct. Have you given the roof penetration a serious look over?
 
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Old 10-26-10, 08:04 PM
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Yes, there is access to the attic, we (me and my dad) crawled and located the area but assumed it's coming within the tube. I am gonna suit up once more, climb inside and see if it's coming from within the tube or the surrounding area.
 
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