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My A/C unit has ducts that run to the second floor, into the attic, to feed some ceiling vents.

Today, after several days of brutal humidity, I noticed paint blistering, and in one spot bursting, around where the ductwork comes upstairs.

When we bought the house (also in the summertime), we noticed some blistering, and the inspector thought it might be from the roof. The seller replaced the roof, we had it re-inspected, and it was judged to be sound.

This water is in the same place. Since we hadn't had any rain today, I suspected it was the A/C, so I cut out the most-affected piece of drywall. Sure enough, there is an uninsulated metal pipe installed right up against the drywall, and the condensation is easily felt.

My question is: what can I do about this? For now, I've cut out the affected drywall and drilled a few other holes in affected areas, hoping to get some airflow through there at least.

But what should my near and long-term strategy be? Do I need to replace this metal section of pipe? (It looks like there's an elbow there; the space is not big enough for the plastic insulated flexible ductwork that we have downstairs). How can I prevent this issue from happening? Would a permanent vent into this space help?

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The best way to combat the condensation problem is to insulate the riser duct, aka metal duct, that is causing the problem. There are several ways to do the job. The best way is to use a vapor-barrier & spray-on foam insulation, but that can be costly and messy, unless done by a professional. Another is to use Rock-wool to insulate the duct, but often there is not enough room in the wall to use it. The insulation that I prefer to use is a Duct-wrap. It looks like bubble-wrap with small "bubbles" sandwiched between 2 layers of foil-lined plastic. It all depends on how much money you want to spend and how much space there is between the duct and the drywall.
The short-term solution, is to install a duct-wrap. It is available at home improvement stores in the building products department in the insulation section. It generally comes in various lengths at a 4 foot width & can be cut to size with scissors (or a razor-knife, if you are careful). To install it, simply measure the distance between the studs and add at least 12 inches to tuck the insulation behind the duct OR add 4 inches & staple it to the studs like traditional fiber-glass insulation. Cut the duct-wrap to size. You will also need to get metal (duct) tape to seal the seams between the upper piece and the lower piece of duct-wrap. It has a paper-backer to keep the glued side from sticking to itself. You will need to peel the paper-backer off when you install it. The Long-term solution would be up to you. You could hire someone to take apart your existing ducts and insulate them or you could do the job yourself, either way that is the only solution to your problem. And by the way, cutting a hole on the wall will only make matters worse by giving the humidity a direct path to the now un-insulated ducts.

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The humid air I believe is coming from outside, through the ridge vent, into the attic crawlspace, which is right near where the issue is happening. It looks like I'm going to have to completely re-do the drywall in the closet, since there's no way I can get to everything without cutting it out. –  Arkamis Jul 21 '13 at 13:19

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