Another sweating duct thread!


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Old 07-26-12, 06:26 PM
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Another sweating duct thread!

I just discovered part of an attic crawlspace that I never knew was there while replacing a medicine cabinet. There are two ducts inside, each leading to their own vents. One duct elbow (the taped one in the pics) was detached when I first found it. I reattached it and discovered a ton of condensation happening primarily at the elbow. I taped the crap out of it, figuring it was leaking enough air to cool the outside surface of the vent, but it's still getting very wet when the A/C is running.

The duct in question is a straight, unbranched shot from the blower and is therefore moving much more air than the other one. My first guess is that that's why one is sweating and the other isn't. Also, if you can tell by the pictures, the sweating elbow is more of a direct 90* bend than the other.

In any case, what's the quickest way to remedy this so I can seal up the wall and put my medicine cabinet in? I have an extra roll of pink interior wall insulation. Can I simply wrap the ducts in that?

The crawlspace/attic is considerably warmer and more humid than the rest of the house, if that factors in.

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Old 07-27-12, 05:23 AM
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All duct in the attic and non conditioned spaces needs to be insulated with R-6 duct wrap and sealed this will stop the issue
 
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Old 07-27-12, 04:07 PM
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Thanks, airman.

I wrapped and sealed the ducts today (not fun). I tested the A/C and now I'm getting condensation on the register boxes that the ducts connect to. Also, I can't tell if there's condensation where the ducts go into the attic floor, as I couldn't insulate and seal all the way down.

I think I now know why the previous homeowner had disconnected the one duct. Now that I think of it, would it be an option to air condition the crawlspace to reduce/eliminate condensation via a Y-fitting, or is this just crazy talk?
 
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Old 08-11-12, 05:36 AM
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The only way to prevent condensation is to prevent warm/moist air from coming in contact with cold surfaces. This is done with duct wrapping insulation with a vapor barrier and commercial grade duct tape. If this is not possible, you must pull new R6 or better insulated flex duct and get rid of the old metal duct. Any metal register boxes must also be insulated/sealed from crawl space air. You obviously have a DIY project done by someone without knowledge of proper ducting procedure. A professional would NEVER do what your pictures show.
 
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Old 08-11-12, 06:54 AM
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He can't run flex it's in a wall
 
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Old 08-15-12, 06:19 AM
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Then he is going to have to open up the wall and do the job right or continue to have the condensation problems. Repairing a bad job is always twice as hard/expensive as doing the job correctly the first time. His only other option as I see it, would be to liquid foam over the entire ducting area using a foam that will not expand and break the walls. That would be a touchy job working through a medicine cabinet opening though.
 
 

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