AC plenum in crawlspace sweating?

AlberichJuly 15, 2012

I had a brand new Trane ac and plenum installed last summer in my crawlspace, and to date it has been a nightmare dealing with a poor install. The (national) company who sold and installed the unit has already been out 7 times to "fix" issues. Last time, they took almost everything apart, put it all back together with new insulation and parts, and sealed everything with mastic "goop" and tape. The air in the house is now finally cool and dry--so it feels like they (finally) got the job right.

However, one week later I just discovered that water is puddling on the outside of the insulated sheet metal plenum and dripping off the connections to the ducts. I can't tell if it's happening on the inside too . . .

I live in the southeast, and the weather is hot and humid. However, it's only been in the upper 80's this past week. The crawlspace under the brick house is on a slope ranging from 7 to 4 feet in height and is not insulated. There are 8 vents in the brick that we open in the summer and close in the winter, and there is a sheet of plastic ~4mm thick on the floor.

We didn't seem to have this issue before with the previous AC, which was made out of duct-board.

Can anybody please tell me what is going on, how this issue can and should be fixed?

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If the metal plenum is sweating, then there is an insulation problem. They might have used just a sheet metal plenum with out any internal which case they should have wrapped it on the outside. I would take some pictures and give the installing company a call.

Hope this helps

    Bookmark   July 15, 2012 at 9:51PM
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Thanks for your reply, but they did cover the outside of the plenum with insulation. Therefore, the water/condensation is collecting on the outside of the insulation. What can I do about that, and how would I know if there was moisture underneath the insulation as well?

    Bookmark   July 16, 2012 at 2:01AM
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It seems likely to me the problem may be excessive moisture in your crawlspace. If the crawlspace is vented as most are, consider that you have hot humid air surrounding the outside of the building. Now you bring that humid air into the crawlspace where it is nice and cool..The humid outside air reaches saturation point in the crawlspace and anything just slighly cooler like you ductwork is going to sweat. Check the humidity in your crawlspace. Solution could be sealing the crawlspace to installing a dehumidifier. I believe that the the latest trend in new crawlspace construction is to seal and insulate them to prevent moisture and mold problems.

    Bookmark   July 16, 2012 at 10:26AM
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Consider the possibility that outside air is leaking past poorly-installed insulation and water is condensing on the duct. That wets the insulation rendering it useless so you get condensation on the outside vapor barrier.

    Bookmark   July 16, 2012 at 1:23PM
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