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Community and Q&A

Ducted mini split sweating

GreenLeopard | Posted in Mechanicals on

Hello, I’m in zone 2A and recently had a 2 zone ducted Midea minisplit installed. The system is running well except for some sweating issues on hot/humid days.

I have 4 registers where airflow must go through a 14×5.5″ joist space, 6″ flex duct is joined to the end of custom made metal boxes insulated with R4 on the inside, R6 flex duct is joined via a metal collar on the end of the box, the end is insulation only (no metal).

The issue is massive sweating on the bottom and top of the box near the duct on hot/humid days. I thought the R4 is insufficient and the cause, but my installer says it may be thermal bridging from the collar and metal tape to the metal box, causing the metal side to be too cold and condense.

We thought maybe using a plastic or custom collar may help, or should I add more insulation? Can I place insulation on the outside of the box, or will that be a double vapor barrier and cause more issues? Spray foam the outside? Use polyiso or fiberglass?

Thank you for any guidance and help, even the installer is stumped on this one.

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  1. GreenLeopard | | #1


  2. GBA Editor
    Martin Holladay | | #2

    Q. "Can I place insulation on the outside of the box, or will that be a double vapor barrier and cause more issues?"

    A. Of course you can place insulation on the outside of the box. The box is part of your duct system. When your air conditioner is operating, cold air is flowing through the galvanized box. The galvanized steel gets cold. You get condensation.

    So insulate the galvanized box. I would use rigid foam on all sides, installed in an airtight manner, with seams sealed with canned spray foam.

  3. GreenLeopard | | #3

    Thanks Martin, did you read that the inside of the box is insulated already with fiberglass? My concern was creating some kind of double vapor barrier by insulating the outside as well that would trap moisture somehow? If this won't happen in this case then I'll definitely do as you suggest.

  4. GBA Editor
    Martin Holladay | | #4

    The galvanized steel is vapor-impermeable. It's already a total vapor barrier. There is no way that the addition of exterior rigid foam is going to change the vapor permeance of the assembly. It's a vapor barrier either way -- with or without the rigid foam.

    My guess is that the fiberglass insulation installed on the interior of the galvanized box is sloppily installed. Exterior insulation is better in any case -- and it's easier to verify that exterior insulation is properly installed, without any defects.

  5. Anon3 | | #5

    Buy the sticky insulation from home depot, that's the easiest way to make it air tight. This is a solved problem.

  6. GreenLeopard | | #6

    I see, thank you for educating me. Very much appreciated, have a good day!

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