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Community and Q&A

Sweating ducts In conditioned attic

MikeF223 | Posted in General Questions on

Hello all, first time poster here. 

I have been renevating our home and one of the steps that I did was add two bedrooms to the attic area and close off the attic and make it a conditioned space. 

The roof sheathing has a combination of closed and open cell foam placing R value about R-38, which is good my region. 

I had an LG ducted minisplit installed in the conditioned attic space and they used metal ducting in this area. 

I was checking on something up there today and found the ducting near the output of the unit sweating pretty badly. 

My concern is I may have an air leak in the foam somewhere causing this or should I be tracking down another issue. 

Ducting doesn’t have any insulation on it and being conditioned area I feel the contractor didn’t think it needed it. 

Any help or thoughts would be greatly appreciated. 



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  1. Expert Member
    Akos | | #1


    Might be worth while to check the dew point of the air near the unit compared to the rest of the house. There are online converters for going from RH and temperature to dew point. Use the same meter as RH measurements can vary a lot from meter to meter.

    If the dew point is higher by the AC unit then it could be just a case of not enough conditioned air going to the area where the unit is mounted.

  2. Jon_R | | #2

    A common dew point for fully conditioned summer air is 60F. If they aren't insulated, the exterior of AC ducts is less than that.

    You could buy a spray foam kit and insulate the duct.

  3. Expert Member
    Dana Dorsett | | #3

    Giving the mechanical room even a modest amount of supply air from those ducts would remove the moisture from the conditioned attic and the problem becomes self correcting- the more the AC runs, the drier the attic air becomes.

    1. joshdurston | | #4

      Agree with Dana, sealing and insulating an attic doesn't make it conditioned, unless you actually heat and/or cool it.

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