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Community and Q&A

Fixes for Failed Duct Leakage Test

1910duplex | Posted in Mechanicals on

Hi all,

I took your advice and paid extra for a duct leakage test when I got a ducted minisplit installed.
It failed due to a panned return (which only had sheet metal on one side) and lack of sealing at the registers.

The duct blasting company said there should be some kind of silicon ring added in the system near the ducts, and that sheet metal should be done on all four sides of the return.

The installation company said they want to move the return duct from below the floor of the attic to above the floor and put mastic between the registers and the plaster at the ceiling ducts.

Is either of these fixes the better way to go, or are they both okay?

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  1. user-2310254 | | #1

    I’ll give your question a bump. Is this the only return?

  2. 1910duplex | | #2

    Yes, there's only one return, as the system only serves upstairs. So return is in the hallway, there are registers in the three bedrooms and the bath, and there are transom windows to get the room air into the hall.

  3. user-2310254 | | #3

    So currently the supplies are in the (unconditioned?) attic and the single return is inside the conditioned space. Is that correct? Can you attach a couple of photos?

    1. 1910duplex | | #5

      I guess you would say the attic is semi-conditioned. It does not have a heat or cooling supply, but it has 2 inches of closed cell foam against roof decking, covered by r-23 or r-16 rockwool batts, and the rafters are furred out with 1.5 inches of polyiso; the gable walls are covered with 1 inch of closed cell foam and fire-resistant thickness of drywall. There are three small old windows in the attic, which are covered with plastic in the winter; they also did air sealing at the top of the plates (balloon framed house) at the time of the foam job. The party wall between our side of the duplex and the other side is not insulated, but they also installed some level of insulation at their roof line (I think just fiberglass batts under drywall).

      Some pictures are below of what the attic was like before the ducts were installed. The Duct blaster service said the duct runs (sheet metal) were all well sealed, that it just seemed to be where the flex duct went down into the attic flooring and then through the plaster to the register that was the issue, and, as I said, the panned return was not properly done.

      Do you want pictures of the return mechanicals in the attic?

  4. walta100 | | #4

    The way I see it if you were smart and found a way to keep all your ductwork and equipment inside the conditioned space duct leakage is mostly irrelevant.

    Yes your leaky supply duct work will not deliver the exact amounts of air to exactly the points in the house your plan called for but my guess is it will be close enough that when you adjust the dampers all your rooms will be about the right temp.

    If you home has a more or less open plan as most homes as is the tendency today and if you are the type of people who tend to leave their doors open or ajar return ductwork also tends to be irrelevant so long as the flow is not restricted.

    I accidently ran a return duct experiment for about a year. My contractor happened to have forgotten to install 2 pans on the ends of my return ductwork so almost all the return air was coming from the basement. After the repair was made my comfort level was unchanged with the exception that after the repair some of the return grills would resonate until I changed the angles of the louvers.

    If you have leaky ducts in a vented attic insist the installer repair his work at his expense and retest.


  5. 1910duplex | | #6

    We don't have a vented attic. Attic is insulated at roofline with flash and batt and along walls, and top plates are sealed. Installer is going to repair at his expense and retest, the question is more which is the right way to repair -- with mastic at the registers in the rooms and moving from an incomplete panned return to a return above the attic floor, or to just put the sheet metal on all sides of the pan and the silicon ring the duct contractor talked about?

  6. user-2310254 | | #7

    I'm not an expert, but this article ( indicates that supply leaks are worse than return leaks. If it were my house, I would want to avoid using a leaky pan as a return. I'd seal up the leaks or find another way to return conditioned air to the plenum. I'd be less concerned about small amounts of supply air spilling into a mostly sealed attic.

    But let's see if one of the experts has an opinion on this.

  7. walta100 | | #8

    The way I see it you need the ducts in your attic conditioned to leak because if they don’t, the dew point of the air in the attic is likely to be higher than the temp if the attic walls from time to time. When this happened the walls will get wet if they get wet enough and stay wet for long enough mold will grow and things could rot.

    When you chose the conditioned attic you chose to take the bitter pill of supplying enough conditioned air to the attic to keep the attic at more or less the same temp and humidity as the rest of the conditioned spaces.

    If your ducts don’t leak enough you will need to add registers in the attic or risk mold.

    I have to ask have you removed any and all of the old insulation from the attic floor?


  8. 1910duplex | | #9

    There was never any insulation at the attic floor. Before we did the flash and batt there was a wooden floor in the walk-up attic and no insulation anywhere.

    I don't think we're going to have a dew point problem in the attic, as the attic is hot and humid in the summer and cool and dry in the winter.

    Before this system was installed a couple of months ago, there were window units in some upstairs rooms and nothing anywhere else, and hot humid conditions did not cause damp walls anywhere in the house above ground, windows open or windows closed.

    The basement is another issue, and we run a dehumidifier down there.


  9. walta100 | | #10

    "the attic is hot and humid in the summer and cool and dry in the winter."
    Remind me what, is all that insulation you paid big buck for doing if the attic conditions are more or less the same as the outdoor conditions.

    If the R 1.5 plaster/drywall ceiling has a bigger temperature difference from top to bottom than your insulation then insulation has a lower R value than the ceiling.


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