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Community and Q&A

Insulating an attic-based minisplit indoor unit & ductwork

Bdgray | Posted in Green Building Techniques on

Our home has a newly installed Mitsubishi ducted minisplit. There are now two ducted units in the attic: both the “Concealed Slim Duct” SEZ-KD12NA (there is a 3rd unit within the conditioned space that I’m less worried about). After a long-fought battle with my wife, the indoor units were installed in our attic. I know doing this was dumb but aesthetics won out over efficiency. I’ve posted on this forum separately about moving the conditioned space to the roof rafters. $60K bids basically killed that idea, at least for now. Right now I just want to band-aid the problem.

This weekend I will be applying CC sprayfoam to encapsulate the ductwork (two component system bought online).

I have three questions:
1) Should I also insulate the minisplit inside unit? If so, how? If I do anything, I assume it should be something removable like a rigid polyiso “box”.
2) On the ductwork, is there a recommended thickness to cover it? I was thinking 2-3″ and a bit more at the ceiling penetrations.
3) For the CC spray foam, will I need to cover the finished foam with anything for fire retarding?

I plan to do this work myself unless advised otherwise. I’ve worked with sprayfoam before. It’s nasty stuff but pretty straightforward.

Thank you,

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  1. Jon_Harrod | | #1

    Hi Brian,

    1) Yes, you should definitely enclose the air handler. A polyiso box will work as long as it is well-sealed (especially at the floor connection and duct penetrations) and crush-resistant. I like the idea of a fixed box (seams taped, spray foamed at the floor and duct penetrations, reinforced with 2 x 2s) with a removable access panel for service.

    2) 2-3" closed cell foam will meet recommendations for duct insulation in all climate zones.

    3) Technically you should have an ignition barrier (DC315 or similar paint) over the spray foam (and the polyiso, if it's not a product like Thermax with an integrated thermal/ignition barrier). The paint may be available locally through a specialty store. You can apply it with a brush or a commercial-grade sprayer.

    With the foam, be sure to follow recommendations for respiratory protection, ventilation, and re-occupancy.

  2. Bdgray | | #2

    Thank you Jon. Two follow up questions:
    1) Do you think it would be ok if the poyiso is in direct contact with the air handler (which is a minisplit, not a traditional airhandler)? Having viewed everything this morning, it looks like it will be quite easy to simply cover the split, the main branch and the return with taped rigid foam. I would then save the CC spray for the take-offs and maybe a thin layer over the seams of the final layer of rigid.
    2) Is there any reason to not to keep adding more r-value if the ductwork can accept it? The handler and ducting is suspended ~6" above the top of our existing insulation (~12" of fiberglass). There is really no practical limit to how much rigid I could tape around the main branch and return. If I wanted a foot of rigid encasing this, I could do so (at some point I'd start compressing the batt insulation beneath but I don't think that would be a bad thing).

    I would certainly feel better about this attic decision if it were insulated and sealed out the wazzoo.

    Thank you,

  3. Jon_Harrod | | #3

    The SEZ manual (which you can access at talks about insulating the air handler for condensation control, so it seems like direct insulation contact is okay.

    I see no problem in going to higher R value on the ducts or air handler, subject to diminishing returns on your efforts. I would just make sure you leave a way to access the unit needs if it service later on.

  4. BuildersPrideCons | | #4

    You can spray intumescent paint on the foam for fire retardant. 1/2 to 1” closed cell is more than adequate for ducts, you can buy closed cell sheets of armacell and cut to size for air handler 1/2” is enough, or buy off brand k flex it’s 1” same price as 1/2” armacell should cost you about 100 to do each handler

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