GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Air-Sealing Around Casette-Style Minisplit Heads

idahobuild | Posted in Energy Efficiency and Durability on

I am considering using the sheet rock on the ceiling as an air barrier (tied in to exterior walls) for our new house.  I know they’ll be a lot of detailed sealing to do around fixtures; and I’m willing to methodically take on that challenge.

I am wondering how to seal around the cassette-style, mini-split, HVAC system while maintaining accessibility to that equipment for maintenance down the road.

Any thoughts on this would be greatly appreciated.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Akos | | #1

    I would build an air tight cubby a good bit larger (to leave room for pipes and service) than the unit out of plywood or rigid insulation. You can mount this cubby in-between your ceiling joist and seal the ceiling air barrier against it.

    These units are not noise free, I would not mount one into a bedroom and if possible use gravity drain instead of the built in condensate pump.

    1. idahobuild | | #2

      Helpful. I'll see if I can detail that out. Thank you.

    2. idahobuild | | #4

      Great input. The gravity drain makes'll be close to a bathroom.

      I am considering a ducted mini-split to supply 2 bedrooms, guest bath and hallway. I suppose I could do the same thing; with the added precaution of insulating the short ducts and providing passage of the ducts through the 'cubby'. Approx. layout atch'd. Am I missing anything for this config?

      1. Expert Member
        Akos | | #6

        How much of this is already built? You can simplify your life by raising the ceiling in the bed/bath area to 9' and installing the air handler into a dropped soffit in the hallway area. In a house with decent air sealing, good insulation and reasonable sized windows the register can be just above the door and provide pretty even temperature in the room.

        If you must go into the attic, the cubby can also work. Make sure to leave plenty of space for the refrigeration connection and add structure at the top to hang the air handler.

        I would home run the ducting for each register so there are no splices in the attic and air seal the register boots and duct connection by encapsulating it in spray foam with one of the smaller two part spray foam kits before the attic insulation goes in.

        I would look at a membrane product for ceiling air barrier that can be installed before the drywall goes up. This would give the option for a blower door test to check for air leaks. Much easier to fix air leaks here plus any cracks in drywall down the road won't effect leakage.

        1. idahobuild | | #7

          The current plan is to wrap the exterior in 2” of XPS foam, buck out the doors and windows, air seal the OSB sheathing (tape or some peal/stick option) and apply batts or blow-in to the cavities. I haven’t modeled it out yet because I want to get some of the details sorted before getting the manual J,D,etc. done (I’ve been in contact with a couple of folks). I will have a 3rd party do those and ask a local HVAC to build to the spec.

          We are currently in the planning stages of the build. I am thinking through the details so that I can document them for construction—no architects available in my area (too busy). We do have a company helping with the drafting of the plans.
          Ceiling height is currently set at 9’ throughout the single level house (approx. 2065 sqft).


          OPTION 1:
          I initially considered using spray foam in the attic; under the roof deck, to about R-40 (I think R-38 is code in ceiling in ID) for a good air barrier/thermal barrier. However, when the quotes started to come in, I began to investigate other options. One alternative that I’ve considered was to raise the heels of the trusses to allow for blown-in insulation above the ceiling (including over the tops of the walls) and air sealing at the drywall (tied into the exterior air barrier via a seal under the trusses).

          OPTION 2:
          I had thought of using a membrane on the ceiling (under the trusses—as I think you are suggesting). In my mind, I guess that I can’t reconcile that putting up the membrane is much different that using the drywall as an air barrier. Either way I think it is an exercise in diligently plugging holes. So, I am thinking that using the drywall, air-barrier option would work. That’s what drove me to ask this HVAC-related question (it and related parts are ‘penetrations’ in the air barrier).

          Can I not use a blower door if there isn’t a membrane at the ceiling? I thought that a well-sealed drywall layer would have the same affect.

          WITH OPTION 2 (membrane or drywall)
          I could drop a 1’ soffit in 3 places: 1) guest hallway, 2) great room at the kitchen wall and 3) the laundry room. Then, if I’m understanding your suggestions, I’d need to keep the registers coming out of the soffits into their respective supply area (see atch’d floorplan). Not sure if that would work in the Great Room and provide adequate air circulation---which is what I think you are getting at with the statement that ‘…house with decent air sealing, good insulation and reasonable sized windows the register can be just above the door”.

          Home runs back to for each register does make sense and I’ll incorporate that into the details—along with the spray foamed registers idea.


          1. Expert Member
            Akos | | #8

            When it comes to hvac, 3 zones are about 3x the cost of a single zone. Ducts are cheap, equipment and install is not. Your layout is pretty spread out, so two zones might be worth it to simplify ducting.

            Plus based on your description of build goal, the bedroom zone will never have enough heat load to need even the smallest ducted/ductless indoor unit. Best to combine it with other areas of the house. If the two other bedrooms won't be used often, it would make sense to have those on its own zone and the rest of the house on a single zone.

            For slab on grade new build, I would look at plenum trusses for the roof and put the HVAC and ducting in there. This would let you have a clean flat ceiling everywhere while keeping the HVAC inside conditioned space. It does mean more expensive trusses and some extra OSB but not a huge cost for a new build.

            The reason for using a membrane for the ceiling air barrier is compatibility with standard build workflow. To be able to test and fix air leaks, you need the air barrier up but the rest still accessible enough to touch up leaks. If you are using drywall, this means getting the drywall crew out twice to the job site, once to hang and tape the ceiling and the 2nd time for the rest of the drywall. This adds cost and lead time, it is simpler to use a membrane for the ceiling.

            For a single story structure, armed with a pneumatic staple gun and a roll of quality tape, installing a ceiling membrane air barrier is a weekend DIY job. Once this is up the HVAC, ducting, plumbing and electrical can go in at which point you can do a blower door test to check your air barrier and fix any leaks.

            If you want to improve building efficiency, the lowest cost and most effective way is to simplify your building envelope. It doesn't mean you have to build a Monopoly house type of gabled box but try to keep walls simple. For example, your main bath looks to be a couple of feet less than the bed beside it. Loosing this jog means fewer corners, simpler roof line and slightly larger bathroom.

            This might be a good read for a similar house plan:


            P.S. XPS is not the greenest or cheapest option for wall insulation. It uses a pretty high GWP blowing agent that is release as it ages. GPS/EPS or polyiso are a better option. Around me roofing polyiso seems to be the best $/R value, plus since it is higher R value than XPS you need less thickness.

        2. idahobuild | | #9

          I have some questions, but I want to do some reading on a couple of the topics first. I hope you don't mind if I take a day or two to look into the plenum trusses and central air handler--including a call to the local truss guy.

          More to come regarding trusses and zones...

          When I looked into the Intello membrane, if my memory serves me, it was quite costly. In the mean time do you have any recommendations for a more cost effective membrane option?


  2. greenright | | #3

    Keep it simple- install insulation blanket squares cut to size above the cassettes and staple them to the joists. It can be even done to cassettes where you do not have access from above in which case you install the blanket, staple it and then lift the cassette in place from below. You will have to cut the drywall larger on the side where the line set goes in and then patch it once the connections are made. You absolutely want to seal well around the cassette as if you are not using a remote sensing thermostat and rely on the cassette to sense the temp via the air flow it will suck cold/ hot air from the attic and grossly overshoot the set point. Ask me how I know….

    1. idahobuild | | #5

      Seal well. Thanks Greenright. See #4 above.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |