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Air/Vapor Control Method with Future Work in Mind

BrunoF | Posted in General Questions on

air / vapor barrier for shared wall of future finished bonus room?

My house build is moving along but we are over budget and as a result, we will opt to finish off the bonus room above the garage in a few years.  I did have the HVAC crew run the lineset for a mini-split but that is as far as I am going to go at this point.

The overall house is sheathed with taped OSB as the primary air barrier but there isn’t any sheathing installed between the main house and the future bonus room.  Once finished, that wall will have drywall on both sides and be climate controlled but in the meantime what should I use for air sealing that won’t lead to a lot of additional work later?


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  1. Expert Member
    DCcontrarian | | #1

    Put the building envelope in its ultimate position. It won't cost any more and will simplify everything.

    1. BrunoF | | #2

      So go with drywall on the initially “attic” side?

      1. Expert Member
        DCcontrarian | | #3

        You may not even need drywall on that side. In fact drywalling it may cause your inspector to treat it as finished space which triggers all sorts of requirements for things like electrical, smoke detectors and HVAC.

        But you want it inside the building envelope. You want the exterior of that room, including the floor over the garage, to be insulated and air sealed to the same standard as the rest of the house. There's really no energy penalty to having additional unconditioned space within the building envelope.

        Moving the building envelope after the house is finished is a royal pain. You don't want to go there.

        1. BrunoF | | #4

          If I bring it in the airspace and insulate it it will essentially be finished off. I’m already over budget so we can’t finish the bonus room yet; was just looking for a good way to ensure the integrity of the rest of the envelope. The bonus room has knee walls, vaulted ceiling sections and a shed dormer…I don’t think I can air seal those ceilings without drywall.

          1. Expert Member
            DCcontrarian | | #5

            Let's make sure we're both talking about the same things when we say "finished."

            To me "finished" means drywall finished and painted, trim installed, finished flooring, lighting and receptacles. I guess it also means having HVAC.

            What it doesn't mean necessarily is inside the building envelope. You can have both finished and unfinished space inside the building envelope. I suppose you can even have finished space outside the building envelope, like with a screened porch or something.

            You can save some money by not finishing space that was meant to be finished space. You will save zero money by trying to move the building envelope and move space that was meant to be inside the building envelope outside of it. You'll probably end up spending more, and potentially make a mess of your house in the process.

          2. Deleted | | #7


  2. freyr_design | | #6

    Just use proclima intello plus.

    I don’t know what your primary air control layer is on the rest of the structure but as DC says I would detail that out now to be contiguous with that room. You don’t even necessarily need to insulate if it is your exterior wrap that is the air barrier but I would make sure the exterior and air barrier are in place now. Then for the wall in between use the intello and insulation to segment it off for now.

    1. Expert Member
      DCcontrarian | | #8

      I'm going to push back here.

      You can't just arbitrarily move the building envelope and turn an interior wall into an exterior wall. As a practical matter, where I am the energy code requires R20, which means a 2x6 wall, interior walls are normally 2x4. But more important, how are you going to have a continuous thermal layer? How are spots like rim joists going to be detailed?

      Yeah, you could probably do it if you tried. But it's not going to save any money. Which is the whole point of what the OP is trying to do.

      The broader point is that it's almost impossible to save money by making design changes mid-build. Really the only opportunity is to cheapen the finishes or omit details.

      1. freyr_design | | #9

        I see where you are coming from and shooting from the hip in that I have very little data, you are probably right that getting it to insulation won’t cost a whole lot more, but getting to drywall could.

        It also doesn’t seem all that arbitrary, as it is over a garage, which may or may not be in the thermal envelope. I think if you sat down and looked at a section you could figure out how to make it work in a couple of hours. Insulating a rim joist doesn’t seem like that big a hurdle, and furring out a wall to 2x6 is not hard either.

        I think it boils down to whether the builder that is building it thinks it will save money, as he will have a much better idea than you or I. And if he says it will but he needs an air barrier for a wall to make it work, intello is a good option as you can just leave it in place.

        The amount of info you have is not enough to say if it will save money or not.

        1. Expert Member
          DCcontrarian | | #10

          >The amount of info you have is not enough to say if it will save money or not.

          Exactly. And we should be urging him to talk to his builder. For all we know his contract has a penalty clause for change orders and any change is going to make construction more expensive.

          I got a bad vibe from the original question. The assumption was that this change would save money, he needs to back up and ask whether that's true to begin with.

          1. freyr_design | | #11

            This is a good point, I had just assumed it had already been discussed but if not it certainly should be.

          2. BrunoF | | #12

            This isn’t about saving money, it is about spending it later. I was hoping to be able to finish off the bonus during the initial construction but it will be a few years until we spend that money. I also do not have any penalty clauses for changes with the builder; it is a simple, fixed-fee deal so I can make adjustments as needed.

  3. Expert Member
    Michael Maines | | #13

    Is the garage below fully drywalled with 5/8" type X and fire-taped? If not, you'll need drywall on the bonus room side for fire safety. And if that is your thermal boundary for now, you should insulated it with batts that can be removed and re-installed later.

    Or do you mean that the bonus room is fully insulated, you just don't want to cover the insulation for now? That will lead to condensation on your sheathing if you don't at least install a variable permeance membrane.

    1. BrunoF | | #14

      The garage below will be fully insulated and dry walled to the FR code as required. The future bonus room will be left as open framing with the exception of the continuous floor sheathing which is already in place.

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