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Dealing with unconditioned garage

Jackie_M | Posted in General Questions on

Hi all,
Looking for some clarity on our plan for insulation/air barrier for our unconditioned garage. Climate zone 6. 
Key points:
-garage is attached to two walls in the home and attic above the garage is unvented and semi-conditioned. 
-house, including garage, will have significant (R-24 on walls and R-36 on roof) exterior insulation and an exterior air barrier (Intello X). On the inside of the home, we will have r-22 Batt insulation on all exterior walls. 
-The attic above the garage ceiling connects to an attic above the master bedroom and as I mentioned, the attics will all be unvented and (semi) conditioned with Zehnder and heat pumps. This so that we could air seal the exterior of the home rather than ceilings. 

My question is what do I do with the walls and ceiling that adjoin the garage? We are considering a second air barrier where we would apply Intello on the inside of the garage and then add batt insulation for those walls and the ceiling the join the garage. I am thinking the batts would go in the stud cavities in the master bedroom, mudroom and rafters rather than in the garage walls. I am wondering if these second layers of air barrier and insulation are necessary and if they may cause any issues with moisture. Intello X is vapour open and insulation will all be Rockwool. 

Appreciate any input. 
Jackie

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Hi Jackie,

    How are you meeting code with R-36 on the roof?

    On your question, you do want to create a continuous air barrier between the garage and the living space. You can use caulk, tape, foam, and gaskets to seal the wall between the two areas. A blower door test might be helpful as well. (See https://www.greenbuildingadvisor.com/article/questions-and-answers-about-air-barriers for more information.)

    You also want to insulate the wall, but the fiberglass batts (and rockwool) are air permeable and won't help with air sealing.

    Intello X is an exterior-grade product. Maybe you mean Intello Plus, which is half the cost? (I understand Certainteed Membrain is cheaper still but a bit fragile.)

    You might want to read this article as well: https://www.finehomebuilding.com/project-guides/drywall/airtight-drywall.

    Maybe one of the professionals will chime in.

  2. Walter Ahlgrim | | #2

    I treated all the wall between the house and insulated garage much like any exterior wall except I skipped the exterior foam insulation.

    Some of your choices make me wonder why.

    R24 wall with a measly R36 on roof. If you said R24 and R60 I think great choice. If you said R19 and R38 OK it is a code minimum build.

    Semi conditioned attic? Vented attics are great. Conditioned attic can be OK. When you play the semi conditioned game the air in the space will fall below its dew point from time to time. Then the moisture will condense if it happens often enough things will get moldy and rotten.

    I see no reason to condition an attic unless someone is desperately trying recover from poor design choices like HVAC in the attic or half story buildings.

    Walta

  3. Jackie_M | | #3

    Thanks for the feedback. I can clarify a couple of things. The roof is actually R-58 - R-36 exterior insulation and R-22 batts. We have limited space on the roof to add without hitting other roofs so 9-inch of exterior comfortboard was all that we could fit. The walls total R-46.

    As for the unvented attic, I can appreciate the debate in this area, but we wanted to do proper air sealing and felt that for our retrofit, from the exterior was going to be most effective. We are using Zehnder (two systems) to ensure that humidity is controlled and will heat/cool as needed to ensure we don't have issues with condensation. Given our climate zone and the published studies that I've read, we are (admittedly just) at the correct ratio to avoid our interior walls hitting dew point and condensation occurring.

    Intello X is an interior/exterior membrane and it's what we have for the exterior so I was just thinking that it will make more sense to use this for the garage than purchase a whole roll of Intello Plus. Price difference is minimal.

    As for the attached garage, it sounds like we are on the right track. Our exterior insulation will wrap the garage, but then we will add batt insulation to the interior, attached walls. And we will air tight the garage as a separate envelope.

    Thanks for the input.
    Jackie

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