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Garage Insulation Question

rpatrick | Posted in Energy Efficiency and Durability on

Hi All,

I have built a garage with a room in the attic and am scratching my head on the best way to insulate it. The challenge I am having is that I need the Garage and the room to be independently heated and cooled – i.e. I can see wanting the garage heated/cooled while the room is unconditioned, and I can see wanting to heat/cool the room while the garage is unconditioned.

I also need to put A/C equipment in the attic space outside the finished room so I want to have a un-vented, conditioned attic space.

Attached is a diagram for reference. The thick pink lines all represent surfaces I know I need to insulate. The questions are how to insulate the surfaces labeled a, b, c and d.

Since I may want the ROOM to be heated/cooled while the GARAGE is not, I suspect I need to insulate surface ‘a’ (in fact I think code requires this as it assumes GARAGE is not conditioned but that is beside the point). But then I need to either insulate surfaces ‘b’ or ‘c’ or both. This is my real question – which of these or both should be insulated?

I also think that surface ‘d’ can be un-insulated as this is essentially completely contained above the ROOM and would be considered part of the conditioned attic above the ROOM – right?

As a bonus question, I am using open-cell foam for the insulation for air-tightness. Do I need to add a vapor barrier anywhere either inside or outside the foam?

I really appreciate any help you can provide – the cost of foam is crazy right now and aside from the cost I don’t want to waste it anyway.


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  1. Expert Member
    PETER G ENGLE PE | | #1

    Your location (climate zone) will help us answer properly.

    FWIW, open cell foam is not a particularly efficient or cost effective way to do air sealing, especially if you can DIY some of the air sealing work yourself. Then you can use any fluffy insulation and save all of that foam and carbon cost.

    You can insulate surfaces a and b to separate the garage from the room upstairs. No need to insulate c. And, you don't need to run the pink line all the way to the ridge. You can go across d instead and save insulation, using deep fluffy insulation on d. This is cheaper and saves energy. You can also install air channels between the joists spanning the triangular attic spaces so that you can still do a vented attic above the insulation. If you haven't found it yet, this article may help:
    Whether and what sort of vapor retarder you need depends on where you are located.

  2. rpatrick | | #2

    Hi Peter - thanks for your reply.

    Location is Central Virginia which I think is zone 4 (?)

    I am curious why you say open cell is not good for air sealing? Not doubting you but also acknowledging this is not what the manufacturers say - any more info you can point me to appreciated.

    I will take a look at the article on knee walls.

    Again thanks for your pointers!

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