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Community and Q&A

Garage/Attic insulation dilemma

Michael Fannin | Posted in General Questions on

I’m finishing an 896 square foot garage in SW Washington (zone 4C) that has in-floor radiant heat. It has a 10’ ceiling built with attic trusses and therefore a 12’ x 32’ inhabitable space running down the center above the garage which we plan to use for heated storage.

I have already insulated the attic space to R-31 in the ceiling (batts and foam board) and R-21 in the sidewalls. I plan to insulate the garage and would like to use the radiant heat to keep the garage at between 55 to 65 degrees for working during the winter months.

Because of the heated livable space above and the moderate temperatures planned for the radiant heating in the garage, I’m unsure if I should insulate the entire ceiling to R-30, the ceiling below the inhabitable floor space, or just the ceiling of uninhabitable attic space. In any case, my intent would be to have the vapor barrier to the winter side of the inhabitable attic space and to the winter side of the garage in the uninhabitable attic spaces.

Any recommendations are appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, you don't want to include a vapor barrier in your wall or ceiling, especially if "vapor barrier" means polyethylene. All the code requires is a vapor retarder. You mention that you are using "batts and foam board" for insulation, but I don't know whether you are installing the rigid foam on the exterior side of the batts or the interior side of the batts. In either case, you don't want an additional vapor barrier.

    You mention that you will be keeping your garage between 55 degrees and 65 degrees, but you don't mention the temperature range that you plan for the "heated storage" area above the garage. If the heated storage area will be kept at the same range of temperature, then there is no reason to include insulation in the ceiling between the two spaces -- unless you want the flexibility to stop heating the storage area at some point in the future.

    I don't know if you are planning to provide a heat distribution system in the heated storage area, but it's possible that the hydronic system in your garage floor will provide enough heat to keep the storage area warm, without any additional heat source upstairs.

    If you leave a portion of your ceiling uninsulated, be sure to get your air barrier details right. You will need blocking between the floor joists under the insulated walls upstairs, and the blocking will need to be carefully sealed with caulk.

  2. Michael Fannin | | #2

    Thanks Martin,
    I meant vapor retarder, referring to the kraft paper face on the batts. The actual insulation consists of R-38 batts on the ceiling and the sloped walls contain vent baffles, R-30 batts, and 2" foam board (R-10) on the interior side of the batts. The vertical walls are R-21 batts.
    Heating will be provided by a woodstove in the adjacent studio on the upper floor and a small wall heater. I have not installed and currently do not plan to install heaters in the attic rooms as of yet. I will use one of the two rooms in the attic for a bedroom while I build our main home, but eventually I don’t feel there will be a need to heat these spaces. None the less, I installed ducting and a dedicated HRV system to help control interior moisture in the upper floor (including the attic rooms).
    I’d like to maintain the upper floor temperatures between 63 to 68 degrees, thus my dilemma on how best to install the insulation. I guess what I should be asking is should I keep the vapor retarder facing the garage over the entire ceiling or should I install foam board blocking under the attic walls and flip the batts in the center section under the living spaces? Or should I install un-faced batts and use vapor barrier paint on the garage ceiling?

  3. GBA Editor
    Martin Holladay | | #3

    Vapor diffusion almost never leads to problems in walls, roofs, or ceilings. The only exceptions are greenhouses and swimming pools in very cold climates.

    In your case, with two adjacent rooms, one of which is at about 60 degrees F, and the other of which is at about 65 degrees F, your worry about vapor diffusion is wildly off the mark. There couldn't possibly be any diffusion problems in a ceiling assembly separating two indoor heated spaces at almost the same temperature.

    Therefore, it doesn't matter which way the kraft paper faces.

    For more information on vapor diffusion, see Vapor Retarders and Vapor Barriers.

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