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Community and Q&A

Insulating the garage ceiling – bedroom above

4BrJ9ioRhU | Posted in Green Products and Materials on

I am in a 4000 DD cold climate with humid summers
An HVAC system is installed with no HRV. A/C is run in the summer

The room gets cold during the winter. Currently there is a duct between the ceiling and the garage that is covered with drywall (i can’t tell whether it’s suppy or return). the height of the bulkhead is maybe 3 feet.

The plan so far is to remove the flimsy, decrepit garage doors with new thermally insulated ones, and to do the best to eliminate air drafts.

1. If the duct is not insulated, is it usually sufficient just to insulate the duct (and not the garage ceiling/floor)? or would insulating the floor as well make a dramatic difference in addition to insulating the ducts.

2. My usual instinct would be to “foam” the supporting joists under the bedroom floor, but after reading the large comments section in this site about people getting sick from iso and polyurethane foam due to the blowing agents off gassing. I don’t know if I will ever reccommend foam products that cure onsite (minus the cans “great stuff) The question is if I don’t foam, is it possible to flash the underside the rafters as a vapour retarder?? if so using what material?

3. If the flashing is a vapour retarder, can I use cellulose insulation in this assembly? what would hold the cellulose “up” I don’t see how it is possible to use dry densly packed cellulose. Maybe someone here could provide a proper method.

4. should i insulate the garage walls? (connected to the outside) or is this not neccessary since it is not a heated space.

5. finally, even though cellulose is environemntally friendly, is it appropriate for a non-heated garage to bedroom location.? if not, what are the best methods(s) or best options to insulate a garage celiing.

thanks for the help.

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  1. GBA Editor
    Martin Holladay | | #1

    Some of your terms are confusing -- I don't know what you mean by "bulkhead" or "flashing" -- but it seems clear that you have an uninsulated floor under your bonus room, and you also have an uninsulated duct running through unconditioned space.

    1. Insulate your garage ceiling. If you don't want to install spray foam insulation, you can install dense-packed cellulose. Install drywall on your garage ceiling (with or without rigid foam between the drywall and the ceiling joists) before calling your cellulose contractor.

    2. Your duct should be insulated with duct insulation wherever it passes through unconditioned space (for example, your garage).

    3. There is no need to insulate your garage walls if you don't heat your garage.

  2. 4BrJ9ioRhU | | #2

    if I install dense packed cellulose, where is the vapour retarder in the space? shouldn't it be on the warm side of the insulation? that is what i meant by flashing. it was probably a poor choice in words. I should have said vapour retarder.

    Is cellulose the best option if I choose not to foam?? what methods do people usually choose for existing buildings with a room on top of a garage?

  3. GBA Editor
    Martin Holladay | | #3

    A plywood subfloor has a permeance of 0.5 to 0.7 perm when dry. That's your vapor retarder. In any case, vapor diffusion into an insulated floor never causes problems.

    In my opinion, dense-packed cellulose is the best insulation choice if you choose not to install closed-cell spray polyurethane foam. Whatever insulation you choose, you must pay attention to air tightness and you must be sure that your insulation is in full and continuous contact with the subfloor.

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