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How Important is Insulating Floor Between (Heated) Garage and Living Space?

Chris Conti | Posted in Energy Efficiency and Durability on

Hi all,

I’m building a structure (described in depth in this thread) that includes an insulated, heated garage on the first floor and a living space on the second floor. The entire building envelope is going to be insulated (walls will get 6″ of fiberglass in the stud cavities plus two inches of rigid foam on the outside).

In order to avoid the need for a center beam and columns, my lumberyard recommended 16″ engineered I-joists for the floor, allowing me to span all the way from one side to the other without support.

The problem with those I-joists though is that, from my conversation with an insulation contractor, they are very difficult to effectively insulate, with the only really effective options being either spray foam or blown in cellulose or fiberglass, any of which would be very expensive.

My question is, given that the garage will be insulated and heated but with a lower necessary temperature than the living space above, and given that heat rises, is it actually necessary to insulate the floor between the spaces at all?

Thank you all in advance for all your help!

Best,

Chris

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Replies

  1. Jim Tyler | | #1

    Other readers will be much better qualified to crunch the numbers and tell you if it's worth it - in the end it's probably a judgement call for you. If you already plan to heat the garage and you don't mind turning up the thermostat in the living space above if needed, then skip it. If you can afford to lose a little ceiling height in the space over the garage, you could frame a dimensional lumber floor over the garage ceiling, maybe perpendicular to the I-joists, and install batt insulation in the bays.

  2. GBA Editor
    Martin Holladay | | #2

    Chris,
    You always want a very good air barrier between a garage and any living space.

    If you plan to heat your garage to the same temperature as the other rooms of your house, there will be no delta-T across the ceiling of your garage, and therefore no need for insulation in that ceiling -- as long as you remain committed to heating your garage.

    As soon as you (or a future owner of the building) decides that heating the garage is an expensive waste of fuel, insulation in the ceiling might become desirable.

    Finally, check with your local building department. Most building codes would probably require insulation in this location.

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