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Community and Q&A

Insulating a floor above a garage

Joshua Lloyd | Posted in Green Building Techniques on

So this is happening more and more in my office now where we are designing townhomes with a garage below the living space and now that we are building these units in a cold climate I am beginning to have greater issues with it due to the plumbing that needs to run through the floor cavity that is above the garage.

Typical construction is 12-16″ open web floor joists which the contractors want to use R-30 fiberglass batts that I find unacceptable because of the large gaps that will be in place. My ideal method would be to hold plumbing tight to floor and install a rigid material between the floor joists to create a chase and then pack the cavity tight. However I know due to cost, the contractor/developer will not want to pay for anything more than R-30. So how do you satisfy a code official that states the insulation needs to be tight to the underside of the floor deck?

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  1. GBA Editor
    Martin Holladay | | #1

    There are three issues here:

    1. What building scientists say about insulating this type of floor.
    2. What the code calls for when insulating this type of floor.
    3. What to do about plumbing (or whether plumbing even belongs in a floor assembly at all).

    Here is a link to an article I wrote that addresses some of these issues: How to Insulate a Cold Floor. In that article, I advised, "To keep plumbing pipes from freezing, install them as close to the subfloor as possible, without insulation between the pipes and the subfloor. It’s also a good idea to install a layer of sealed rigid foam directly under the pipe. The rigid foam should extend the full width of the joist bay."

    If you follow this method, you can install spray polyurethane foam or fiberglass batts below the rigid foam. If you use fiberglass batts, it's good to install a continuous layer of rigid foam on the underside of the floor joists. Regardless of your chosen method of insulating, it's essential to pay close attention to airtightness, especially at the perimeter (rim joist area).

    Perhaps you can show Joe Lstiburek's article and my article to your local code official to convince him or her that these approaches represent the best current practice.

  2. Joshua Lloyd | | #2

    Martin, I did read your article and thanks AJ, was looking for the one you referenced.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Joe L. says that insulation works better if it does not touch the floor. He just wants the edges closed off well. He says the floor is warmer that way because the space just below the floor is closer to the room temperature so the floor is warmer that way.

    There are I joists that run quite large that are not open truss style.

    Joe's yapping all about this topic is here...

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