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Community and Q&A

Insulating 3 foot-deep floor cavity over garage

MarcBuild | Posted in Energy Efficiency and Durability on

Hello all,

First question on here from a rookie undertaking a major home reno. I’m currently pondering how best to insulate about 10 cavities that run 3 ft under my master bedroom. I’ve got 3 ft of the garage underneath, the builder had it insulated as such: Drywall on the garage side, half to an inch of polystyrene board under all floor joists covering the entire area, then an R12 paper-backed batt simply laid over the poly in between joists.

My plan was to leave polystyrene as its impossible to remove and it likely provides a decent thermal break for floor joists and fills the cavity with R30 or 32 Rockwool, would this be sufficient to insulate this space? I believe R 32 would leave me with a half-inch gap to the subfloor, would it be best to fill the entire cavity? Could I potentially achieve this by placing Roxul Rockboard 40 between polystyrene and the batt? Thanks in advance.

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  1. Expert Member
    BILL WICHERS | | #1

    If you have the floor above open so that you can access the joist cavities, I’d consider blown insulation here. Blown insulation is cheap, and easy to install (no cutting anything to fit, not fluffing batts). If you can access those cavities, I’d fill them with blown cellulose and call it good.


  2. user-2310254 | | #2


    It sounds like you have an attached garage, correct? Is it possible that garage air is being pulled into the house when your heat or AC are running? If so, I would consider using spray foam to better isolate the garage from the living space.

    I did something like this on my townhouse before moving in. The garage rafter bays were wide open across the entire ground floor, and I was uncomfortable with the idea of potentially breathing in garage fumes.


  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Marc.

    If you are planning to leave the rigid foam in place, you must be planning to insulate from above, in which case, I agree with Bill. Why not fill the floor up with blown in insulation, preferably cellulose? You should also make sure the garage ceiling is well air sealed, which you can also do from above, if that is the easiest way to go about it.

  4. MarcBuild | | #4

    Thanks for the replies gentlemen, I'm thinking of going with blown cellulose, thoughts on how best to close off the openings in this case, I have attached an image here that shows my access point from the room that will be an office.

    1. Expert Member
      BILL WICHERS | | #5

      Did that pic get flipped so that what appears to be the floor is actually the ceiling? I’d thought you had existing rigid foam on the underside of the joist cavities. If you have sufficiently thick rigid foam in place you can just use that to support the cellulose. Be sure you have big fender washers on the support screws though so that the extra weight doesn’t cause the fasteners to pull through the foam.


  5. MarcBuild | | #6

    Hi Bill

    What you see above the strapping is my second-floor master bedroom, some thick HDPE is sandwiched under the floor joists between on the garage side, so under the joists in those cavities between the garage side drywall and the joists (you can't see that from this angle).

    So essentially you suggest I would just close off those cavities with rigid and fill them up?

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