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Community and Q&A

Rigid Foam Polystyrene under Wood Floating Floor

M4SdpZMuYW | Posted in GBA Pro Help on

I have a room above my garage that is always cold (or hot), I’m in Southern NH. I had a quote from an insulation company to take down the drywall ceiling of the garage (and insulation) and replace it with spray foam for $2,000.00 (gulp). Since this is way out of our budget I thought I could tackle the problem from the floor of the room since we have to replace the carpet. I was thinking of adding a layer of Rigid Foam Polystyrene (taped seams) over the existing plywood floor and then using an engineered wood floating floor above the Foam?? Can I use a floating floor above the Foam??

I will also be adding blown insulation into the knee walls and ceiling of this room and using spray foam around top plates, sill plates and around any vent openings or cracks.
Thank You!!!

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  1. GBA Editor
    Martin Holladay | | #1

    I advise you to check with the flooring manufacturer, but my understanding is that most manufacturers of floating flooring require a layer of OSB or plywood to be installed on top of the rigid foam before installing the flooring.

  2. M4SdpZMuYW | | #2

    okay thank you Martin!
    I would then have to float the plywood too because I couldn't nail it to the Foam, right?

  3. GBA Editor
    Martin Holladay | | #3

    It's certainly possible to fasten plywood over foam by using long screws through the foam to the floor joists or subfloor below.

  4. M4SdpZMuYW | | #4

    Great, I will look into that! Thank you! Happy Holidays!

  5. jklingel | | #5

    Mary: I would suggest using a higher density foam, in the order of 40 or 60 psi. That is just a gut feeling, though; no data. This has been discussed on if you care to read there. I see zero problems w/ your plan. Subfloors are not supposed to deviate more than about 1/8" in 10' from "perfectly flat", according to the two manufacturers I consulted when I did a similar job. The point is, make sure your subfloor is as good as you can get it.

  6. Billy | | #6

    One question is how thick will your foam will be over the subfloor. You are not going to get sufficient R value in New Hampshire unless you use 3 inches or more of foam... and think about what this will do to your thresholds, ceiling height, window heights, etc.

    My suggestion is to get some more spray foam quotes and hopefully you can get a better price. Plus, there will be benefits to sir sealing from underneath and spraying the rim joists and ends of the joist cavities.

    You also could add some polyiso foam boards, taped, on the garage ceiling before you put up drywall.


  7. M4SdpZMuYW | | #7

    Thanks John and Billy, your advice is much appreciated!!!

  8. mrbreadpuddin | | #8

    You could also take down the garage cieling and put batt insulation between the joists, then rigid foam under the joists, then strapping and drywall.

  9. M4SdpZMuYW | | #9

    The reason I started this question was that I didn't want to take the garage ceiling down so I was trying to insulate from the floor side instead of the Garage ceiling side...even though I know it's better to insulate the ceiling.

  10. wjrobinson | | #10

    Mary, I would cut into the drywall and look at existing insulation. If there is space above insulation by the subfloor, then you could cut a strip of drywall to access blowing in a dense pack of insulation. Then do your foam layer, standard foam is fine, and lay floating floor right on it since your budget is tight. A customer of mine has done something similar.

  11. wjrobinson | | #11

    Lastly, add window ac unit and a small room heater w tstat to supplement your existing system. Your existing system most likely has not enough supply ducting to the space, happens often.

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