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Insulating with rigid foam over subfloor

redmonds328 | Posted in General Questions on

I am doing some renovations in a small area.  There is existing 1/2″ plywood subfloor.  I want to insulate the floor while still preserving head space in the room.  I want to make a subfloor – styrofoam board insulation – Luan sheathing sandwich above the subfloor then topped by Carpet Tiles.  What is the minimum recommended width for the styrofoam insulation and the Luan to protect from damage from walking on the floor.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Redmond.

    If headroom is an issue, why aren't you insulating the flooring system, between the joists? What is blow this area?

  2. redmonds328 | | #2

    Hi Brian. small crawl space, lots of HVAC ducts and plumbing under floor I do not want to deal with if I can avoid it. I am hoping 1/4" XPS + 1/4" Luan Underlayment will be sufficent help my floor + the carpet tile.

  3. Andrzej Kobus | | #3

    Get XPS foam 40 psf, screw plywood on top to create a subfloor. EPS foam is too soft so it will not work. This is a typical basement floor solution.

  4. Expert Member
    Akos | | #4

    You don't need any special insulation, standard box store EPS or polyiso will work. I like polyiso as you get a bit more R value/inch. You do need a solid subfloor above, the 5mm luan is not enough. You need at least 5/8 T&G above it, you can then install any type of flooring on top of that.

    P.S. If your HVAC is bellow here, you are putting the insulation in the wrong spot. It should go on the crawl space walls (don't use polyiso there though as it can't be in ground contact). This would minimize your duct loses and warm up your floor at the same time. See Photograph 5 for details:

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