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Installing Laminate Flooring Over 1.5″ XPS?

alex_coe | Posted in General Questions on

I have an unheated slab on grade in CZ 4b.

My plan is to glue down 1.5″ XPS and tape the seams to create a vapor barrier, then install laminate directly to the XPS.

The XPS product I have available is 20PSI compressive strength.

Will this be sufficient, or would I need to at least use a 1/2″ OSB subfloor layer?

If I used a thinner flooring product like LVP would I need to add the OSB subfloor in that case?

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  1. Expert Member
    BILL WICHERS | | #1

    I think you'll want the subfloor here. If you were installing full thickness wood planks as a finished floor, you'd probably be OK without a subfloor. Some engineered floors may be OK too. Laminates tend to be relatively thin and flexible though, so they won't spread out loading over the top surface of the foam, which means you're more susceptible to deformation of the foam layer and a resulting depression in the finished floor. A subfloor acts to spread out concentrated loads over a larger area, which minimizes deformation of the foam layer. Thicker hardwood floors would be able to spread loads out by themselves, but thinner materials that are more flexible won't be able to do that very well.


  2. Expert Member


    As Bill said, unfortunately foam doesn't deal with point loads very well. You can see this pretty clearly when you screw down a subfloor through foam. Just the pressure of the screws is often enough to compress the foam below and cause lippage between the plywood sheets. Unless the finished flooring is rigid enough to spread loads, it needs a solid substrate under it.

    1. Expert Member
      BILL WICHERS | | #3

      >" Just the pressure of the screws is often enough to compress the foam below and cause lippage between the plywood sheets."

      That's a good point, and would make tongue and groove plywood a good product for this type of application, since it would tend to lock the edges of those plywood sheets together to keep things even.


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