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PSI for XPS Foam in Radiant Floor System

bonai | Posted in General Questions on


Planning to do my own hydronic radiant system.

I already have a 3/4″ OSB subfloor nailed/glued. I wanted to install over the OSB subfloor either some XPS or EPS foam panels and cut grooves with a router, install sheets of aluminums and run my PEX tubing.

I like the premade Roth radiant panels and wanted to copy their design,  their panels have EPS foam with a compressive strength of 90 PSI.

I am planning to have this installed in this order: OSB/foam/Engineered floating floor.

I want to avoid using sleepers because it is just too expensive right now.

What would be the ideal PSI for that project? I cant really find anything rated over 20 PSI.


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  1. Expert Member
    BILL WICHERS | | #1

    You would normally install a layer of plywood or similar material over the foam, essentially building a second subfloor layer. You don't normally install flooring directly onto the insulating foam.

    Owens Corning's Foamular is available in a 25PSI rated foam (the "250" variant). I think there are some even higher rated specialty variations too, although I've never worked with any rated higher than 25 PSI.

    Note that a 25PSI rated foam is good for 3,600 pounds per square foot. This is well over the commonly used 40 pounds per square foot design live load for residential floors, and even the 300 pound concentrated load commonly used for residential floors. It's unlikely you'd even need the 25 PSI rated foam for a normal floor application.


  2. Expert Member
    NICK KEENAN | | #2

    The issue you might run in to, if you're using snap-click engineered flooring, is that the tongue is relatively weak. If someone steps on the joint between tiles and the substrate flexes the tongue can snap off. Now, I have no way of linking that to a specific PSI and no idea of how you might calculate it.

  3. Expert Member
    Michael Maines | | #3

    I'd estimate that more than 1/32" of deflection at the tongue would be enough to dislodge or crack it. A 250-lb person might spread their weight over 10 square inches when walking, or 25 psf. 25-psi foam is rated for the load at 10% deflection, so with that load, 2" foam might compress 0.2". The flooring will spread the load out a bit but people can also be heavier than 250lbs. Either way, far too much deflection for click-lock flooring directly over 25 psi foam. 40-psi foam is very resilient and might work without a subfloor. But I'd use a subfloor if possible.

  4. bonai | | #4

    Thank you all.

    Plywood is so expensive these days that I would like to avoid using it.

    Dupont has a product called Styrofoam Highload 60 PSI, I will call tomorrow to get a pricing and if this is reasonable will go that route.

    1. Expert Member
      BILL WICHERS | | #5

      You might try 1/4" hardboard as a cheap alternative to at least add a little bit of structure between the finished floor and the foam. 1/4" hardboard was still around $10/sheet when I last checked a month or two ago. It won't be as good as 3/4" plywood, but it's better than nothing.

      Remember too that if your floor fails down the road because you saved a few hundered bucks on subfloor materials, you'll be out a lot more money to replace the finished floor. This might not be the place you want to try to save money.


  5. Expert Member
    Akos | | #6

    If you have OSB down already, it sounds like not an above slab install.

    If that is the case you can go with one of the many staple up installs from underneath with either heat plates or heat fins. Way easier to install and no need to mess around with foam above.

    If you don't have access to the joist bays bellow, go for one of the pre-made track systems. DIY is doable for a small area. Any larger area, you'll spend way too much time to make something you can easily buy for a couple of $/sqft.

    You can also pour a thin slab over it, 1.5" of sandmix is cheap and pretty quick to put down. Any click or gluedown floor can be easily installed over the new slab.

  6. bonai | | #7

    Home depot carries plywood in 4x8 1/4", cheapest I could find is $37 CAD + taxes / sheet....

    Dupont 60 PSI is $80 + taxes in 4x8, almost same as 3/4" plywood

    I will just go with 3/4 plywood and use a router and aluminum plates.

    Under subfloor doesn't work as I need 6 o.c channels for max Btu output living in a cold region.

  7. Expert Member
    Akos | | #8

    Before saying that a staple up won't work, what climate zone you are in and what is the construction of the house? You need to figure out what the actual heat load of your place before designing for a floor heat system that delivers 25BTU/sqft. I've heated century homes with minimal insulation with staple up without issues in Zone 5. Very few reasonably sealed code min houses need more than 10BTU/sqft.

  8. bonai | | #9

    Thanks for checking I did a LoopCAd design of the entire house and have a pretty accurate heat load number, we have lot of windows and live in Winnipeg Manitoba, so North of a zone 5. The design temperature is -27.4F just to give you an idea.

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