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Is it possible to install rigid foam just below laminated flooring?

user-1092095 | Posted in Green Building Techniques on

Imagine a pier foundation house. The foundation beams and joists are already on place and the floor joists are only 5¨. Not enough for our insulation ambitions. Over the Floor Joists there will be a plywood or OSB. What if, instead of insulating between the floor joists (as originally intended), we insulate above the OSB/plywood, which is above the floor joists? It’s like the idea of sheathing a wall with rigid foam on the outside (in the sense that we would have no thermal bridges), instead that we would be doing an inside sheathing of the floor.

Obviously, the main concerns are that the floor will eventually sink on some weight spots (where people walk frequently or where heavy furniture is), so, I was thinking of two possible solutions for that:

1. Putting a second layer of plywood/OSB just below the laminate flooring (that would make a perfect OSB-EPS-OSB sandwich just between the floor joists and the laminate flooring). I like the idea of making this a floating OSB layer, and not screwing the sandwich. What do you think of this? What thickness would you use?

2. Using Hi-density Rigid Foam. Would that be enough? If so, what density EPS for example?

This house has really low budget so we are minutely making the math and if this solution turns out to be expensive the owner will probably have to do with her 5¨ of insulation-fillable space within floor joists and the thermal bridging of the joists.

Thanks to everybody.



  1. peaceonearth | | #1


    The obvious issue to me would be the reduction in headroom with the added thickness, but beyond that I can't see a real problem (but others may). You must already know you have the headroom. You might still be able to insulate between the joists in order to get additional R value. I think there are some versions of rigid foam that would have adequate compressive strength, if you are concerned about that, the layer of OSB/plywood above should be a solution.

  2. user-4243359 | | #2

    I constructed such a floor assembly in my studio four years ago and it has worked out well. The studio is in a timber framed barn with unconditioned but tempered (stays above freezing) garage space below. I first laid down 3" of 25 psi XPS in two staggered layers, then 3/4" T&G OSB sub-floor panels. No fasteners in the OSB. On top of the OSB was a carbon-film heating mat, then a floating laminate floor. I used OSB sub-floor panels instead of plywood to reduce the chance of the panels curling.

  3. GBA Editor
    Martin Holladay | | #3

    The approach you suggest is done all the time. In most cases, the top layer of OSB or plywood (the subfloor installed above the rigid foam) is secured with screws through the foam to the floor framing (or in some cases, to the slab) below.

    Others have had success omitting the screws. If you are tempted to install a subfloor without fasteners, I suggest that you use tongue-and-groove subfloor panels. Beware of "potato chip curling" of your subfloor panels -- it can happen.

    Most types of EPS or XPS can easily handle normal loads placed on this type of residential floor.

    If I were you, I would also install some type of insulation between the joists -- fiberglass, mineral wool, or cellulose.

    It's usually a good idea to meet minimum R-values for floor assemblies. Here are the minimum code requirements (according to the 2012 IRC):

    Zones 1 and 2: R-13
    Zones 3 and 4 (except Marine 4): R-19
    Zone 4 Marine and Zones 5 and 6: R-30
    Zones 7 and 8: R-38

  4. jkstew | | #4

    If you glue the top OSB panel to the foam you've got a SIP which is a very strong structure. If you don't bond the top OSB panel to the foam it won't be nearly as strong. For example, a telephone book is easy to bend because all the pages can slide over each other, however If you glued all the pages together it would be extremely hard to bend. Same idea.

  5. user-1092095 | | #5

    Thanks to everybody, guys, you've been really helpful.

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