I am planning to take the 1" EPS + plywood sandwich approach to my basement subfloor (pergo on top). I've read conflicting things around the plywood -- is 1/2" ply sufficient? Do I need to do two layers of 1/2" to get to a full 1"?

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    I'm not sure why you wouldn't go with standard 3/4" (nominal) tongue and groove plywood. I wouldn't want movement at the seams. – isherwood Nov 7 '17 at 17:09

I don't recommend any wood based subfloor (plywood, osb, particle board, mdf, etc.) in direct contact with your concrete basement floor without first putting down some sort of moisture barrier. I just ripped out an old OSB subfloor (glued right to the concrete floor) in my tiled basement and it had large patches of thick black mold in many places. Many laminate floors suggest use of under-layments that can act as a moisture barrier. If you ever get water in your basement you will regret a wood based subfloor down there (regardless of a moisture barrier installed or not the subfloor will retain moisture for long periods of time). Why not just use a floating floor approach with the underlayment below the pergo?

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    Extruded polystyrene is an adequate barrier, no? Especially if the seams are taped. – isherwood Nov 7 '17 at 17:08
  • Yep I'd consider EPS as a decent moisture barrier, but it works better as insulator. My point is throwing osb or plywood on top of EPS will work but if a pipe breaks or your basement floods, that subfloor will mold for sure. Pergo is not a nail down/ staple application so what's the point of the plywood? – Gdogbiskit Nov 7 '17 at 18:35
  • thats a good question.. I want the EPS for insulation, this area gets absurdly cold. I figured pergo right on top of EPS was not going to be rigid enough? – XeroxDucati Nov 7 '17 at 19:41
  • You should call Pergo and make sure it won't void their warranty, but I think you could forgo the plywood and do eps alone if you got the high compression kind . Not all EPS is created equal (it all depends on how hard they compress this stuff at the factory). Some have compressive strength up to 100 PSI which are used for load bearing and under slab applications. I couldn't imagine this stuff flexing the tongue and groove of Pergo even under high loads. Dow makes tongue and groove EPS too. owenscorning.com/NetworkShare/EIS/… – Gdogbiskit Nov 7 '17 at 22:03
  • Just to be clear, there is nothing technically wrong (violating Codes or anything as far as I know) with plywood/OSB over EPS either, I just don't like wood based subfloors in a basement for the reasons outlined above. – Gdogbiskit Nov 7 '17 at 22:20

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