basement flooring system – 2 layers OSB: What thickness? – GreenBuildingAdvisor

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Questions regarding a basement flooring system.  General description on build is below the question(s).

Want to install a sub-floor consisting of 1” rigid foam board (EPS) and 2 layers of OSB (opposite directions, two layers screwed together, etc), but have the whole system ‘floating’; that is, I want to avoid attaching the OSB to concrete slab with Tapcons. 

I have read a decent amount of posts here and elsewhere that have nuanced descriptions of this concept. Ity is clear that if you are going with 1 layer of OSB an(*and Tapcons), then you use ¾”.  But when using 2 layers, the OSB thickness is not always mentioned. 

a)       can I get by using 2 layers of 5/8” OSB, or do I need to use two layers of ¾”.

b)       t/g OSB? Or leave a gap between the edges for expansion (I read that in one post)

For the two layers and my sq-ft, I am looking at ca. 90 sheets (total).  So there’s some cost savings to go with the 5/8” if (IF) it is sufficient. 

https://www.greenbuildingadvisor.com/article/planning-and-prepping-for-basement-flooring (option2)

In addition, I also plan on building some interior walls (non-load-beariung), and the bottom plates would sit on top of (and be screwed to the OSB layer, but not all the way into the concrete).   I read one place (that now I cannot find to cite) that you should attach the bottom plates directly to the concrete. But that was one article whereas all others I have seen suggest I can just screw into the two layers of OSB.  Som I would love some opinion on this too.

General: New construction.  Zone 6b. Walk-out basement.  3 walls ‘fully’ below grade. Concrete slab with R10 (2” foam board under total slab,) and a PE radon barrier. About 1500 sq-ft.