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I have a new room built but floor is lower than adjoining hall by 1 1/2 inch.

Can I put rigid foam down over the plywood floor and top with plywood to match heights?

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That will work as long as you use the dense form of foam board. The white board typically does not have enough compressive strength, at least to my knowledge. The blue or pink colored versions have enough compressive strength withstand the pressure. I have had 2 different occasions to do this type of install. One install was 4" of blue foam with woven wire mesh and radiant floor heat with 3" of concrete first and 2" of bluestone and setting bed over it. The other was 2" of blue foam over a concrete slab, with a layer of 2'X2' squares of OSB with plastic dimple board as a floating floor over the foam. These where done 10 years ago and still holding up fine, so yours should do well.

For your install, plywood will work fine. Use T&G and screw through the foam and the into original floor to keep the large sheets flat. Every 8" on the edges and 12" in the center. This will also help eliminate the hollow sound and feel the floor will have over the foam.

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    The OP wants to match a difference of 1.5 inches. One choice would be 1" foam and 1/2" plywood. Another choice would be 3/4" foam and 3/4" plywood. Which combination would you recommend? – Michael Karas Sep 20 '15 at 11:10
  • I kinda assumed the 3/4" with the 3/4" ply, since the 1/2" does not come in T&G...Sorry, I should have been more specific. As a note, it is better to be a little low, than a little high. Of course dead on is always good. If needed, T&G is available in 5/8" too. – Jack Sep 20 '15 at 22:58

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