Wood – Is it appropriate to put polyethylene between hardwood and subfloor

Normally when flooring changes you would use a proper transition. for example where your tile meets your other room, you will want to leave a small gap between the wood and the tile and then use a transition to cover the gap. I would guess if you have a wood floor in the middle of a tile floor you're going to have to leave an expansion gap all the way around, which will be a good place for crumbs and debris to collect.

You might be able to use silicone to fill the expansion gap, as it would be able to flex when the wood expands/contracts. I don't think I would put grout there, because it will most likely crack when the wood moves.

Here is an example using a transition.

And without a transition.

1. Your current plywood is suitable for hardwood installation. I do not ever like to use 1/4 subfloor as it is brittle and when screwed sometimes doesn't lay flat. If I added to your subfloor I would at least put in 1/2 inch but like I said you don't need it with 16" OC. Note that a thicker sub might have benefits but at the same time you are adding weight to the joists.

2. Depends on area of country and moisture levels. CDX is fine in most places.

3. I would not try to match the current subfloor. I would do the opposite.

4. I do not ever attach additional subfloor layers to joists. When the joists swells - and they will in 80% of the world - your sub will be pulled or pushed and then you will have a screw or nail that is working its way out and possibly squeaky floors. The whole reason for the second layer is extra inflection levels. By attaching to the same thing you get the same thing just thicker.