We're installing an engineered wood flooring in a room that already has a plywood subfloor. The engineered flooring will be 1/4" shy of the height of the hardwood in the other rooms, so I'd like to make up that difference underneath. My original thought was to use 1/4" XPS but that seems like it may not work.
If I go the 1/4" plywood route, what kind of connection do I need between the two subfloors? Can I just float the 1/4" on top? Or do I need to affix them together? If the latter, would some 1/2" brads be enough? Do I need to glue and screw?