Leveling floor between two rooms

[DIYersep]

I am wanting to increase the height of a wood floor (floor boards on joists) to match an adjacent concrete floor. Both floors are level and sound.

I think I have two options. The first is to use two sheets of board (chipboard type) 20mm and 18mm (the difference in the height is 38mm). The second option I am considering is to use 25 x 38mm battens at about 14” spacing laid at 90 degrees to the existing floor boards with 12mm ply on top. In this case I would insulate between the battens to improve the thermal efficiency. I am slightly concerned that the ply would flex to an unacceptable degree? The final covering will be carpet. Which would be the better option and of course is there a third possibility?

 

[cdbe]

20mm insulation (PIR) with 18mm chipboard on top, no battens. Although you can get 38mm chipboard flooring (for commercial mezzanine floors) it's probably quite heavy.