Create one type of underfloor - if needed!

When you have one area containing two (or more) types of underfloor you have to start with other preparations: making sure you have one type of underfloor. Example: extension with new concrete underfloor connected to a room which has existing floorboards. Both types of underfloor (can) have a different temperature or moist content and your natural wooden floor will react accordingly: one part of a board when laid on both types could 'go-all-over-the-place' and you could end up with a cupped or warped floor.

Simplest solution in this situation is to install plywood or OSB sheets (cut in small sheets 30 by 60cm maximum) in the whole area. Glue the small sheets with flexible adhesive to the concrete and nail them into existing floorboards. On the plywood/OSB (=sheet material) use a foam or rubber underlay for sound-insulation and your underfloor is ready for the wooden floor.

Question:
I've read your guidance on installing over 2 types of subfloor. In our case most of the floor is newly laid concrete with two areas of wood floorboards. You suggest a dpm over the concrete and then hardboard over the whole area. What thickness of board & what type (?ply) do you recommend? How should it be fixed?
We're concerned about the overall thickness of the floor covering, it is the hall with five doorways off, with four different floor coverings on the other side.

Answer:
All depends on if you plan to install the wooden floor ongoing (i.e. without thresholds in between the rooms).
Is so, thin sheets of hardboard (3mm) would be sufficient.
If you install all areas as different rooms i.e. using thresholds in between then you use on type of underlayment for those rooms containing concrete (with DPM) and another type for rooms with the existing floorboards (foam underlayment)

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