underlay/DPM for wood floor over existing floorboards

[jbuild]

laying an engineered, click-fit type wooden floor over existing floorboards.

Which underlay type do you use and is DPM required?

Downstairs there is an airgap under the exisiting floorboards that vents to the outside via airbricks

Upstairs exisiting floorboards are of course above the downstairs ceiling,

I have laid tongue and groove glue flooring over concrete before so used a DPM.

Whats the advice for laying over boards? I am concerned about damp building up on the old floorboards but equally, damp building up on the new floor. Use DPM upstairs and down or not?? any other advice?

Thanks

 

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