I have installed rockwool insulation blankets under the floor and I am
now experiencing some worrying problems with condensation as the
insulation itself gets very humid at times with water dripping onto
the DPM laid on top of the underfloor (no concrete).

The floor is made of 18mm plywood nailed on top of the joists. The
insulation is placed between the joists. The crawling space is about
600-800mm deep.

I am thinking of reducing the crawling space (and so the volume of
air) so that the existing ventilation will be more efficient.
Someone mentioned to install a vapour check membrane underside the
plywood (floor).
Will the above work? Any other suggestions?

Thanks

Alex