Internal insulated plasterboard - lower space - how to avoid condensation?

Hello,

I am installing Quinn Therm insulated plasterboard on 9-inch hollow block walls of a converted garage.

I am worried about condensation, and therefore mould, under the insulated slabs. And the place that especially worries me is the lower part, at skirting level. I am told that I can not rest the slabs on the floor (to avoid picking up moisture into the slab? anyway that is what online sources say too), so the slabs will be about 1-2 cm off the floor.

The slabs themselves, plus the skimming on them, should be a decent moisture barrier. But when they are raised from the floor, there is no moisture barrier in the hole between the slabs and the floor. Moist air can freely flow in - the skirting won't really hold it back. It will then condense on the wall below the slab or in the space between the wall and the slab (that particular wall is uneven at its lower side so there will be some space).

How do I prevent this? Is there, perhaps, any suitable filler for the space between the slab and the floor? Expanding foam might not be moisture-proof as it is an open foam, and silicone seems to be unsuitable for such big volume (besides, it's a bedroom, silicone would create an unelcome smell). Put in expanding form anyway, then fix (glue?) polythene to it under the skirting, and hope for the best? Or is there a better way, ideally something well tested in the industry?