Vapour barrier above floorboards to prevent new carpet getting damp?

[Daveyp1471]

Hi, last year I noticed my underlat I a few corners and carpet were damp, there were no leaks not animal wee so put it down to temperature changes as we often get water under the floor boards. I had new air rocks installed and cleared out all the older ones. We are also getting the chimney re pointed as think rain water maybe seeming down. My concern is that we want to replace the carpet and underlay but can't face it getting moisture transference again so was considering fitting a vapour barrier above the floor boards and under the skirting. I'm hoping the new airbrick a will prevent mould underneath.
Does this sound a good idea?

[welsh brickie]

I would lay 3mm thick ply, and screw it down first, and tape the joints. a vapour barrier will move and sweat