Hi all. Just about to insulate between the joists in cellar which has dining room above. Earlier in the year I took down the old cellar lath and plaster ceiling. The cellar is pretty damp which is controlled through decent ventilation. The dining room above which is open plan with lounge next to it (lounge has solid original flag floor) suffers from high humidity levels and I can't work out whether this is from flags or from cellar (or both). Is it wise to install some sort of vapour barrier when I do the insulation? I am favouring cutting PIR boards to fit between joists, securing with batons where necessary and ensuring any gaps are filled with foam . The bottom of joists will still be visible so I can check their condition over time.