boarding a loft and water vapour

[Peter J L]

Am I being over-zealous or simply paranoid?
I intend to have my loft boarded out entirely. The roof is well ventilated, and will stay that way. There is fibre insulation to the depth of the ceiling joists and the boarding would lay direct on the joists. This is a 100mm depth (they are short spans and there will be only very light storage). What precautions should be taken to avoid trapping water-vapour beneath the boarding? It is intended to use moisure-resistant chipboard.
Should I vapour-check the ceilings of the rooms beneath - in which case is there a vapour-check paint that can be overpainted with emulsion? Or should I ensure gaps between the boards? Or am I over-reacting?
Your views would be greatly appreciated.
Thanks

[welsh brickie]

lay more insulation so that it compresses when you lay the floor on top
no gaps no moisture.

[Perry525]

Water vapour can move through plasterboard and open cell insulation like fibreglass unhindered. It will also pass through the gaps between the floor boards.
Laytex or gloss paint will stop it. Years ago it was common to paint kitchens and bathrooms with gloss paint for that reason.
Ceiling joists are in tension and are much stronger than you imagine.