Underfloor vapour barrier

we have our house floor and ceiling insulated but not a vapour barrier,
its about 1.2 metre clearence under there, i have heard some people say dont do it as moisture will sit under the polythene and some people swear by it, thoughts?

Is it dry underneath? Moisture problems in the house? If you don't have any problems, it might not be necessary.

polythene on the ground is fine.
some people fit it to the floor itself and that will hold moisture in and rot the floor.
one thing to watch with polythene on the ground is to put small drain holes in the low points. so any pipe leaks or rain blown under will drain away.
if its wet under there sort out the drainage first. under the house should the the high point so water drains away from the house.
even if its dry under the house its still worth doing. all thats happening is the water is evaporating away (nz average 300ml per square meter per day ? ). unfortunately some of that water ends up going up through the floor.

i would certainly put ground sheet down especially if you have underfloor insulation, wet insulation will rot out the floor.(except polystyrene)

awesome thanks tweake

Yes,Is good but do in summer or timber flooring/joists may dry out too fast.