Currently my loft has 200mm of glassfibre insulation between the joists on top of lath and plaster ceilings. The joists are visible because I need to know where I can put my feet if I go up there! Up to now ther haven't been any condensation problems because the loft is very well ventilated and the glass fibre is permeable. I now want to upgrade the insulation, which means that I'm going to have to put some sort of flooring up there. This means condensation will become an issue so I've got to put some sort of vapour barrier in.

Would foil faced PU laid tightly between the joists be adequate (there would probably be a small vapour path around and possibly through the joists) or should I cover the lot with polythene (tricky because of the light fittings etc.)?

Bill

Thanks for the comment, but the raised walkway is the problem as it will be solid and condensation will form on it, drip through the insulation and damage the ceiling.

I don't store anything up there, in fact I go up there as infrequently as possible, but occasional access is needed and cross laying insulation guarantees a foot through the ceiling!

Tony's walkway has a ventilation gap under it and wouldn't cover the whole loft so won't attract condensation.

What about laying 8in floor boards (or the smaller loft boards) down with say 50mm gaps between each then cross lay the insulation over, although I suspect your ceiling below with possibly many generations of Dulux on it probably checks the vapour pretty well.

J