In article ,
"Christian McArdle" writes:

Consider what your vapour control strategy is. When insulating, you should
have a vapour barrier beneath, to prevent hot moist air becoming cold
saturated air and leaving mould causing condensation soaking the insulation.

I don't believe this is a significant problem unless you have
lots of cracks or holes through your plasterboard and an
unventialated loft.

The problem would be much more apparent with fibre based insulants.
Normally, this is done using foil backed plasterboard, but this isn't
suitable for a new build. The best solution in retro-fit applications is
often to lay large sheets of plastic sheet between and over the joists,
taping up any holes you need to cut around struts etc.

I think this is well OTT. Also, I would not put a vapour barrier _over_
the loft timbers. Seems like a recipe for them getting wet and being
unable to dry out.

If you have problems with a damp loft, look for draught holes through
from the house, lack of loft ventilation, and pumping over in the central
heating header tank (making it warm/hot/steaming).

--
Andrew Gabriel