wrote in message
oups.com...

Hi,

Im in the process of insulating the roof of my garage.

I am planning on using 50mm Celotex between the rafters. Due to cost,
I was planning on using 25mm polysterene from B&Q over.

Would I still need the vapour barrier plasterboard ? Or if I ditch the
polysterene could I just tape up the joints where the Celotex meets the
rafters and use standard plasterboard ?

why plasterboard a garage roof/ceiling? Plasterboard won't perform well in
an unheated building. Where foil backed PB is specified in construction
it's to prevent warm moist air from an inhabited space condensing out in a
cold space. You seem to be expecting the vapour barrier to prevent moisture
vapour entering your garage. Not necessary due to ventilation of the garage
space and would be a bad thing for your roof structure.

Jim A