On 29 Nov 2006 04:07:48 -0800, a particular chimpanzee named
randomly hit the keyboard and produced:

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.

Why are you insulating the garage roof? Is it for frost protection
(with some low-level heating) or are you going to heat it
significantly? If the former, then you probably won't generate enough
condensation to worry about. If the latter, then you're going to need
more insulation anyway.

You'll get better value from your insulation if you use the Celotex
over the rafters and the polystyrene between. There is heat lost
through the rafters and the more insulation you can cover them with
the better.
--
Hugo Nebula
"If no-one on the internet wants a piece of this,
just how far from the pack have you strayed?"