In article
, Man
at B&Q writes
On Apr 3, 9:27*am, fred wrote:

When putting celotex between rafters it is even more important to
exclude water. If it creeps into the unventilated trough formed by the
celotex, between the rafters, it wont be able to escape and we all know
where that leads.

The point is that you simply don't do it without arranging adequate
ventilation. It's not all about letting water ingress from outside
escape.

Yes, the area above the Celotex needs ventilated but that trough cannot
be ventilated (otherwise the Celotex would be bypassed) so it is
imperative that moisture doesn't get there in the first place.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs