George Miles wrote:
I dont think the problem is condensation,
but that if rain blows in under the slates then there is nowhere for it to go
and no ventilation to dry it out
so the timbers will rot

Generally the advice when insulating with Celotex is to leave some of the
timber exposed, so moisture can escape. Do they not do that with spray foam?

I've heard some US horror stories, but they were where the foam hadn't been
properly mixed and toxic gases built up. And of course the nightmare of
removing it afterwards.

(I don't even like foaming in Celotex for that reason)

Theo