RJH wrote:
Reading this, I'm not sure why you don't just celotex the ceiling (and
verticals) with, say, 75mm. You could loose board over too, without the need
for stilts over wool etc. If I had my time again it's what I would have done.

Hmm, that is a good thought. I had previously looked into using EPS-backed
chipboard, but there's no a lot of available products and reckoned it would
be easier just to lay rolls on top.

I hadn't considered replacing everything with celotex. I can see that has
merits. One issue is the space - no part of the loft is more than 4' high
and the roof slopes from 4' to 0', so it's not much fun to work in there.
Access is also awful (can't get large items through, very limited turning
circles). Cutting celotex is a bit of a pain too. I had hoped throwing
some encapsulated rolls down would be least-work.

A second concern was that installing foil-backed insulation is likely to
make RF conditions worse - mobile signal and wifi. But that's likely the
case with any insulation to block solar radiation, which is a big issue.
The second can be addressed with more access points, I could probably live
with worse mobile signal. So I should probably stop worrying about this and
go with the flow.

However losing some of that height for insulation would also be a pain and
celotex has advantages here.

I might think about a mixed strategy: replacing the tall parts with celotex
and boards on top and the awkward bits with rolled insulation. Probably put
celotex behind the batts on the existing stud walls too.

Methinks I should go and grab some samples to try...

Thanks (and to RJH too) for a good idea

Theo