On 18 Jun 2004 20:52:59 +0100, a particular chimpanzee named John
Richer randomly hit the keyboard and produced:

Thanks - the relative performance was just what I was looking for.

The conductivity ('k-value' or 'lambda-value') is the relative
performance. AFAIR, the k-value for mineral wool is 0.037W/m^2K,
whereas for Celotex it's 0.023W/m^2K

So this suggests that if I do get to 50mm celotex, then I have done
enough. The attic rooms have sloping ceilings with loose fill
insulation poured between slates and lath-and-plaster sloping walls (a
previous owner's contribution to the house) so they will dominate the
heat flow.

I think I suggested at least 100mm thick Celotex. Although something
is better than nothing.

Your previous owner's contribution appears to be to create a high risk
of condensation in your roof by blocking the ventilation path between
the insulation and the roof felt.
--
Hugo Nebula
"The fact that no-one wants a piece of this
shows just how far you've strayed from the pack"