"Hugo" == Hugo Nebula writes:

On 18 Jun 2004 15:42:44 +0100, a particular chimpanzee named John
Richer randomly hit the keyboard and
produced:

I have a loft that I want to insulate better, but also use as
storage space - so I don't want to lay deep insulation roll
loosely on top.

The best insulation is currently Celotex (Kingspan is equivalent).
It's almost twice as effective as mineral wool. 100mm of mineral
wool isn't particularly effective, and you should be looking for
at least 200mm thick, or 100mm Celotex between the joists.

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

When you start getting to these thicknesses of insulation, the
heat loss through the joists starts to become significant and you
get better 'value for money' out of your insulation if you can
cover your joists.

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.