On Wed, 21 Jul 2010 22:55:14 +0100, John Rumm
wrote:

I use either a snap off blade knife with extended blade, or a general
purpose jack saw. The latter is better for fiddly bits and cutouts. The
former makes no dust.

I've fitted it in my wall now and will think about doing the roof
another time. It has occurred to me that if you did the whole loft
insulation in celotex you would need ~150mm, so you would need two
layers any way, so whether I use celotex/celotex or rockwool/celotex,
there are still going to be two layers and the joists will be covered
either way, so I guess I don't need to worry about losses through the
joists.

I did have some cut-offs of 50mm celotex and trod on them; you are
right they do not crush. The foam is tougher than it looks.

BTW at what point does a sheet become a second? There were some marks
on the foil of mine but isn't that inevitable? I did reject one sheet
of 25mm which was squashed where it had been tied to the pallet and
was completely bent along its length. I did get a couple of "you won't
see it when it's in the wall" comments before they finally agreed to
swap it. I think some of the 50mm sheets look wider at the edges than
in the middle. Is that a side-effect of the manufacturing process, is
there always some curl at the edge? They were from a subsidiary of
Travis Perkins so it wasn't some fly by night shop I bought them from.

I didn't have snap-off knife so I used a kitchen knife (don't know
what type but it was a long one!) and it worked ok. Next day I bought
a snap-off knife. I see you can get 9mm, 18mm, 20mm blades. I guess
this is the height of the blade? What is the advantage of a taller
blade? Is it more rigid and therefore stronger?

TIA