Hi Fred

It helps if you have some Celotex/Kingspan lying around as you can see
how tough it is in compression.

As a test, I just took a spare small piece of 'tex and placed it over a
couple of 18mm wide battens before adding a small board on top and
jumping up and down on it. It did creak and deform by about 0.5mm at the
battens so it was at the limit but that was 80kg+ over only 50cm2 of
narrow contact on the battens. Scale that up to 50mm joists at say 500mm
centres and the test was equiv to 3200kg/m2 so it suggests that a few
100Kg/m2 would be well within safe limits.

As to alignment, if you lay the 'tex and mark the joist positions on the
top foil surface with a marker pen then it is relatively easy to keep
track of their positions. If you were then to align the OSB with the
edges within 25mm of the joist position then I would say that was safe
enough. I wouldn't even screw the boards down (unless it proved
necessary) but would float it and tape the seams with good quality duct
tape. Just keep the 'tex joints away from the joists.

How does that sound? It seems a lot less work that cross joisting.

Thanks for this; I'm aware that my reservations are in part due to lack of
real experience with Celotex. I've tried to press my thumb into a sheet in
Wickes; that's about all ;-/

What thickness of Celotex are we talking about here anyway?

That all sounds reasonable, and as you say much less work than cross
joisting. I'm still not too sure about the relative staggering of the
Celotex and OSB panels though - especially if they're both of a similar size
to go through the loft hatch. I will do a few layout doodles and see what I
can come up with. I would quite like to be persuaded that this is the way to
go ;-).

Cheers
J^n