The easiest way to insulate is probably to use space blanket, which is
a roll insulation encapsulated in plastic to avoid mess. It is rolled
out between the joists and supported on netting or similar. My problem
with this is that the standard size is 370mm wide, and assumes that
your joists are at 400 centres, whereas mine, annoyingly, are at 430
centres, so I'd have a gap. This leaves me the option of using
messier, unencapsulated roll insulation, or what I'll probably do
which is buy seconds of 50mm Kingspan, cut to size and fix in place
with PU foam.


Rockwool slabs are less messy than the rolls. 1200x600 but as easy to
cut as Celotex