Simon Hawthorne wrote:
Hi all
snip
Got an old house. Almost zero insulation in the loft. Planning a
loft conversion in a couple of years when the barn conversion is
complete (another story).

Anyway, just measured the rafters in the loft and the dims a

2.5" wide
3" tall
Space between rafters averages at approx 15.5" (some 14, some 16)

I need to put some insulation up there - but don't want to waste money
by putting something down only to find it unsuitable when we come to
convert.

The rafters are utterly unsuitable for anything more than walking
around on carefully, and maybe storage of empty boxes, and will need
replaced or augmented to put rooms up there.

As this is the case, the insulation is unlikely to be suitable - you may
not want to insulate the two rooms off anyway as eventually you'd want
the whole roof to be warm.
Personally, I'd get some 150mm insulation at B&Q or wherever, and
split into three 50mm layers, before laying.
You should probably do some sums to see how much money you'll save
in heating, and how much the insulation will cost over a couple of years.