Hi, I’ve just insulated my loft hatch using 50mm (2” approx) insulation board from Celotex.
However, I’ve managed to achieve an energy rating equivalent to 100mm board, by adding a shallow frame of wood around the inside of the hatch/door. I used 20 x 20mm strip wood, to create an air gap between the insulation board and the hatch itself.
This gap makes all the difference, as heat can travel directly through materials, but has a hard time jumping a gap!
Oh, and if you don’t seal the exit with draft excluding foam, then your work has been in vain!