insulating between joists with celotex

[Carm10]

i want to insulate over the joists of the eaves in my attic with celotex/kingspan. Just one quick question.... If the joists are 1000mm is it ok to put 1000mm boards of celotex? ( i am getting a joiner to board it up afterwards).

Or should i go for less insulation like 8cm, but to leave me 1-2 cm so when we put the boards up everything wont be compressed?

TIA

In the first sentence i meant BETWEEN the joists, not over!!!

There needs to be a gap for ventilation, and airflow on the outside of the insulation. Read celotex's website, I think they have an installation guide on there.

it says it needs ventilation for flat roofs but in the kingspan insulation it says that it has to be flushed to the joists, if it goes over the joists you will have to add to the joists so the weight of the floorboards falls into the joists and not the insulation.

Do you mean the joists, so you can board the loft, or the rafters so you're insulating the roof?
Roof - need ventilation and both products have a particular product for it
Floor - doesn't need ventilation and you could just use normal insulation
In both cases the celotex should fit snugly between the rafter or joists