Looking to install PIR insulation between ceiling joists in our attic. Cold attic space. The ceiling is that immediately above bedrooms/stairs landing.
Joists are 100mm depth and 600mm centres.
We currently have sub-recommended depth fibreglass wool between the joists (only around 100mm thick as is a 1970's property) that I'd like to completely replace with the likes of Kingpass, Celotex etc. 100mm panels.
The ceiling plasterboard panels are directly attached to joists.
I'm looking at buying foiled PIR as it's got the vapour control built in.
Do I have to leave any air gap between the ceiling plasterboard and the PIR?
I.e. Can I simply take out the old wool -> cut the PIR -> push between joists so it is flush with top of joist -> aluminium tape where boards join -> done?