Have I got rhis about right?

--------------------------------------------------------------------------
roof tiles
|\\\| |\\\| |\\
\| 25mmventilation space
|\\\| ::::::::::::::::::: |\\\|::::::::::::::::::: |\\
\| :::::::::::::::::::
|\\\|::::::::::::::::::: :|\\\|::::::::::::::::::::|\\
\|::::::::::::::::::: 50mm Kingspan
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^ 25 mm
PIR foam & dpm
===================================== Plasterboard
So... if the joists are 3",
I'd leave a 1" ventilation gap at the top,
pack a 2" layer of kingspan between the lower 2 " of the joists.
25 mm PIR accross the underside of the joists,
DPM
Ceiling plaster.


Thanks
Tony