box profile cladding --- insulation

i am soon to be building my shed/office roof.

my three layers are going to be 9mm wpb plywood
on top of this i plan to put that cheap polystyrene sheets for insulation
and on top of this i plan to put box profile roofing sheets.

now i think there should probably be a vapour barrier (?) either above or below the polystyrene insulation layer.

is this correct ?
should it go above the insulation or below it ?
what is the cheapest way of doing a vapour barrier ?

the vapour barrier should go below the insulation, why not use fibre glass for your insulation, as it might work out cheaper

If you plan on finishing the inside of the room, then use foil backed plasterboard, which provides adequate VB. For better insulation performance, you could use celotex/kingspan type insulation, which being foil backed, also acts as a VB. (Tape all the joints with silver tape)

The really cheap way though, is to staple polythene to the rafters before attaching normal plasterboard. works a treat but you need to ensure all joints are taped and be careful it doesn't get damaged during installation.