box profile cladding --- insulation

[catfordbags]

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 ?

 

[drewroof]

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

 

[Deluks]

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.