I plan to insulate the walls of my still-to-be-built 'garden office'
as well as I can, and wondered whether someone could help on a couple
of questions...

I understand about a vapour barrier having to be on the warm side of
the insulation; and about the difference between the conduction,
convection, and radiation of thermal energy i.e. heat.

What I am confused about is what can be next to the said vapour
barrier, which will be foil.

Going from the outside, I plan to have:

1) weather-resistant paint as used on houses in Sweden
2) 10mm timber
3) a thick layer of polystyrene, probably XPS, probably 2 x 100mm
4) plasterboard
5) common-or-garden internal silk emulsion paint

(I won't mention the studs. These will prob be in two layers, one
perpendicular to the other).

My questions a

a) should there be an air gap between the polystyrene and the
plasterboard?
b) if so, where should the foil layer be? I.e. should I use foil-
backed polystyrene or foil-backed plasterboard?
c) as well as 1-5 and the foil, and not counting the studs, should
there be a layer of anything else in there?

(I'm aiming for an inside-to-outside R-value in excess of about 4.5
RSI x 8 inches, giving R 36).

Thanks for any help with this.

John