John Nagelson wrote:
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

Way too much - the walls are to be 225mm thick? - are you planning on using
8 inch timber framing?

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?

No - you can purchase PB with polystyrene already affixed to the back of
it....come to think of it, there's no vapour barrier in it neither.

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

If it were mine, I'd buy 3X2 scant and 75mm jablite (polystyrene) and a roll
of visqueen.....it's only ever going to be a temporary structure anyway, and
there'll be little difference between using 75mm and 200mm of polystyrene,
inthat it's still going to be a cold shed.

And don't forget that you'll also need underfloor and loft insultaion
otherwise insulating the walls will be a waste of time.