On 29 Feb, 10:42, John Nagelson wrote:
On Feb 28, 5:48 pm, "Phil L" wrote:

John Nagelson wrote:
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?

Two lots of 4 inch. Over the top by some people's standards, I
realise. Call it a folly or a waste of capital investment, but I want
something very warm and with very low running costs.

4) plasterboard
5) common-or-garden internal silk emulsion paint
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.

How would you get a vapour barrier onto the warm side? Use vinyl paint
facing into the room or...?

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, in that 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.

Sure; it will be very warm once I've finished with it! As for
temporary, well true in the planning sense but it will be similar to
many buildings classified as permanent in Scandinavia.

What would you put on top of the visqueen?

I'd value your opinion on which is preferable, if I do it my way, i.e.
from outside:

1) polystyrene/foil, gap, plasterboard
2) polystyrene, gap, foil/plasterboard
3) just forget about the gap, in which case it doesn't matter what the
foil is stuck to

I'm told that when foil gets dusty, it stops reflecting so much
radiant heat. And various places say you need a gap by it; but I'm not
sure why.

Cheers,

John

It would be more usual to use celotex/kingspan, which is a better
insulator than polystyrene (but more expensive also). Taped with foil
tape to give a vapour barrier.
Simon.