qu about insulation of outside wall

[John Nagelson]

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 are:

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

[Phil L]

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

8 inch timber framing?

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.

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.

[John Nagelson]

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.

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

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

[sm_jamieson]

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.

[Donwill]

Don't forget about building in some thermal inertia unless you are going to provide a heating source which is on 24/7. A shed with intermittent heating and very little thermal inertia will show rapid changes of temperature causing condensation on the colder bits. Ideally I would have a very well insulated shell as you are proposing and have some material with a high specific heat inside. eg Concrete blocks. Don

[John Nagelson]

Thanks - I will have a look at this. Seems to have very high resistivity. I've seen it with foil both sides. I guess this is for manufacturing reasons?

John