Vapour barrier

[alan_m]

I have a outhouse which was previously, 40 years ago, an outside toilet.

Currently it has a concrete tiled roof with (breathable) membrane underneath the tiles. Internally the roof joists and the membrane can be seen.

I want to plasterboard straight on to the bottom of the joists with some insulation between the plasterboard and tiles. I may batten beneath the joists and fasten the plasterboard to the battens (giving adding another

50mm between the plasterboard and the tiles).

Do I need a polythene moisture barrier, and if so which side of the insulation?

The area to be boarded is small at approx 3m x 0.8m and this outhouse is unlikely to be heated apart from a small wall mounted electric heater for frost protection - controlled by a thermostat set for just above frost temperatures.

[John Rumm]

On the warm side - so just behind the PB (or use foil lined PB).

Chances are there will not be that much warm wet air in the building by the sounds of it, but for the effort involved to tack a plastic sheet behind the PB you might as well.

[The Natural Philosopher]

you need some sort of moisture barrier inside the insulation. consider foil backed plasterboard.

Or best of all, celotex wedged between joists and foil taped over the joins

[Jim Stewart ...]

vapour barriers are on the whole over rated and not required ....

[Mike Rogers]

Yes, between plaster board and insulation to prevent interstitial condensation.

Mike

[alan_m]

Thanks all,

I'm intending to do this as cheaply as possible as I already have 4 half sheets of plasterboard left over from previous jobs plus enough insulation, again from stock in my spare room.

I've now purchased some material for the moisture barrier from SF for around a tenner and boy does it stink! I had it in my kitchen for around

30 minutes before deciding to chuck it outdoors :)

Also from my stock a couple of lengths of stud timer - very slightly banana shaped now but it will do. This is not the best wood in the world but on the label it says it is suitable for buildings AND BRIDGES! I do hope when I next cross a bridge that the builders found something a lot better :)

[John Walliker]

Is this something to do with breeding racehorses?

John