I've got to decorate a room before Christmas where the owner has had issues with black mould on one wall, the room isn't used regularly as kids have flown the nest but she likes to keep it in good order and will during the winter months use a dehumidifier regularly. It's an upstairs room and the wall in question is on the elevation that tends to catch the 'weather', construction is solid wall, granite, I'm happy that it's not penetrating damp but is a condensation problem. Now, what I've suggested is 1" timber battens at 60cm centres, 25mm celotex in between, plasterboard over and skim. I've seen that wallrock do a thermal paper (polystyrene) which they recommend is papered over with standard wallrock. From looking at wallrock's site, this could be a viable and cheaper option, However, I recall as a youngster my old man putting up polystyrene wallpaper and painting it (I'm going back to the 60's here) and I can vividly recall taking a strange satisfaction from pressing the surface with my finger to make small 'dents' before eventually progressing to making full 'handprints'. Does anyone know if the new stuff is still as fragile as the stuff my old man put up in the 60's, I'm guessing that it will still be fragile in that if you knock it with a piece of furniture, it will easily tear. Anyone have any experience with it?