by Perry525 »
Thu Mar 03, 2011 6:58 pm
As you will understand a solid 9 inch wall is basically cold.
Being a bedroom and probably upstairs you have the water vapor produced by breathing and sweating and the water vapor that transits into the bedroom, from cooking, washing and drying things on and above radiators, because water vapour is always attracted to cold and air holding water vapour is lighter than dry air it moves into the cold walls.
However, the water vapour will always make its way through the wall to the outside where it is colder. Water vapour only causes a problem when the outside is warmer than the inside and then the water vapour can build up inside the wall on the surface of the wall facing the polystyrene.
In the Britain it would be rare for a day to be that warm and if it were you would probably have the windows open and the water vapour would exit via the window.
If your stick polystyrene backed plasterboard to the walls, water vapour will merely pass through the joints in the boards and make its way outwards.
Plaster, plasterboard and brick walls are transparent to water vapour and it moves through these quickly and easily.
Polystyrene is to all intents water proof and water vapour proof.
An inch of polystyrene will slow the movement of heat through those walls and make the room a lot nicer to use.
Do it!
