Vapour barrier on single thickness wooden outer wall?

Question

I am insulating a single thickness outer wooden wall using polystyrene foam and then covering the inside with plasterboard. Do I need a vapour barrier and if so does it go on the wood side of the insulation or the plasterboard side?
UK so wet and damp sometimes and cold.
Thanks for your help and advice.

Answer 1

the vapor barrier goes between the insulation and the living space! u can also have a barrier under the siding that is on the wall......

lic. gen. contractor

Answer 2

The vapor barrier needs to be applied on the outside of your insulation. It is very important that the vapor barrier serves two functions; and that is to keep moisture out, and allow water vapor to escape that gets trapped in the wall system. To this end my preference is 15 # felt, installed horizontally, overlapping the rows at least 12 inches, and installing from the bottom up.

Let me add that the consequences of a double vapor barrier can be devistating. The trapped moisture is a breeding ground for mold. It is important to allow any moisture that insinuates itself into the wall system to be drawn into the home and exrtracted by virtue of your air conditioning system. That is why it is critical to keep the vapor barrier to the outside.