[pmmunro]

Mike,

It's very possible that you know a lot more about building insulation than I do and the material of the outer wall may have a considerable bearing on the matter, but normal practice in buildings which have masonry walls and dry-lined interiors is to put the vapour barrier(s) as close to the inside surface as possible.

This is because most of the water vapour in a house is generated by the occupants and their activities. If the vapour permeates the inner wall and insulation, the thermal gradient and the dew point are likely to meet just where it is least desireable - where supporting wooden studs are fixed to the masonry, thus inducing rot in the perishible fabric. When using polystyrene insulated plasterboard, for example, it is recommended that a latex paint is applied to the interior surface and there is either a polythene or metallic foil between the plasterboard and the polystyrene. If further insulation is to be placed between the studs which support the plasterboard, another vapour barrier should separate the polystyrene from the studs and their insulation.

There is possibly a risk that putting in two vapour barriers, one each side of the insulation could trap and moisture which enters inadvertently.

If the walls of your shed are timber, I would consider treating the inside surfaces against rot, if this has not already been done, before fitting any insulation.

For more information, see the Mitchell's Building series of books and publications by British Gypsum and the Building Research Establishment.