In message , "Bob Smith
(UK)" writes
I was thinking of putting some of that 2mm polystyrene roll on the outside
walls before I wallpaper.

Has anyone ever used this before?

no, but we did have it on some of the walls in this house when we bought
it. We have removed it during redecoration and not replaced it.

Does it actually stop condensation?

I've never noticed any problem with condensation on those walls it has
gone from.

I
have a condensation problem in winter, behind wardrobes, drawers, pictures
etc.

I suspect you need to improve ventilation.
It's a 30's semi, not as draughty as it was , but not draught proof by
any means, but we have an extractor in the bathroom and kicthen
--
Chris French, Leeds