If you feel you need to stick the polystyrene to the metal, and in the small bit that I did I did so feel, expanded polyurethane foam sticks it very well indeed, You can also run it around the edges of the polystyrene to seal the panels. The foam guns are much easier to control than the cans with the plastic tube on top and there is considerably less waste.
Richard Tanner NB Cartref [Non-text portions of this message have been removed]
