Foil Backing is extremely out-dated now and is no longer needed on our Pilkingtons OW™ silvered mirror range.
It was used from the 30's right upto the mid-nineties when the protective paint coating on the reverse of the mirrors was no way near as good as today's advanced back-paint technology.
When fitting mirrors in damp/moist areas with adhesive the last thing you want is foil backing.
Firstly, you are adhering to the foil backing not the actual back of the mirror, i.e. relying on the adhesive on the foil backing to hold the mirror in place.
Over a period of time foil backing peels away from the back edges of the mirror and holds dampness, that is one of the reasons for silver rot.