When installing mirror panels in a damp environment they must be fixed to a clean, dry, sealed substrate and the edges should be sealed to prevent any water ingress.  

You can use a neutral cure silicone sealant or a two part epoxy grout for glass. This must be regularly checked and replaced to ensure that the silvered side is not damaged by moisture. 

Some delamination and blackening of the silver can occur if moisture does creep behind the panel.
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