boiler insulation

[Z.R. Struzik]

Dear Friends, 
I have about 2.0-2.5mm space between the boiler and the
jacket and I wonder whether insulating the boiler would 
be a good idea and what the best material for this would be.
I am considering ceramic sheet like the one used for the 
smokebox insulation or mineral or glass wool.
The only thing that I am concerned with would be possible 
moisture accumulation and corrosion to the boiler.
Many thanks for any advice!
Happy steaming,
Zbigniew