Hmm, just looked at tri iso site andnot convinced by the sales pitch. "equivalent to 210mm mineral wool when installed in a pitched roof" which is quite a different scenario to a fridge, even if you belive those figures. Foil type insulation works by reflecting RADIANT heat back, but it has to be reflected back TO somewhere - in an attic this is useful as it goes back inside the thermal envelope of the house, but in your fridge box it will be hard up against either the box or liner, so no radiant heat transfer occurs. on the contrary, it actually conducts the heat through very effectively, which is the opposite of what you want... think high school physics!

the best material in terms of thermal resitance per thickness is gennerally thought to be extruded poyurethane (note extruded not expanded - this is good but not as good as extruded) and happily will suffer no ill effects from the marine environment.

I think some of the kingspan products are based on this, and are of course readily available from builders merchants.