It’s true that all
insulation loose their resistivity over time & this is why generally when we design w/ polyiso we use ‘aged’ or long term values. I suspect that it will age much more slowly when nestled between your
hull & ice box than in a roof where temps can easily get up to 160-180 deg.
Trust me, if there was anything better & more affordable, we’d be using it in our buildings.
Lou K, AIA, LEED, EDGE Energy Expert