I went and bought 2" Styrofoam board to use for insulting.
(edit)
Having used auch extensively for years Styrofoam (coolite) and polyurathane(sp?) foam both are
fine bits of kit.
Things to pay attention to include protection from physical damage, deterioration in full sunlight,
careful use of solvents (mineral oil based products) and knowing freshly blown foam emits gas
for some days after moulding - the urathane "builders foam" aerosol packs do not generally
cause bees any noticeable discomfort, over getting bogged in it.
And yes, left locked in by foam an active colony will chew through 50mm in around two days.
I attach one hive body built from a urathane product, note the lining. A work in progress, disregard
the construction supports/seals.
Cheers.
Bill