I have two schwank radiant tube heaters in my shop for 2 years now both are two stage burners the tubes closest to the burners are aluminized steel are fine with no surface rust. But tubes at ends non aluminized have lots of rust that falls off like powder on cars and equipment otherwise i love...
Here in iowa for wood built pole sheds foam insulators only use open cell if you get a leak it will seep out and you will be able to see it. Open cell will hold moisture and start rotting the wood.