[SmokinDuck]

I would think that if you were to wrap the rigid insulation with a heavy plastic and seal all the seams of the plastic with some sort of heavy tape, like the tape that they use on Tyvek house wrap, you should be alright. Kind of like gift wrapping it.

You might also be able to get rigid insulation that is impervious to absorbing moisture which would solve the problem.

Kick ass project by the way.