The Foam is closed cell, however it will absorb water over time. Usually it's meant for emergency floatation. As for using it in a small cavity, work in small batches and make sure that there is an opening for any excess expansion.

Peel ply is usually used on the last coat of fiberglass. If you want to save more resin weight vacuum bag the laminate.

Jason

> That is cool, Ive thought about it but the budget said no...

> Careful with that foam. Closed cell? It could absorb water and really
> create a problem if not. Also, should there be a problem underneath it it
> would be difficult at best to fix. I like the peel-ply idea! Is it only
> used on the final fill coat?