Expandable foam is common in boats with sealed tanks. However, it tends to absorb and hold water, unlike the polystyrene sheets we recommend.

It also turns any repair into something resembling an archeological excavation, with a tunnel having to be bored through the expanded foam to reach the area of damage. I've done it several times and it ranks near the top of my "least fun boat projects ever." 

(Sure, for a repair you might have to dig out the foam sheeting, too. But it'll come out in big chunks, not in sandstone-like granules. And it won't be bonded to every surface in sight.)

   Thanks for your reply John. I decided to follow your advice and use foam sheets.  You are a wise man!  The foam sheets seem like a pain to cut to fit. I'm not quite there. I think the boat is plenty stiff and doesn't need the foam support.