M_Boats: Buoyancy

I'm still working on my 15ft Monty and noticed that the old styrofoam is breaking up . That's in both the front and back areas , at the back there was just two small blocks about 8x10 inch by 1ft . I'm thinking of either filling the areas with some of the building foam in aerosol cans , or using sailing dinghy buoyancy bags . Anyone done either or have any ideas why one shouldn't do either Thanks in advance for any comments KLaas