They don't use the foam to back the fiberglass in racing boats for structure, they use it for flotation. They build those hulls then fill the gaps with expanding foam which seals them off from all water and makes the boat virtually unsinkable.


If you lay fiberglass onto foam there's a big chance you are going to melt the foam during the curing process. Foam generally doesn't like any kind of chemical interaction. Hell spraypaint will melt styrofoam.