I have a Kohler bathtub from 1988 and I do not think it has a foam coating underneath. It might just have that spray on rubber coating to deafen sound of water. I am not sure what its material is, porcelain or porcelain glaze over metal? It's always freezing as F--- if you try and sit down in the water in winter, and the water gets cold fast and does not hold heat. I think a professional company put it in, but they did not think to insulate it - maybe because of mice. This was before sprayfoam existed. I'd love to give it a sprayfoam coating or something under there to make it a bit more insulated, I think it would make a world of difference.