OK, I know this thread is ancient but I just had to add my two cents worth for people who come along later as I did. The idea that cast iron retains heat longer than fiberglass or acrylic tub materials is simply a myth. Cast Iron, even with porcelain over it is a better CONDUCTOR of heat than any of the modern tub materials other than steel. Steel and cast iron will conduct heat at more or less the same rate with differences that would only matter in a lab. Cast Iron will transmit heat to the surrounding air and other materials faster than fiberglass or acrylic, which are both better insulators than cast iron. This is the exact opposite of retaining heat. Anybody who doubts this: Take a cast iron pan, fill it with hot water. Take a plastic pan of equal size and fill it with hot water of the same temperature. Put the pans side by side with a thermometer in them. Check them an hour later. I guarantee you the water in the cast iron pan has lost more heat and is colder because cast iron is much better able to radiate heat to the surrounding air than plastic. There is no way that any metal tub whether steel or cast iron is going to keep the water in it warmer than fiberglass or Acrylic will. Yes cast iron does radiate some of the heat from the water back to the bather above the water line, which some people like, but it does this because it's a better conductor of heat, not because it holds heat. The sole advantage of cast iron is, admittedly, much greater strength and if you want maximum strength steel is stronger still. I've demo'd multiple cast iron tubs into pieces with a sledge hammer. It shatters easily. Try that with steel.