We're going to be putting in a new tub, and I'm not sure what to get. Cast iron? Enameled steel? Acrylic?
My concern with the cast iron (other than lugging it up the stairs) is the rust problem.
How well does acrylic hold up?
Cast iron tubs are very heavy and expensive. Rust is not a problem with cast iron tubs as long as the enamel top layer is intact (if you get a chip in it, you can get enamel repair kits). My wife did not like the old cast iron tub in our bathroom because it was cold when standing in it during a shower and made bath water get cold fast. Evidently she was not taking baths in the “cast iron method” which is to put super-hot water into the bathtub and allow the heat to transfer into the tub, this will cool the water down but keep it at a reasonable temperature longer (in theory at least, YMMV).
Enameled steel it basically a cheaper version of the cast iron, it is a lighter because the metal is much thinner. I doubt the “hot bath trick” described above for cast iron tubs will work in a steel tub. Like cast iron tubs, you should not have to worry about a steel tub rusting or cracking.
Acrylic (plastic) tubs come in a different quality levels. Some are really cheap, thin plastic that have the potential to crack over time (particularly if they are poorly supported/improperly installed). However some acrylic tubs use thicker stronger plastic (often with fiber glass embedded for tensile strength) that are very sturdy.
I recently had to make this decision; we replaced our cast iron tub with plastic Kohler model. I knew I did not want to lug a cast iron tub up to the 2nd story. I thought about a steel tub but they are still 120-150 pounds. I was worried about getting a cheap plastic one that would break so I settled on a $700 plastic Kohler tub that is pretty thick.
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Further Reading
Cast iron is the most desirable for a bath tub. It's withstood the test of time for centuries. Enameled steel has a cheap drumming sound to it. Americast (an American Standard acrylic product) is far better than those cheap fiberglass tubs, but also chips. Fiberglass flexes and, unless you've bought the entire wall kit (which is cheezy), will continue to give you fits where the tub perimeter meets the tile. There's always trade offs.
If it's your forever house, I'd do the cast iron. Of the lot, it has the most durable finish and, unless you're prone to drop hammers in it, will probably outlast you. Usually a cast iron tub fails - not because of the material - but due to improper installation, using harsh cleaning products on the surfaces, or damage from impact. All other materials will meet the same fate.