No, you've probably read about insulating acrylic tubs. Cast iron has enough thermal mass that when you get it hot from the water, it stays hot for a good long time. Your plumbers are correct.

There were a few posts about insulating cast iron, but I never understood why you would do it. The info posted by Greendesigns & your plumbers are correct. The built-in insulating factor is the only real selling point for cast iron vs. steel or even 'cast' or acrylic when it comes to tubs IMO.

Thanks so much for the quick replies! I guess it's one less thing I need to think about now. The 65-year-old cast iron tub we removed wasn't insulated, so I'm sure this one will work the same way.

Thanks!

We insulated around our new cast iron tubs just as an added measure. Our plumber said we didn't need any either, but others here had added some to theirs so we decided to as well. No harm in adding it. DH just used home craft-faced insulation--stuffed it around the tub as best he could while walls were open and the bathrooms were still down to the studs.

My building has cast iron bathtubs, and the water stays warm for about 5 minutes when bathing. I can see from the plumbing access that there is no insulation around the exterior of the basin. This will have to be corrected in some way.