I did not know that. Insulation sounds like a good idea. I did find a site specific to the UK (https://www.diydoctor.org.uk/projects/condensationcisterns.htm ht

I did not know that. Insulation sounds like a good idea. I did find a site specific to the UK (https://www.diydoctor.org.uk/projects/condensationcisterns.htm https://www.diydoctor.org.uk/projects/condensationcisterns.h...) that calls it an anti-sweat valve and says it's available in the US. :) I've never heard of an insulated toilet tank over here. I wouldn't be surprised if they start showing up, though. My current house was built in the 80s, when the region was flush with oil money. I get the impression that building anything energy efficient was seen as a political statement, aside from the added cost to the builder.