Harry K wrote:

With no cold water pipe insulation, we have 4 potential savings in
water heating: the small warm slug of still water before flow begins,
the large flowing gain to the dew point, the small flowing gain from
the dew point to room air, and the effect of having warmer cold water
at a sink or a shower, which can allow using less hot water in a mix
to achieve a certain temperature. The latter may come from a lot more
cold water pipe in the house, unrelated to the pipe that goes into
the water heater. Harvard physicist William Shurcliff has written
about these savings...

Good discussion of the -theorectical- savings. Just how much savings
dollar wise do you think it comes down to in practical life?

The biggest might be from condensation on active pipes. If we spend
(say) 50K Btu/day warming 1000 pounds of 60 F well water to 110 in
a water heater, and insulating the cold water pipes lowers the 60
to 50, we have to spend 1000(110-50) = 60K Btu/day, ie 20% more, eg
10K/3412 = 2.93 kWh/day or 1070 kWh/year, eg $107 at 10 cents/kWh.

You won't see it on your power bill...

I disagree.

Nick