Just fitted some insulation
(https://www.screwfix.com/p/hot-water-cylinder-jacket-18-x-80mm-x-1219mm/43483)
over a hot water cylinder which already had about 25mm of foam
insulation. Of course, the many pipe entries meant it was not easy to
line up the edges of the bags, and I had to cut a hole for the cylinder
thermostat, as the instructions say it should not be covered.

After fiddling around and making sure it wasn't covered, I wondered why
it shouldn't be covered. Surely it doesn't make any difference if it's
covered of not if it's on contact with the metal of the tank.

Oh, and the cylinder insulation doesn't seem to be designed for
top-entry immersion heaters (which we have) as much of the insulating
action seems to rely on the top of the tank being covered completely
with overlapping edges. I had to ensure the immersion heater and
connecting cable wasn't covered.

--

Jeff