15mm internal diameter with a 25mm [1"] WALL THICKNESS - so the total diameter of the pipe insulation is 65mm [millimetres] = approx. 2 5/8" [U.S.] - for protection of a 15mm - 1/2" copper pipe.
On a 22mm [millimetre] - approx 3/4" - `Mains`- Drinking Water copper pipe - again the WALL THICKNESS of the pipe insulation should be 25mm .
Because the water being carried in a `Mains` - Drinking water pipe - coming in from below the ground is much colder than the temperature of water that has been stored in a tank [Generally - unless tank has just been filled] - it is `allowable` to have for example 22mm [3/4"] copper pipes from a Cold Water Storage Tank within the roof space insulated with `good quality` insulation with a wall thickness of 19mm [3/4"] - giving a total insulation diameter of 60mm = approx. 2 3/8" , although in an area where very low temperatures are experienced - THAT would be `False Economy`.
In general - because water within a `Mains` - Drinking Water supply pipe is VERY COLD in Winter - it has a `Head Start` regarding Freezing - so that is why the Wall Thickness of the pipe insulation has to be MUCH thicker than `Standard` pipe insulation - But also in areas where the temperatures drop to very low - ALL pipework within Roof Spaces AND Void areas needs to have suitably INCREASED Wall Thicknesses of insulation.
I hope that this is `Helpful` Chris.