Water pipes as a rule should not be located in exterior walls, but unfortunately this might be the case in some houses. Uninsulated hot water pipes can cause your hot water to lose over 20% of its heat. Uninsulated pipes can also cause condensation.
For pipes in exterior walls, there needs to be insulation on the pipe itself, and there should be as much insulation as possible between the pipe itself and the outer surface of the wall.
Cold and hot supply lines should be insulated as well. Three common forms of insulation are spiral insulation wrap, fiberglass batts, and tubular pipe sleeves. Their effectiveness depends on whether they’re installed correctly or not.
If you install the insulation yourself, it is a fairly simple process, and the main concerns are to make sure the insulation covers everything well, and that you don’t compress it by wrapping it too tightly.
Well insulated pipes can save you 4-5% off your annual heating bill.