Insulating hot water lines that run through a heated basement or crawl space sounds logical, but in fact it will same you little or no money. The infrequency of their use lets them cool off whether they are insulated or not, and the heat is converted to useful room heat anyway. The exception is pipes serving cycling loads like washing machines: they can profitably be insulated. Also pipes serving much used bathrooms could profit from keeping the water in the pipes hot for about 20 minutes between bathroom visits. In some areas that have no freezing weather, hot water tanks are often very far from the bathrooms and run through unconditioned attics. In this case it is possible that it takes a good percentage of the total volume of the hot water tank just to fill the pipe from the tank to the shower. These cases can justify pipe insulation because there is a lot of pipe loosing water heat to a cool winter time attic. Insulating the cold water lines will stop them from sweating in the summer (make sure that they are taped on air tight with a vapour barrier on the outside) -- that's a productive use for pipe insulation in all humid climates. What can save water in all of these applications is a recirculation system that does not dump water down the drain while waiting for the hot water to get to that far bathroom, but recirculates it back to the hot water tank. Check out Recirculating Pumps.