Since some water pipes are located either outside or in unheated areas where the ambient temperature may occasionally drop below the freezing point of water, any water in the pipework may potentially freeze. When water freezes, it expands due to negative thermal expansion, and this expansion can cause failure of a pipe system in any one of a number of ways.
Pipe insulation cannot prevent the freezing of standing water in pipework, but it can increase the time required for freezing to occur—thereby reducing the risk of the water in the pipes freezing. For this reason, it is recommended to insulate pipework at risk of freezing, and local water-supply regulations may require pipe insulation be applied to pipework to reduce the risk of pipe freezing.
For a given length, a smaller-bore pipe holds a smaller volume of water than a larger-bore pipe, and therefore water in a smaller-bore pipe will freeze more easily (and more quickly) than water in a larger-bore pipe (presuming equivalent environments). Since smaller-bore pipes present a greater risk of freezing, insulation is typically used in combination with alternative methods of freeze prevention (e.g., modulating trace heating cable, or ensuring a consistent flow of water through the pipe).