How Pipes Freeze and Burst

When water freezes, it expands. That’s why a can of soda explodes if it’s put into a freezer to chill quickly and forgotten. When water freezes in a pipe, it expands the same way. If it expands enough, the pipe bursts, water escapes and serious damage results.

Why Pipes Burst

Surprisingly, ice forming in a pipe does not typically cause a break where the ice blockage occurs. It’s not the radial expansion of ice against the wall of the pipe that causes the break. Rather, following a complete ice blockage in a pipe, continued freezing and expansion inside the pipe causes water pressure to increase downstream -- between the ice blockage and a closed faucet at the end. It’s this increase in water pressure that leads to pipe failure. Usually the pipe bursts where little or no ice has formed. Upstream from the ice blockage the water can always retreat back towards its source, so there is no pressure build-up to cause a break. Water has to freeze for ice blockages to occur. Pipes that are adequately protected along their entire length by placement within the building’s insulation, insulation on the pipe itself, or heating, are safe.

Regional Differences

Generally, houses in northern climates are built with the water pipes located on the inside of the building insulation, which protects the pipes from subfreezing weather. However, extremely cold weather and holes in the building that allow a flow of cold air to come into contact with pipes can lead to freezing and bursting.

Water pipes in houses in southern climates often are more vulnerable to winter cold spells. The pipes are more likely to be located in unprotected areas outside of the building insulation, and homeowners tend to be less aware of freezing problems, which may occur only once or twice a season.

Pipes in attics, crawl spaces and outside walls are all vulnerable to freezing, especially if there are cracks or openings that allow cold, outside air to flow across the pipes. Research at the University of Illinois has shown that “wind chill,” the cooling effect of air and wind that causes the human body to lose heat, can play a major role in accelerating ice blockage, and thus bursting in water pipes.

Holes in an outside wall where television, cable or telephone lines enter can provide access for cold air to reach pipes. The size of pipes and their composition (e.g., copper or PVC) have some bearing on how fast ice forms, but they are relatively minor factors in pipe bursting compared with the absence of heat, pipe insulation and exposure to a flow of subfreezing air.

When is it Cold Enough to Freeze?

When should homeowners be alert to the danger of freezing pipes? That depends, but in southern states and other areas where freezing weather is the exception rather than the rule (and where houses often do not provide adequate built-in protection), the “temperature alert threshold” is 20 degrees F.

This threshold is based upon research conducted by the Building Research Council at the University of Illinois. Field tests of residential water systems subjected to winter temperatures demonstrated that, for uninsulated pipes installed in an unconditioned attic, the onset of freezing occurred when the outside temperature fell to 20 degrees F or below.

This finding was supported by a survey of 71 plumbers practicing in southern states, in which the consensus was that burst pipe problems began to appear when temperatures fell into the teens. However, freezing incidents can occur when the temperature remains above 20 degrees F. Pipes exposed to cold air (especially flowing air, as on a windy day) because of cracks in an outside wall or lack of insulation are vulnerable to freezing at temperatures above the threshold. However, the 20 degrees F “temperature alert threshold” should address the majority of potential burst-pipe incidents in southern states.

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