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, it’s caused by the increased pressure created between the frozen section and the faucet. 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.

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 typically do not provide adequate built-in protection or builders carelessly place pipes in unprotected areas), the “temperature alert threshold” is 20 degrees F.

However, freezing incidents can occur when the temperature remains above 20° 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. 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.

Homeowners in the south need to be alert to the danger of freezing and bursting water pipes when the outdoor temperature threatens to drop to 20 degrees F. That’s the temperature at which ice is likely to start forming in water pipes located in an unheated portion of a house. Once ice forms into a blockage in a pipe, continued growth of ice in the pipe can lead to excessive water pressure. It’s pressure of the water that has no place to go when ice builds that causes the pipe to burst, rather than ice pushing against the wall of a pipe.

Although 20 degrees F is well below the freezing temperature of water, two factors make this the critical outdoor temperature:

1. The temperature of an unheated portion of a house is almost always at least a few degrees above the outdoor temperature. For example, an insulated attic may be at 37° or 38° F when the outdoor temperature is 32 degrees F.
2. Water “super cools” several degrees below freezing before any ice begins to form. In research tests at the University of Illinois, water pipes placed in an unheated, insulated attic consistently started forming ice when the outdoor temperature dipped just below 20 degrees F.

Builders in the north, where temperatures in the teens and lower are common, routinely install more building insulation and usually do not place pipes in unheated portions of a building. There are several precautions homeowners in the south can take when the temperature is expected to dip into the low 20s:

• Seal all openings where cold air can get at unprotected water pipes. It’s especially important to keep cold wind away from pipes, which speeds up the freezing process.
• Cover pipes with foam or fiberglass insulation sleeves, the thicker the better.
• Install heating tapes or cables on vulnerable pipes. Be sure to follow manufacturer instructions carefully to avoid starting a fire.
• Leave cabinet doors open under the kitchen and bathroom sinks to allow warmer room air to circulate around pipes.
• Let faucets drip slowly to keep water flowing through pipes that are vulnerable to freezing. Ice might still form in the pipes, but an open faucet allows water to escape before the pressure builds to where a pipe can burst. If the dripping stops, it may mean that ice is blocking the pipe; keep the faucet open, since the pipe still needs pressure relief.