How Freezing Causes Pipe Failure and What You Can Do to Prevent It

By Jonathan Simon, Lubrizol Advanced Materials Inc.

Frozen pipes don’t fail for the reason most people think they do. It’s not the pressure of expanding ice against the pipe wall that causes the failure. Instead, as the video below shows, frozen pipes burst because ice in the pipe forms a dam that blocks the flow of water. As the ice expands, the water pressure builds behind the dam and when that pressure exceeds the burst pressure of the pipe material, a service plumber somewhere gets an emergency call.

Factors that Influence Failure from Freezing

Knowing the cause of pipe failure from freezing provides a better understanding of how well different materials can resist freezing.

But before I get into those factors, let me make one point very clear: There is no such thing as a freeze-proof pipe. All residential plumbing materials will fail if the pressure within the pipe exceeds the material’s burst pressure. The tips for preventing freezing presented in the next section can be helpful regardless of the material you are using.

There are three variables that influence the rate at which a pipe will fail when exposed to cold air.

1. The first is the thermal conductivity of the pipe material, which determines how well the material can resist cooling. Of the three commonly used residential plumbing materials, CPVC resists cooling the best, followed by PEX. Copper has the highest thermal conductivity with the least resistance to cooling.
2. The other factor is the material’s burst pressure. Copper compensates for its thermal conductivity with a quick burst pressure of 3,000-4,000 psi, compared to 1250 psi for CPVC and 475 psi for PEX. When you combine the effect of thermal conductivity and burst pressure, you find that CPVC and copper will fail from freezing at about the same rate and PEX will fail faster than those two materials.
3. With PEX, the elasticity of the material can also factor into the freezing rate. New PEX will expand or balloon somewhat to protect the pipe from bursting; however, this expansion can weaken the pipe wall, increasing the risk of future failure. In addition, as PEX ages, chlorine degradation may reduce the elasticity of the inner pipe wall, which prevents expansion, diminishing any advantages the elasticity of the material provides in terms of freeze-break resistance.

Protecting Pipes from Freezing

Whether installing pipe in new construction or responding to a failure, here are ways you can reduce the risk of piping failure from freezing.

• Always plan for cold weather. 
  Just because freezing conditions are uncommon in certain areas, doesn’t mean they never happen. According to the National Oceanic and Atmospheric Administration, the vast majority of the United States can expect the temperature to drop below freezing at least once a decade. As weather patterns have become more unpredictable, it’s become more important to plan for the worst.
• Avoid installing pipes in unconditioned areas.
  Unconditioned areas, such as attics, crawlspaces, overhangs or exterior walls, are the first to get cold when outside temperatures drop and therefore present the biggest risk for frozen pipes. Running pipe in conditioned spaces with the piping within the building insulation envelope is always preferred and may be required by code.
• Use appropriate insulation practices.
  When pipe must be installed in unconditioned areas, compatible insulation must be used on the pipe to slow the rate of freezing. Heat trace systems may be used in connection with some piping materials to warm the pipe within these spaces. Be sure to use an insulation material or heat trace system that is compatible with the pipe. To determine compatibility with FlowGuard Gold CPVC, use the FBC™ System Compatible Program.
• Install pipes under slab foundations instead of overhead.
  By installing piping beneath the slab, the natural geothermal insulation of the ground will often be adequate to protect against frozen pipes. In addition, running pipes under the slab can have other benefits to builders and contractors, including faster installations and lower installed costs. Always verify that local codes allow installation of water distribution piping under slab.
• Seal exterior wall penetrations
  Holes or gaps in exterior walls for cable or electrical penetrations can allow cold outside air to reach pipes, creating a “wind chill” effect that accelerates freezing within pipes. Sealing exterior penetrations and gaps can help prevent cold air from flowing around the pipes.

Different materials may fail from freezing at different rates, but all materials are vulnerable to failure from freezing and should be protected. To access more plumbing resources, visit FlowGuardGold.com.

Jonathan Simon is the North American residential plumbing manager for Lubrizol Advanced Materials Inc., the parent company for FlowGuard Gold Pipe and Fittings.