As a professional engineer, I often examine building components that have failed. And I'm always struck by the fact that the majority of homes damaged by frozen pipes are new construction. This has led me to wonder what the folks building these houses failed to consider as they constructed buildings that would later prove vulnerable.  

We all know that several days and nights of unusually cold weather often result in pipes freezing and breaking. The resulting failures often cause water to flow from the broken pipe until someone notices it, finds the appropriate shut-off valve, and stops the flow. The resulting damage can be expensive to repair.

The simplest answer to the question of why this happens is because the pipe got too cold. But that doesn't explain how pipes can be vulnerable, or what makes some pipes freeze and break while others do not. My simple understanding is that as water in a pipe freezes and expands, it pushes outward on the pipe walls until they stretch and burst.

But a study from the University of Illinois at Urbana-Champaign indicates that while this can happen, it rarely does. It is more common for the ice to form within the pipe and try to expand along the length of the pipe, according to the study.


Which type of pipe material is best to prevent freezing? It depends on the specifics of each application and installation. The following are some attributes and drawbacks of some popular pipe materials.

COPPER. Metal pipes and water itself are fairly good conductors of heat. Therefore, a copper pipe that supplies an outdoor faucet might benefit from interior conditions if it extends along the underside of the first-floor framing in an unfinished basement. Some residual heat from the basement will warm the pipe, and the copper will conduct this heat along its length. This may be enough to keep the water in the pipe from freezing. However, because copper is a good conductor, heat traveling along a copper pipe can also be lost rapidly. For a copper pipe that is insulated, if that pipe extends to the outdoors, the insulation may actually prevent heat from reaching the pipe where it may otherwise have been exposed to a relatively warm basement. Further, if there is a gap in the insulation and cold air can blow across the exposed length of pipe, heat can flow rapidly from the warmer parts of the pipe and be lost to the local cooling.  Consequently, a pipe may freeze because it is insulated or because its insulation is incomplete.  

PLASTIC. It's also important to consider the relative strength and flexibility of the different pipe materials. For example, if a pipe material could simply stretch as the water pressure increased, perhaps a rupture could be avoided entirely. Plastic pipes include rigid (PVC or CPVC) and flexible (PEX), and they don't have the same characteristics as copper when it comes to freezing.

PVC. Polyvinyl chloride pipes (PVC) and post chlorinated polyvinyl chloride (CPVC) pipes are rigid pipes, with thicker walls than copper pipe. These rigid-plastic pipes do not conduct heat very well, so a flaw in the insulated jacket will be less important to a plastic pipe than it would be to copper. PVC and CPVC pipes do possess some ability to expand under elevated pressure, but they may also become brittle at low temperature, and therefore be vulnerable.     

PEX. PEX or crosslinked polyethylene is typically more flexible than PVC or CPVC. Further, PEX is generally run in a single continuous line from source to fixture, so there are very few intermediate connections. So long as the fittings are well attached, if the PEX can stretch sufficiently to accommodate an increase in internal pressure, a PEX pipe may freeze and thaw without ever rupturing. However, the overall system vulnerability may still be governed by the components of the system, especially the connections, and even PEX is not immune.

Water is incompressible, so where water is confined between the expanding ice and a closed fixture, the pressure increases until the pipe ruptures. The rupture will occur at the weakest part of the pipe holding the confined fluid. Therefore, the pipe most likely bursts away from the actual freeze location, and water pressure in a confined volume is a critically important part of the underlying cause.

Some Solutions
That said, the best way to prevent a pipe from freezing is to keep it warm enough so the water within remains above the freezing point.

The most common way to warm a pipe is to leave it exposed to warm interior air, most often by locating pipes within interior walls—where the temperature on both sides of the wall is usually well above freezing. Alternatively, if a pipe must go in an exterior wall, a builder can locate the pipe so that it lies between the warm room and the wall insulation. In this case, even though the pipe will be cooler than the adjacent room, so long as there is enough insulation to the outside of the pipe to keep it warm, it likely won't freeze.

Finally, in certain circumstances, one may have to install heat trace on a pipe to heat it directly.  Heat trace is a resistance heating tape that wraps around the pipe and uses electricity to keep it warm—a kind of electric heater for the pipe. Unfortunately, this is expensive to install and operate, and becomes ineffective if there is a power outage.  

For pipes that are not actively kept warm, it is possible to slow cooling by insulating the pipe. Insulation helps prevent freezing in two ways. First, the insulation works by slowing the rate of conductive cooling—that is, water losing heat directly to the pipe walls, and the pipe walls losing heat directly to the surrounding air. Second, insulation can also provide protection against convective cooling, that is, cooling that results from cold air blowing on the pipe and carrying heat away at an accelerated rate. Where a pipe is exposed to an air leak, the cold air blowing against the pipe will rapidly increase the rate of cooling and subsequently increase the pipe’s vulnerability to freezing, a kind of wind-chill effect. 

Another way to prevent bursting is to relieve the pressure on both sides of a freeze. In many instances, outdoor water faucets may be winterized by closing a valve inside the house, somewhere along the length of the pipe, and then opening the outdoor tap. Even if the tap is opened only long enough to partially drain the line, once it is closed, there will result a volume of air between the faucet and the water that remains in the pipe. In this case, the expanding ice will tend to compress the air volume and prevent the pressure from exceeding the strength of the pipe and its connections.