Why Pipes Freeze and Which Ones Are Most at Risk

A frozen pipe rarely splits where the ice is. That single fact reshapes how you think about a cold snap, because the part of your plumbing you can see frosting over is usually not the part that fails. Understanding where freezing starts, what makes a pipe rupture, and which runs in your house are exposed lets you judge your own risk before the temperature does it for you.

This guide is about diagnosing vulnerability, not stopping it. For the steps that keep pipes warm, see our guide on how to prevent pipes from freezing (125). If a pipe is already frozen and a faucet has gone dry, see our guide on how to thaw frozen pipes safely (126).

Why Freezing Water Bursts Pipes (It’s the Pressure, Not the Ice)

Pipes burst because trapped water pressure has nowhere to go, not because expanding ice pushes the walls apart. This is the detail most explanations get backward.

Water expands by roughly 9 percent when it turns to ice. Inside a pipe, that growing ice forms a plug. Here is the part that matters: the plug itself does not split the pipe by pressing outward. Instead, as more ice forms, it compresses the still-liquid water trapped between the ice and a closed faucet or valve. Upstream of the blockage, water can retreat toward the source and relieve itself. Downstream, between the ice and a shut faucet, the water is sealed in. Pressure climbs in that trapped column until the pipe fails at its weakest point.

That weak point is often well away from the ice, on a section that never froze at all. The research on pipe bursts, including the University of Illinois Building Research Council findings, describes a closed system as the requirement for a burst. With water sealed on both ends, the only release is the pipe wall itself. The pressure involved is far beyond what household plumbing is built for, which is why a single hard freeze can split copper, PEX, or PVC alike.

Two practical lessons follow. First, the frosted, bulging section you find is a warning, not necessarily the eventual break. Second, a faucet left slightly open during a freeze gives that trapped water an escape path, which is part of why a trickle helps. The mechanics behind that trick are covered in our guide on whether to leave faucets dripping in a freeze (130).

The Temperature at Which Exposed Pipes Start to Freeze

Exposed, uninsulated pipes generally begin to freeze when the outside air falls to about 20 degrees Fahrenheit or below. That figure is not the freezing point of water. It is a practical alert threshold for plumbing.

Water freezes at 32 degrees, but pipes inside a home rarely match the outdoor reading. They sit behind drywall, under cabinets, or in spaces that hold residual heat, so the air around them lags well behind the thermometer outside. Researchers at the Building Research Council at the University of Illinois tested this directly. They found that uninsulated pipes in an unconditioned attic began freezing once the outdoor temperature dropped to roughly 20 degrees Fahrenheit. A separate survey of plumbers working in southern states put the onset of burst calls in the same range, with problems appearing as temperatures fell into the teens.

The American Red Cross uses that 20-degree mark as the point to start taking protective action. Treat it as a trigger, not a guarantee. A poorly insulated pipe directly against an exterior wall, or one hit by wind, can freeze at a higher reading than a sheltered interior run. Material, insulation, and location all shift the real number for any given pipe, which is why two homes on the same street can fare very differently in the same storm.

How Wind Chill and Uninsulated Runs Speed It Up

Wind does not make a pipe colder than the actual air temperature, but it does make a pipe reach that temperature faster. This is a common misread of wind chill, and getting it right changes how you assess a windy cold snap.

The National Weather Service is clear that wind chill applies to people and animals, not to objects. A pipe, like a car radiator, cannot cool below the true air temperature no matter how hard the wind blows. If the air is 15 degrees and the wind chill feels like minus 10, the pipe still bottoms out at 15 degrees, not lower.

What wind actually does is strip away the thin layer of slightly warmer air that clings to a pipe’s surface. It keeps replacing that insulating boundary with fresh cold air, so the pipe and the water inside lose heat much more quickly and reach the freezing point sooner. A run that might have survived a calm night at 22 degrees can freeze in a windy one because the cooling happens before any heat can reach it.

Uninsulated runs in unconditioned spaces compound the problem. The Department of Energy notes that pipes in crawl spaces and attics get none of your home’s ambient heat, which leaves them exposed to whatever the outdoor air and wind deliver. The combination of an uninsulated pipe, an unheated space, and a steady wind is the fastest path to a freeze, faster than the bare temperature alone would suggest.

A Room-by-Room Map of Your Most At-Risk Pipes

Your most at-risk pipes are the ones that combine cold exposure with little or no heat reaching them. Walking your home with that filter in mind tells you where to worry, in rough order of risk.

Start outside and at the edges, then move inward:

• Outdoor lines: hose bibs, irrigation and sprinkler lines, and pool supply lines sit fully exposed and freeze first. Winterizing these is covered in our guide on winterizing outdoor faucets and hose bibs (160).
• Where the water service enters: the point where the main line passes through the foundation is a classic freeze spot, especially if the entry is drafty.
• Attic and crawl space: pipes routed through these unconditioned spaces get no ambient heat and often have minimal insulation.
• Garage: an unheated attached garage with supply lines running through it, or a water heater out there, is a frequent failure point.
• Basement: usually warmer than the spaces above, but rim joists and pipes near foundation vents or cracked windows still freeze.
• Kitchen and bathroom cabinets on exterior walls: the supply lines under these sinks are cut off from room heat by the cabinet itself, which is why opening the cabinet doors is a standard freeze tactic.

The Red Cross lists these same locations, unheated interior areas and pipes in exterior walls, as the ones most susceptible to freezing. The pattern is consistent: the further a pipe is from your living-space heat and the more outdoor cold it touches, the higher it ranks on your map.

Why Pipes Against Exterior Walls Freeze First

Pipes inside exterior walls freeze early because cold reaches them from one side while house heat is blocked on the other. They sit in the worst possible spot, close to the outdoors and cut off from the rooms that stay warm.

A pipe routed through an interior wall is surrounded by conditioned space on both sides, so it stays near room temperature. A pipe in an exterior wall has cold pressing in from outside, often with thin or settled insulation between it and the siding, and a layer of drywall holding the room’s warmth back. That asymmetry means the water inside chills steadily even while the thermostat reads a comfortable number.

This is also why these freezes are hard to spot and harder to treat. The pipe is hidden behind a finished wall, so you get no frost to see and no easy way to apply heat. When a pipe inside a wall, near a gas line, or in an inaccessible space keeps freezing winter after winter, that is a situation to assess with a licensed plumber rather than something to chase yourself. Rerouting or reinsulating a pipe buried in a wall is skilled work, and opening a wall blindly to thaw a frozen run can cause more damage than the freeze.

Reading the Early Warning Signs Before a Full Freeze

The earliest sign of a freezing pipe is a faucet that slows to a thin trickle when the temperature drops. Catching that stage, before flow stops entirely, is your best window to act.

Watch for these signals during a cold snap, roughly in the order they appear:

• A faucet that runs weak or sputters when it had normal pressure the day before, which means ice is starting to narrow the pipe.
• Frost forming on the outside of an exposed pipe in a basement, garage, or crawl space, a clear sign the metal is cold enough for ice to form inside.
• A faucet that gives nothing at all, the marker of a fully blocked, frozen line.
• A pipe that looks swollen or bulged, or a visible crack or split, which signals the wall has already been stressed or breached.

If you catch a slow trickle and a single faucet has lost pressure while others run fine, you likely have a partial freeze on that branch, and warming that area early can stop it from completing. If flow has stopped entirely, the line is frozen and needs careful thawing, which our guide on how to thaw frozen pipes safely (126) walks through. If you find a bulge, a split, or any water escaping, treat it as a burst even if nothing is spraying yet, because a split hidden by an ice plug can flood the moment it thaws. Our guide on what to do if a frozen pipe bursts (127) covers that emergency.

Frequently Asked Questions

At what temperature do pipes freeze?
Exposed, uninsulated pipes generally begin to freeze when the outdoor air falls to about 20 degrees Fahrenheit or below, based on field research from the University of Illinois Building Research Council. Water itself freezes at 32 degrees, but pipes sheltered inside a home stay warmer than the outside air, so 20 degrees is the practical point to start watching. A poorly insulated pipe against an exterior wall, or one exposed to wind, can freeze at a higher reading.

Why does a frozen pipe burst away from the frozen spot?
Because the break comes from trapped water pressure, not from the ice itself. As ice forms a plug, it compresses the liquid water sealed between the plug and a closed faucet. That pressure builds until the pipe fails at its weakest point, which is often a section that never froze.

Does wind chill make pipes freeze faster?
Wind cannot cool a pipe below the actual air temperature, since wind chill only applies to people and animals. But wind strips away the warm air layer around a pipe and replaces it with cold air, so the pipe reaches the freezing point faster. A windy 15-degree night is harder on pipes than a calm one at the same temperature.

Which pipes in my house are most likely to freeze?
Outdoor lines like hose bibs and sprinkler lines freeze first, followed by pipes in unheated attics, crawl spaces, and garages, pipes where the water service enters through the foundation, and supply lines inside exterior walls or under sinks on exterior walls. The common thread is cold exposure plus little heat reaching the pipe.

Can pipes freeze even if my house is warm?
Yes. A thermostat reading a comfortable room temperature says nothing about pipes tucked inside exterior walls, under cabinets, or in unconditioned attics and crawl spaces. Those runs are cut off from your living-space heat and chill on their own, which is why pipes can freeze while the rooms feel fine.

This article is general information, not professional advice. For any frozen pipe inside a wall, near a gas line, or already leaking, contact a licensed plumber.

Sources

• American Red Cross, Preventing and Thawing Frozen Pipes: https://www.redcross.org/get-help/how-to-prepare-for-emergencies/types-of-emergencies/winter-storm/frozen-pipes.html
• U.S. Department of Energy, Turn Down the Temperature, but Don’t Let Your Pipes Freeze: https://www.energy.gov/energysaver/articles/turn-down-temperature-dont-let-your-pipes-freeze
• National Weather Service, Understanding Wind Chill: https://www.weather.gov/safety/cold-wind-chill-chart
• NOAA, Wind Chill: https://www.noaa.gov/jetstream/synoptic/wind-chill
• Live Science, Why Do Freezing Pipes Burst (citing University of Illinois Building Research Council findings on the 20-degree threshold and the trapped-pressure burst mechanism): https://www.livescience.com/43408-why-do-freezing-pipes-burst.html