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A pipe can burst within about 6 hours of freezing solid, but many stay frozen for days without failing. Time is not really the variable — pressure is. A pipe splits when an ice plug forms with a closed faucet downstream and the growing plug drives trapped water pressure past what the pipe can hold, which can reach thousands of psi. The practical rule from cold-weather research is that 6 hours or more below about 20°F in an unheated space is the danger window. Open a faucet to a trickle and the risk drops sharply.
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Why does a frozen pipe burst at all?
Not where you would guess. Water expands about 9 percent when it freezes, but the ice itself rarely splits the pipe — the split usually happens some distance away, between the ice plug and a closed faucet. As the plug lengthens it acts like a piston, compressing the liquid water still trapped ahead of it. Water is nearly incompressible, so the pressure climbs fast, and the pipe fails at its weakest point: a solder joint, a fitting, or a thin-walled section. This is why the damage often appears in a warm part of the house well away from the frozen run, and why an open faucet — which gives that trapped column an escape — is such effective protection.
What temperature and how long?
Research at the Building Research Council at the University of Illinois established the widely used 20°F "temperature alert threshold" for uninsulated pipes in unconditioned spaces in homes built for milder climates. Above that, pipes in attics, crawlspaces and exterior walls generally hold. Below it, freezing becomes likely once the cold persists. The lag matters: a pipe inside a wall does not reach outdoor temperature immediately, so a brief overnight dip is far less dangerous than a two-day cold snap.
| Outdoor conditions | Risk to pipes in unheated spaces | Rough time to trouble |
|---|---|---|
| Above 32°F | Effectively none | — |
| 20–32°F | Low; exposed hose bibs and uninsulated crawlspace runs only | 12+ hours |
| Sustained below 20°F | Significant for uninsulated pipes in unconditioned space | 6–12 hours |
| Below 0°F, with wind | High, including some interior exterior-wall runs | 3–6 hours |
| Any temperature, heat failed, house unoccupied | Highest risk scenario there is | Depends on building envelope |
Wind matters more than the thermometer suggests. Wind chill does not cool a pipe below air temperature, but it strips heat from the building envelope far faster, so an unsealed crawlspace vent or a gap in a rim joist turns a marginal night into a burst.
How do you know a pipe is frozen?
The first sign is almost always no water, or a thin dribble, from one fixture on a cold morning while the rest of the house runs normally. Others: frost or condensation visible on an exposed pipe, a section that is noticeably colder to the touch than its neighbors, a bulge or a faint bell shape in a copper line, unusual banging when a tap is opened, and a sewer-like smell from a drain, which suggests a frozen drain rather than a supply line. Check the fixtures on exterior walls first, then the crawlspace, then the garage.
How expensive is a burst pipe?
Enough to justify a $6 length of foam sleeve. Water damage and freezing is consistently one of the largest categories of homeowners insurance claims in the US — the Insurance Information Institute reports it accounts for roughly a quarter of all claims filed. A half-inch supply line running unattended can put hundreds of gallons an hour into a house. The repair to the pipe itself is often a $200 to $600 plumbing call; the drywall, flooring, insulation and remediation behind it is where the money goes.
What actually prevents it?
In order of effectiveness for the effort: disconnect and drain garden hoses before the first freeze and shut off the interior valve to any non-frost-free hose bib. Insulate pipes in crawlspaces, attics, garages and exterior walls with foam sleeves, or use UL-listed self-regulating heat cable on the worst runs. Seal air leaks — a crawlspace vent or rim-joist gap blowing directly onto a pipe does more damage than the ambient temperature. During a hard freeze, open the faucet farthest from the meter to a pencil-thin trickle, open cabinet doors under sinks on exterior walls, and keep the thermostat at 55°F or higher even if the house is empty. If you leave for the winter, shut the main and drain the system.
Related answers: at what outdoor temperature pipes freeze, what psi home water pressure should be — high standing pressure gives a freeze less margin — and what a plumber charges per hour for the emergency call you are trying to avoid. Basic kit for the shutoff and repair is in best plumbing tools.