Water in a pipe freezes when the pipe's surroundings hold it at 0 °C long enough to remove its latent heat. There is no single outdoor number: a bare pipe in a drafty exterior wall can freeze on a −10 °C night, while an interior pipe in a heated house survives −40 °C. Duration, wind, insulation and how much house heat reaches the pipe matter as much as the forecast.
- The number that matters is the air temperature right around the pipe, not the forecast high or the wind chill.
- A widely cited US field study found unprotected attic pipes began to freeze when it fell to about −7 °C (20 °F) outside; Canadian homes see trouble at a range of temperatures depending on where the pipe runs.
- Wind does not cool a pipe below the air temperature, but Environment and Climate Change Canada notes it makes objects lose heat faster.
- Canadian cities advise keeping indoor heat up in cold snaps: Calgary says at least 15 °C on the thermostat, Winnipeg no lower than 12 °C, and Montréal at least 10 °C around the water entry.
- There is no burst temperature; bursts come from long, deep cold acting on a pipe with a trapped pocket of water.
01 /At what temperature does the water in a pipe freeze?
Fresh water freezes at 0 °C, and water in a household pipe is no different. What people really want to know is how cold it has to get outside before the water inside the house reaches 0 °C and keeps losing heat. That depends on three layers between the pipe and the weather: the pipe's immediate surroundings, the building assembly around it, and how long the cold lasts.
Water also has to give up a large amount of heat to change from liquid to ice at 0 °C, about 334 kJ per kilogram, which is roughly 80 times the heat it gives up cooling by one degree. That is why a pipe can sit near 0 °C for hours before it freezes through, and why a cold snap that ends early often leaves pipes intact.
02 /How cold does it have to be outside?
It depends on where the pipe is. The best-known reference point comes from US field tests reported by the University of Illinois Building Research Council, which put a temperature alert threshold at about −7 °C (20 °F): that was where uninsulated pipes in an unheated attic started to freeze. Treat it as a rough marker for exposed pipes in unheated space, not a Canadian rule. In a Canadian house, pipes inside the heated envelope ride out much deeper cold, while pipes in cold pockets can freeze well above that marker if the cold is long and the air moves.
| Where the pipe runs | Outdoor cold that commonly causes trouble | What decides it |
|---|---|---|
| Hose bib not shut off and drained | Any night below about −5 °C | Water sits right at the wall, with no house heat |
| Unheated crawlspace, cottage or garage | Sustained cold below about −5 to −10 °C | Space itself drops below freezing |
| Attic or cantilevered floor | Below about −10 °C, sooner with drafts | Thin insulation and moving air |
| Exterior wall, sink cabinet on an outside wall | Several days below about −20 °C | Drafts at rim joists, closed cabinet doors |
| Interior wall in a heated house | Rarely, even at −40 °C | Surrounded by room air; mainly at risk if the heat fails |
Those ranges line up with the bands in the Pipe Freeze Risk Checker, which scores the lowest temperature in four steps: above −5 °C, −5 to −15 °C, −15 to −25 °C and colder than −25 °C, then adds points for location, protection, heat, drafts and history.
03 /How does duration change the answer?
Duration matters because freezing is a race between the heat a pipe loses and the heat that reaches it. A clear night at −25 °C that warms to −8 °C by noon gives a wall cavity time to recover; a week that never rises above −25 °C does not. Wall cavities, crawlspaces and garages lag behind the outdoor air, so on the first night of a cold snap they are still warm from the days before. By the third or fourth night they have caught up.
This site's How Fast Could This Pipe Freeze? calculator makes the time scale visible. For a bare ½-inch pipe in still air, starting from 10 °C water, its lumped heat-transfer model gives these times to freeze solid:
| Air around the pipe | Cool to 0 °C | Then freeze through | Total |
|---|---|---|---|
| −5 °C | 27 min | 6.5 h | about 7.0 h |
| −10 °C | 17 min | 3.3 h | about 3.5 h |
| −20 °C | 10 min | 1.6 h | about 1.8 h |
| −30 °C | 7 min | 65 min | about 72 min |
The freezing phase dominates. Doubling the number of degrees below zero roughly halves the time, because the heat loss is proportional to the difference between the water at 0 °C and the air. The how long pipes take to freeze guide works through pipe size, insulation and drafts in the same way.
04 /Can wind chill freeze pipes?
Wind chill cannot take a pipe below the actual air temperature, but wind can freeze it faster. Environment and Climate Change Canada explains that the wind chill value describes how cold it feels on skin, and that objects outside only cool to the air temperature: at −20 °C with a −30 wind chill, a car or a pipe goes no lower than −20 °C. It also notes that wind affects how fast bodies and objects lose heat.
For a pipe, that faster heat loss is the whole problem. Air leaking through a gap at the rim joist or around a hose bib strips heat from the pipe surface far faster than still air. In the calculator, switching a bare ½-inch pipe at −10 °C from still air to windy air cuts the time to freeze from about 3.5 hours to under an hour. So a forecast of strong wind with deep cold deserves more attention than the same temperature on a calm night, even though the wind chill number itself is not the pipe's temperature.
05 /What indoor temperature keeps pipes from freezing?
There is no single safe setting, because the thermostat measures the room, not the cold cavity where a pipe runs. Municipal water utilities still give useful floors. The City of Calgary asks residents to keep the thermostat at least 15 °C. The City of Winnipeg says to set it no lower than 12 °C at night and when away. Montréal asks that the air around the water entry stay at 10 °C or more. Confirm the current advice with your own municipality.
| Utility | Published advice | Applies to |
|---|---|---|
| City of Calgary | Keep the thermostat set for at least 15 °C | Whole house, including when away |
| City of Winnipeg | No lower than 12 °C at night and when away | Thermostat setting |
| Ville de Montréal | 10 °C or more around the water entry | The room where the service enters |
| City of Toronto | Keep areas warm, especially near the water meter, even when away | No number given |
The gap between those numbers tells you something: they are floors for an average house, not guarantees. A pipe inside a sink cabinet on a north wall can be 10 °C colder than the room thermostat, so opening cabinet doors and sealing drafts often does more than adding two degrees at the thermostat. The where pipes freeze guide lists the pockets that run coldest.
06 /At what temperature do pipes burst?
There is no burst temperature. A pipe bursts when a pocket of water is sealed between an ice plug and a closed fixture and keeps freezing, which requires enough cold for long enough to grow the plug and then more of it. The why frozen pipes burst guide explains the pressure mechanism. In practice, burst reports cluster in long, deep cold snaps and in the thaw right after them, when the plug melts and the split starts to leak.
Hot water lines are not immune. They sit full of water that cools to room temperature between uses, and a hot line in an exterior wall can freeze as readily as a cold one. People often notice a frozen hot line first because the cold side still works, which is one of the clues in the signs of a frozen pipe.
FAQQuestions people ask
Will my pipes freeze at −10 °C?
Pipes inside the heated part of the house almost certainly will not. A bare pipe in a drafty exterior wall, an unheated crawlspace or a hose bib that was not drained can, especially if the cold lasts more than one night.
Do pipes freeze faster in the wind?
Yes. Wind or drafts cannot make the pipe colder than the air, but they carry heat away much faster, so a pipe exposed to moving air reaches 0 °C and freezes through sooner.
What thermostat setting prevents frozen pipes in a cold snap?
Utilities publish floors rather than guarantees: Calgary says at least 15 °C, Winnipeg no lower than 12 °C. The pipe's cavity can be much colder than the room, so also open cabinet doors, seal drafts and trickle the risky tap.
Can pipes freeze when the heat is on?
Yes, if the pipe runs somewhere the heat does not reach: an exterior wall with a draft, a sink cabinet on an outside wall, a cantilevered floor or a garage. Most frozen pipes in occupied Canadian homes happen with the furnace running.
Can hot water pipes freeze?
Yes. Between uses the water in a hot line cools to the temperature of its surroundings, and in an exterior wall it can freeze like a cold line. It is common for the hot side to freeze while the cold side keeps working.