Guide

What Temperature Do Pipes Freeze?

What temperature do pipes freeze at?

Water freezes at 32 °F (0 °C), but pipes rarely freeze until the outdoor air drops to about 20 °F (−6 °C) and stays there for four to six hours. Insulation, wind and where the pipe runs all move that threshold. Exposed pipes in unheated spaces freeze first; pipes inside heated walls survive far colder nights.

Why the answer is not 32 °F

Water freezes at 32 °F (0 °C). A pipe is not a glass of water sitting outside.

A pipe runs through a building that is losing heat in every direction, and some of that heat passes through the walls, floors and voids the pipe runs in. The air in a crawl space under a heated house is warmer than the air outside it. So the pipe stays above freezing for a while after the outdoor air has dropped below it, and the outdoor temperature has to overshoot before the water inside actually turns to ice.

Field research by the Building Research Council at the University of Illinois put that overshoot at around 20 °F (−6 °C) for the most vulnerable case — an uninsulated pipe in an unheated space — and called it a temperature alert threshold. A separate survey of 71 plumbers working in southern states landed in the same place from the other direction: burst-pipe callouts started arriving when temperatures fell into the teens.

That number is a warning line, not a guarantee in either direction. A well-protected pipe can sit through a night at 5 °F without trouble. A pipe running through an uninsulated exterior wall with a gap in the siding can freeze at 25 °F.

How many hours of cold it takes

The threshold and the duration work together. Twenty degrees for an hour is nothing; twenty degrees from dusk until mid-morning is a different proposition.

Approximate time to freezing for a vulnerable, uninsulated pipe in an unheated space. Insulation, a heated space, or a buried run all push these times out substantially.
Outdoor air	Exposed outdoor pipe	Pipe in an unheated interior space
32 to 25 °F (0 to −4 °C)	Overnight, if at all	Unlikely
25 to 20 °F (−4 to −6 °C)	6 to 12 hours	12 hours or more
20 °F (−6 °C)	4 to 6 hours	6 hours or more
Below 15 °F (−9 °C)	2 to 4 hours	4 to 6 hours

Two things shorten every row in that table. Wind, because moving air strips the insulating boundary layer off an exposed pipe. And a house that is not being heated, because then nothing is feeding warmth into the spaces the pipes run through — which is why an empty house in a cold snap is the classic burst-pipe story.

Which pipes freeze first

Rank your own pipes before the cold arrives, because the order is always the same:

Outside the building entirely. Hose bibs, garden taps, irrigation lines, pool plumbing, exposed pipe on an exterior wall. Nothing is protecting these but their own insulation.
Unheated spaces inside the envelope. Crawl spaces, garages, attics, basements without heat. These are the pipes the 20 °F threshold was measured on.
Exterior walls. A supply pipe run through an outside wall, especially where insulation was cut around it, or under a kitchen sink on an outside wall.
Interior walls and heated spaces. These freeze only when the heating has failed or the house has been left cold.

If your house has never had a frozen pipe, that ranking is why: everything in categories 1 and 2 either does not exist, is insulated, or drains down for winter.

Why a frozen pipe bursts

The intuitive explanation — ice expands and splits the pipe — is not what usually happens, and the real mechanism changes what you should do about it.

Ice does expand by about 9%, but a pipe rarely fails at the ice plug. It fails somewhere else along the run, often on the warm side of the blockage. As the plug grows, it seals the pipe. Water still trapped between the growing plug and a closed tap has nowhere to go, and water does not compress. The pressure climbs until the weakest point in the run gives way — a joint, a thin spot, a section already stressed.

Two consequences follow. First, a pipe can be frozen solid and perfectly intact, and only burst later. Second, an open tap downstream of the freeze gives that pressure an escape route, which is why a dripping tap works: not by keeping the water warm, but by refusing to let the pressure build.

The burst usually shows up on the thaw

A split pipe full of ice does not leak. It leaks when the plug melts, which is often hours after the cold has passed and everybody has stopped paying attention. If pipes froze, the thaw is when to be at home and near the stop tap.

What to actually watch on the forecast

The number that decides this is not the afternoon high. It is the overnight low, and how many hours the temperature spends below the threshold.

Low of 32 to 25 °F: drain and disconnect anything outside. Nothing else needed.
Low of 25 to 20 °F: open the cupboard doors under sinks on exterior walls so room air reaches the pipes. Check that the crawl space vents are closed.
Low at or below 20 °F, for more than four hours: set a trickle running on the tap furthest from where the water comes in, keep the heating on overnight rather than setting it back, and know where the stop tap is.
Leaving the house empty in winter: hold the thermostat at 55 °F (13 °C) or higher and have someone check on it. This is the scenario that produces most of the serious damage.

Notice that all four lines turn on an outdoor number, over a duration. Which means the useful habit through a cold snap is a boring one: look at the outdoor reading and the overnight low, in the unit you actually think in, before you go to bed.

Common questions

Can pipes freeze at 30 °F?

An exposed outdoor pipe or a hose bib can freeze at 30 °F if the wind is up and the cold holds all night, but it is uncommon. Pipes inside the building envelope almost never freeze at 30 °F, because the house is still losing heat into the spaces around them.

How cold does a house have to get inside for pipes to freeze?

Keep the indoor temperature at 55 °F (13 °C) or above and the pipes inside the heated envelope are not the ones at risk. That number is the standard advice for an empty house in winter. The pipes that still freeze at 55 °F indoors are the ones outside the heated space — in the crawl space, the attic, the garage, or an exterior wall.

Do pipes freeze faster in the wind?

Yes, if the pipe is exposed to moving air. Wind strips away the thin layer of warmer air around the pipe and speeds up heat loss, so an exposed pipe reaches freezing sooner on a windy night than on a still night at the same temperature. Wind makes no difference to a pipe buried in insulation or below the frost line.

How long does it take for a frozen pipe to burst?

There is no fixed interval. A pipe bursts when a growing ice plug pushes the water trapped between it and a closed tap to a pressure the pipe cannot hold, which can happen within hours of the blockage forming or not at all. Nothing about the outside of the pipe tells you how close it is.

Does letting a tap drip actually help?

It helps, for two reasons. Moving water takes longer to freeze than still water, and an open tap gives the pressure somewhere to go if an ice plug does form — which is the part that prevents the burst. A trickle about the thickness of a pencil lead is enough; the usual advice is to start on nights below about 15 °F.

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