At What Temperature Do Pipes Freeze? The 20°F Threshold and Where It Hits

Water freezes at 32°F, but pipes in unheated parts of a house are most at risk once it is about 20°F or colder outside. That is the “temperature alert threshold” a University of Illinois study set for the southern US, after an uninsulated attic pipe started to freeze each time the outdoor temperature fell to about 20°F.¹ Pipes can still freeze between 20°F and 32°F, especially where wind or gaps in the walls let cold air reach them.

We checked how often US ZIP codes reach that number in NOAA’s 1991–2020 climate normals. The median ZIP code gets 32 nights a year at or below 20°F. And in 59% of the ZIP codes whose average January low stays above freezing, a 20°F night still comes at least 1 year in 10, more than half of them in Texas and the other Gulf states.

Pipe freeze temperatures: 32°F, 28°F and 20°F

Sources: State Farm,⁵ NWS,^9,10 University of Illinois,¹ Red Cross and IBHS.^2,3 A Texas A&M AgriLife article gives the same 20°F figure, and Joel Pigg of AgriLife Extension advises precautions whenever temperatures dip below freezing.⁷

Google’s AI Overview for this question also repeats a rule of 28°F or colder for four hours. That line comes from a FOX 8 New Orleans explainer, which calls it a rule of thumb.¹⁴ We found no study behind the four hours, so we don’t use it.

Where the 20°F number comes from

Most pages that answer this question give 20°F without a source. It comes from Research Report 96-1 by Jeffrey Gordon of the University of Illinois Building Research Council, sponsored by an insurance industry institute.¹

In field tests in the winter of 1995, Gordon ran pipes through an unheated, vented attic. The most exposed one, uninsulated and on the cold side of R-30 fiberglass attic insulation, started to freeze 11 times, each when it was 15.4°F to 20.1°F outside. The attic was only about 3°F warmer than the outdoor air. Gordon called this the worst case and based his threshold on it:¹

a temperature alert threshold could be established at 20°F for the southern United States.

It has two limits. It was set for the South, where building practices, the report says, do not protect pipes from occasional severe cold. And Gordon expected isolated freezing between 20°F and 32°F, with holes in a house’s shell playing a major role.¹

Why a frozen pipe bursts

The ice doesn’t simply push the pipe wall out. First, ice blocks the pipe. Then more ice forms in the water trapped between that blockage and a closed faucet, and its pressure can climb to thousands of pounds per square inch. The pipe then fails at its weakest point anywhere downstream, which can be a joint or a fixture, not necessarily where the ice is.¹

Insulation buys time. On a 14°F night in the same tests, the uninsulated pipe was blocked by about 5:30 a.m. One inch of pipe insulation delayed the blockage by 9.5 hours and 1.5 inches by 23 hours. A pipe with 2 inches of insulation thawed before it ever blocked.¹

Which pipes freeze first

The Red Cross lists the pipes that freeze most often:²

• pipes exposed to severe cold, such as outdoor hose bibs, swimming pool supply lines and sprinkler lines;
• supply pipes in unheated areas: basements, crawl spaces, attics, garages and kitchen cabinets;
• pipes against exterior walls with little or no insulation.

IBHS says plumbing along exterior walls is common in warmer areas, including much of the South, and that homes built before 2010 typically lack frost-proof outdoor faucets.³ Nebraska Extension adds that with severe wind chill, a tiny opening can let in enough cold air to freeze a pipe.⁸

How often your area reaches 20°F (NOAA data)

For each of 40,789 ZIP codes with a NOAA station within 50 miles, we read that station’s 1991–2020 normals: the average number of nights a year at or below 32°F and 20°F, the chance of at least one 20°F night in a year, and the median dates.^12,13

In 84% of ZIP codes, a 20°F night comes in most years, and in 69% it comes essentially every year. The median ZIP code gets 32 nights a year at or below 20°F, and a quarter get 60 or more. Only 12% average less than half a night.

Dates are medians in years when that temperature is reached. Our frost date lookup shows the first 28°F hard freeze for any ZIP code.

Mild winters, rare hard freezes: the riskier mix

Gordon found burst pipes a bigger problem in the South than in the North, where pipes are better protected.¹ IBHS says arctic outbreaks still freeze pipes, especially where extreme cold is rare.³ Our data shows how wide that zone is. In 10,524 ZIP codes (26%), the average January low is above 32°F. Yet in 6,165 of them, 59%, a 20°F night still comes at least 1 year in 10, and in 5,202 at least 1 year in 4. Texas has 26% of those ZIP codes, followed by Georgia (12%), Alabama (10%), Louisiana (10%), Washington (7%) and Mississippi (6%).

February 2021 showed the cost. The NWS Houston/Galveston office lists broken pipes among the impacts of that week, which it called potentially the costliest weather disaster in Texas history. Its forecasts had warned:¹¹

Pipes (including sprinkler systems) will be subject to bursting if not adequately insulated

When the pipe-freeze season starts

Where a 20°F night comes in most years (34,349 ZIP codes), the median first one arrives Nov. 25 and the last Mar. 15. The first 20°F night comes a median 24 days after the first 28°F hard freeze; for the middle half of those ZIP codes, the gap is 21 to 28 days.

Our guides to frost vs. freeze and covering plants for frost explain the 28°F hard freeze, and pool owners should finish closing the pool before freezing nights put its plumbing at risk.

Before a freeze: what the sources tell you to do

Sources: Red Cross,² IBHS,^3,4 State Farm,⁵ Mississippi State,⁶ Nebraska Extension⁸ and University of Illinois.¹ State Farm adds a warning: if your house has a fire sprinkler system, shutting off the water may deactivate it.⁵

Drip hot or cold? The sources disagree

The Red Cross and Mississippi State Extension say to drip cold water.^2,6 State Farm and Nebraska Extension say warm water, preferably from a faucet on an outside wall.^5,8 Gordon’s report notes that hot and cold lines are separate pipes, so each one needs its own flow. It also advises leaving the faucet slightly open even if the drip stops, because the open tap relieves the pressure that bursts pipes.¹

Water flow will delay freezing, but it may not prevent it in all cases.

A drip is cheap: about a gallon an hour, or 2 cents a night, says Mississippi State Extension, and dripping the faucet farthest from the water meter draws water through the whole house.⁶

When the power goes out

IBHS warns that plumbing may freeze in extreme cold, particularly during power outages.⁴ Heat tape stops working then too.¹ Our guide to the generator size to run a furnace and the generator size calculator cover backup heat; read our portable generator safety guide before running one. If you heat with a heat pump, see how heat pumps handle cold weather.

If a pipe freezes anyway

If you turn on a faucet and only a trickle comes out, suspect a frozen pipe, and check your other faucets, because other pipes may be frozen too.² State Farm’s advice is to leave the faucet on and call your plumber.⁵ If a pipe has burst, State Farm says to turn off the water at the main shut-off valve.⁵

Next step

Look up your ZIP code’s first hard freeze in the frost date lookup, and finish the outdoor faucets and insulation before then. Methods are on our methodology page; more cold-weather guides are in heating and cooling.