The short answer
Water freezes at 32°F, but pipes in unheated parts of a home are generally at risk once the outdoor temperature drops to about 20°F or below. That 20°F "alert threshold" comes from field research by the University of Illinois Building Research Council, which found freezing began at or below 20°F in uninsulated pipes in unheated spaces. Pipes exposed to drafts can freeze at warmer outdoor temperatures than that.
In the Chicago suburbs, 20°F nights are routine every winter, and severe cold snaps go much lower. O'Hare recorded −23°F on January 30, 2019.
Why 32°F isn't the whole story
A pipe inside your heated living space won't freeze just because it's 30°F outside. The house keeps it warm. What matters is the temperature around the pipe, which depends on:
- Where the pipe runs: exterior walls, crawlspaces, attics, garages and basement rim joists get much colder than the rooms you live in.
- How long the cold lasts: one cold hour rarely freezes a pipe; a sustained cold night or several days in a row can.
- Air movement: drafts through gaps around pipes, vents or sill plates pull heat away faster. Wind can't make a pipe colder than the outside air, but it makes the pipe reach that temperature much sooner.
- Insulation: pipe insulation slows heat loss but doesn't add heat. In a long, deep freeze, an insulated pipe in a cold space can still freeze.
- Whether water is moving: water flowing through a pipe is far harder to freeze than water sitting still.
A practical checklist by forecast temperature
Use the overnight low in the forecast as your trigger. This is a common-sense plan, not a guarantee. Homes vary, so adjust it if you know you have a problem pipe.
| Forecast low | What to do |
|---|---|
| Above 32°F | Nothing special. Good time to insulate exposed pipes and winterize outdoor faucets. |
| 32–20°F | Hoses disconnected and outdoor faucets drained; garage door kept closed if pipes run through the garage. |
| 20°F and below | Open cabinet doors under sinks on exterior walls; let faucets on problem lines drip; keep the thermostat steady overnight. |
| Below 0°F, or several days below 20°F | All of the above, plus check every faucet first thing in the morning so you catch a frozen line before it thaws and leaks. |
Our guide to winterizing outdoor faucets covers the fall prep step by step.
How long does it take for pipes to freeze?
There's no fixed number. A bare pipe in a drafty crawlspace on a −10°F night can freeze within a few hours. A pipe in a cool but sheltered basement corner might survive days of mild cold. That's why the safest approach is to act on the forecast rather than wait for signs of trouble.
What thermostat setting prevents frozen pipes?
Keep the heat at the same temperature day and night during a cold snap. Nighttime setbacks are when pipes in exterior walls are most likely to freeze. If you'll be away, a common recommendation is to keep the thermostat no lower than 55°F, and to have someone check the house. For longer absences, shutting off the main water and draining the lines is the safest option, and it's what insurers generally expect for a vacant home. Our post on whether insurance covers burst pipes explains why.
Why pipes burst when they thaw
Ice in a pipe doesn't usually split it right where it freezes. As ice forms, it pushes water toward the closed faucet, and pressure builds in the section between the ice and the faucet. That's where the pipe gives way, and you often won't know until the ice melts and water starts flowing through the crack. If a pipe has frozen, thaw it gently and watch it closely, as explained on our frozen pipe repair page. If it's already leaking, shut off the main valve and get it to a plumber. We can connect you with one for local burst pipe repair around Elmhurst.
Do some pipe materials resist freezing better?
PEX tubing can stretch somewhat as water freezes, so it's less likely than rigid copper to split, but it isn't freeze-proof. Fittings can still fail, and a frozen PEX line still stops your water. See PEX vs. copper pipes for a full comparison.