What temperature causes pipes to freeze?
Short answer: Water freezes at 32 degrees Fahrenheit, but interior plumbing in a heated home is generally at risk when outdoor temperatures fall to about 20 degrees or below for several hours. Exposed pipes in unheated garages, crawl spaces, and attics can freeze closer to 32 degrees, especially with wind. Duration matters as much as the low: a brief dip is far less dangerous than 12 hours of cold.
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- Commonly cited alert threshold: 20°F outdoor temperature for interior pipes in unheated cavities.
- Exposed and wind-exposed pipes can freeze at temperatures just below 32°F.
- Time at temperature matters. Six to twelve hours of sustained cold is the danger window.
- Wind strips heat from pipes faster than still air at the same temperature.
- An unheated or vacant property can freeze at much milder outdoor temperatures.
Why 20 Degrees Is the Practical Alert Point
Heated homes keep wall cavities and crawl spaces above outdoor temperature. Field experience and utility guidance both place the practical risk point for pipes in those semi-protected spaces around 20 degrees Fahrenheit, because that is where cavity temperatures can drop below freezing for long enough to form an ice plug.
Factors That Lower the Threshold
- No insulation on the pipe or in the surrounding cavity.
- Air leaks at rim joists, sill plates, and wall penetrations.
- Wind exposure on a north or west wall.
- Interior doors closed, blocking heat from reaching a plumbing wall.
- Thermostat set back while away, or heat off entirely.
- Attached garage doors left open during cold weather.
Actions Tied to Forecast Temperature
- Below 32°F: disconnect hoses, shut off and drain exterior spigots.
- Below 25°F: open cabinet doors under sinks on exterior walls, keep the thermostat steady.
- Below 20°F: let a pencil-thin stream drip from the fixture farthest along each vulnerable run.
- Below 10°F, or any extended cold: check crawl spaces and garages, add temporary heat where pipes are exposed.
Colorado and Denver Considerations
Denver metro sees overnight lows below 20 degrees dozens of times each winter and occasional stretches below zero. The combination that causes local damage is a hard overnight low followed by bright sun and rapid warming, which produces the thaw-time bursts that fill restoration schedules in January and February.
Exceptions and Important Considerations
- Vacant or unheated homes can freeze at outdoor temperatures in the high 20s.
- Pipes in an attic can freeze while the living space below is comfortable.
- Mobile and manufactured homes with underfloor plumbing are vulnerable at milder temperatures.
Related Questions
Typically six to twelve hours of sustained exposure, faster for uninsulated pipes in wind.
No lower than 55 degrees Fahrenheit, and higher if the home has known cold spots or plumbing on exterior walls.
It can, for exposed pipes in a garage or crawl space. Insulated interior lines usually survive short dips.
No. Drip the fixtures at the end of runs that pass through unheated or exterior spaces.
Burst and Frozen Pipe Damage in Denver
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