Household water pipes generally start to be at risk once outside temperatures are below freezing for several hours, but they most often freeze when it’s closer to 20°F (about −7°C) or colder.
Key temperature numbers
Most guidance notes that while water freezes at 32°F (0°C), pipes in typical homes usually do not freeze until outdoor air has been at or below about 20°F (−6.6°C) for some time, especially if pipes are exposed or poorly insulated. Some sources add that pipes normally need several consecutive hours at or below freezing before the water inside actually freezes solid.
Other factors that matter
Pipes in unheated or uninsulated spaces such as crawl spaces, basements, exterior walls, or attics are more likely to freeze and can do so faster than pipes in conditioned areas. Good insulation and keeping indoor heat on significantly reduce the risk, while small-diameter or still-water pipes (no flow) tend to freeze faster.
Practical rule of thumb
Many home-maintenance sources suggest beginning freeze-prevention steps (like letting faucets drip and opening cabinet doors under sinks on exterior walls) once the forecast shows temperatures below 32°F (0°C), especially if they may approach 20°F or lower overnight. If indoor spaces stay well above freezing, such as maintaining the house temperature around the mid-60s°F (around 18°C), interior pipes in those heated areas are generally unlikely to freeze even during outdoor cold snaps.
