How Cold Before Pipes Freeze? The 32, 20, 10 and 0 Rule

Water freezes at 32°F. Pipes usually do not.

A pipe freezes when the water inside it stays below 32°F long enough, and the water is only as cold as the space around the pipe. A garden spigot sits in the open air; a pipe in a heated kitchen sits at 65°F whatever happens outside. So the outdoor low that freezes a pipe depends on what is between the pipe and the weather. The University of Illinois Building Research Council's tests, the source behind the Red Cross "20°F rule", put the line for a bare pipe in unheated space at about 20°F.

Four places, four temperatures

Pipe	Freezes near	Counties that get this cold
Outdoor spigot, open crawlspace	32°F	3,125 (100%)
Bare pipe in garage, attic or outside wall	20°F	2,938 (94%)
Insulated pipe in unheated space	10°F	2,315 (74%)
Inside the heated house	0°F	1,694 (54%)

"Gets this cold" means at least one night in a normal year. Almost every county sees a frost; 94% reach the 20°F line for a garage or attic pipe; 54% drop below zero, where even pipes inside a heated house can freeze if they run along an outside wall or the heat fails.

What makes it happen sooner

Wind is the big one. A pipe in a north-facing outside wall with a draft through a sill gap can freeze well above 20°F, because moving air pulls heat out of the wall faster than still air does. Duration matters too: a few hours at 18°F before sunrise is less dangerous than two days below 20°F. A night at the threshold is a warning, not a guarantee either way.

Reading your county

Every county page shows the count for all four thresholds, with the 20°F row in bold because that is the pipe most houses have somewhere. For comparison: Hennepin County, MN (88 nights below 20°F), Denver County, CO (61 nights below 20°F), Cook County, IL (36 nights below 20°F), Fulton County, GA (5 nights below 20°F).

Where to go next

See the range: Hennepin County, MN, Cook County, IL, Harris County, TX, Miami-Dade County, FL.