Tool

Pipe Freeze Risk Checker

Enter tonight's low

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Where the pipe runs matters as much as how cold it gets.

Tonight's low (°F)

Where the pipe runs

Insulation

Is the house being heated?

How long does it take for pipes to freeze?

An uninsulated pipe in an unheated space starts freezing after roughly four to six hours at 20 °F (−6 °C). A pipe outside the building goes faster, one inside a heated room takes far longer, and insulation roughly doubles the time. Duration matters as much as the temperature itself.

What the estimate is built on

There is one solid anchor in the published research: field work by the Building Research Council at the University of Illinois established that an uninsulated pipe in an unconditioned space begins freezing when the outdoor temperature falls to about 20 °F (−6 °C) and stays there for roughly four to six hours. A survey of plumbers in southern states agreed from the other direction — burst-pipe callouts began when temperatures fell into the teens.

Everything the checker reports is that anchor scaled for exposure. A pipe outside the building has nothing protecting it and goes faster. A pipe inside an exterior wall is partly warmed by the room behind it. A pipe in a heated room is only at risk if the heating has failed.

That is why the output is a band. The scaling factors are reasonable, not measured for your house.

What it cannot know

Stated plainly, because a tool that hides its limits is worse than no tool:

Wind, which matters enormously for a genuinely exposed pipe and not at all for one in insulation.
Air leaks. A gap in the siding blowing directly onto a pipe defeats any amount of insulation around it.
Pipe material and diameter. A narrow copper pipe freezes before a wide plastic one.
Water movement. A pipe in use freezes far more slowly than a dormant one.
Where the insulation is thin. Runs fail at bends and joints, which is where sleeve kits are usually left short.
How many cold nights came before. The spaces around your pipes carry cold over from one night to the next.

Take the result as an ordering of risk rather than a countdown.

What to do at each result

Actions by result. The full checklist is on the prevention guide.
Result	Do
No risk tonight	Nothing beyond hoses off and outdoor taps drained
Unlikely tonight	Open cupboard doors on exterior walls. Watch a multi-night run
Watch it	Cupboard doors open, hold the daytime heating overnight
Act tonight	All of the above plus a pencil-thick trickle on the furthest tap, and find the stop tap

Why a dripping tap works

Not for the reason most people assume. Moving water does freeze more slowly than still water, but that is the smaller half of the benefit.

The larger half is pressure. A pipe usually fails not at the ice plug but somewhere along the run, when water trapped between the growing plug and a closed tap is compressed past what the pipe can hold. An open tap gives that pressure somewhere to go, so even if a plug forms, the pipe survives.

Which means the trickle should be on the tap at the far end of the vulnerable run, and it only needs to be about the thickness of a pencil lead. The mechanism is covered in more detail in what temperature pipes freeze at, and the ordered list of everything else worth doing is in how to keep pipes from freezing.

Common questions

How many hours at 20 °F before pipes freeze?

Four to six hours for the most vulnerable case — an uninsulated pipe in an unheated crawl space, attic or garage. That figure comes from field research on exactly that configuration. A pipe inside the heated envelope will take much longer, and one exposed outdoors with wind on it can go in half the time.

Why does the checker give a range rather than a number?

Because the inputs are a rough description of a pipe. Real exposure depends on the pipe material and diameter, air leaks near it, wind, how much water is moving through it and where the insulation is thin. A single figure would imply a precision the evidence does not support, so it returns a band with the assumptions printed.

Does a frozen pipe always burst?

No. A pipe bursts when the growing ice plug pressurises water trapped between it and a closed tap beyond what the pipe can hold. That can happen within hours or not at all, and it often fails somewhere other than the frozen section. Leaving a tap open gives the pressure an escape route, which is why a trickle helps.

Do several cold nights in a row make it worse?

Yes, considerably. The spaces around your pipes do not fully recover during a short cold day, so each night starts from a colder baseline than the last. A third night at 20 °F is more dangerous than the first, which is why a prolonged cold snap causes more damage than a single sharp night.

Does the checker account for wind?

Not as an input, and it says so. Wind matters a great deal to a pipe genuinely exposed outdoors and almost nothing to one buried in insulation, so a single wind factor would mislead in both directions. If your vulnerable pipe is outside and it is windy, treat the result as the optimistic end.

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