At What Temperature Do Pipes Freeze? The Real Number

At what temperature do pipes freeze? The practical answer is 20 degrees Fahrenheit, not the 32 degrees at which water freezes.

That gap between 32 and 20 is the whole reason people are caught out. A night at 30 degrees is usually nothing, and a night at 18 can end with a flooded ceiling.

The number also is not fixed. It moves with wind, insulation, and how long the cold lasts.

Here is the real threshold, where it comes from, what shifts it, and the point at which you should stop reading and go open a faucet.

At what temperature do pipes freeze? The 20 degree threshold

The number comes from field research at the Building Research Council at the University of Illinois, and it is the figure the insurance industry has used ever since.

Researchers ran residential water systems through real winter conditions and watched when freezing actually started.

For uninsulated pipes in an unheated attic, freezing began when the outside temperature reached 20 degrees Fahrenheit or below.

They called it the temperature alert threshold, and it is meant as the point at which a homeowner should act, not the point at which damage is certain.

A separate survey of plumbers working in southern states agreed from the other direction. Burst calls started arriving once temperatures fell into the teens.

So 20 degrees is the practical line. Below it, the most exposed pipe in your house is on the clock.

Why pipes do not freeze at 32 degrees

Water freezes at 32 degrees. A pipe full of water inside a house is a different situation entirely.

Three things stand between the cold outside and the water in the pipe: the building, whatever insulation is around the pipe, and heat leaking out of your heated rooms.

Freezing is also not instant. Heat has to leave the water first, and that transfer takes hours even once the air is well below freezing.

Water moving through a pipe resists freezing too, which is the entire logic behind letting a faucet drip during a cold snap.

So at a steady 32 degrees, most household pipes will simply sit there. The margin is thin, not absent.

That margin is what insulation, a dripping tap, and a steady thermostat are buying you, and the full prevention checklist is worth running before the first hard night.

How long the cold has to last

Duration matters as much as the number on the thermometer, and this is where most forecasts mislead people.

The rough guide used by utilities is four to six hours at 20 degrees or below for an exposed, uninsulated pipe to begin freezing.

A brief dip to 19 degrees just before dawn is far less dangerous than a full day that never rises above 25.

That is why a hard freeze warning, usually defined as several hours at or below 28 degrees, is worth acting on even though 28 is above the alert threshold.

Watch the overnight low and the number of hours below freezing, not the daytime high. Pipes freeze at 4am, not at noon.

At what temperature do pipes freeze in walls, attics, and garages?

The 20 degree threshold applies to the worst-placed pipe in the house, and most homes have one.

Ranked from most to least vulnerable:

• Outdoor hose bibbs and sprinkler lines: exposed directly to the air. First to go, often above 20 degrees.
• Pipes in an unheated attic, crawl space, or garage: the classic case, and the one the 20 degree figure describes.
• Pipes in an exterior wall, especially on the north side or behind a cabinet that blocks room heat.
• Pipes in an unheated basement, which usually stays warmer than outside but not always in a long freeze.
• Pipes in interior heated walls: the last to freeze, and normally only during a power outage.

The bathroom or kitchen sink on an outside wall is the most common failure point in an otherwise well-built house.

Opening the cabinet doors under that sink on a cold night genuinely works, because it lets room heat reach the pipes. It is free, and it is the cheapest item on the whole list.

Wind, insulation, and what moves the number

Treat 20 degrees as a starting point, then adjust it for your own house.

Wind is the biggest variable. Moving air strips heat from an exposed pipe far faster than still air, so a windy 24 degrees can beat a calm 18.

Insulation buys time, not immunity. Foam sleeves slow the heat loss, which often carries a pipe through a short freeze, but a long one still gets there.

Air sealing matters more than people expect. A gap where the hose bibb or dryer vent passes through the rim joist points cold air straight at the pipe.

An open crawl space vent in January is one of the most common causes of a frozen line under a floor that is otherwise fine.

Losing power or heat changes everything. Without heat leaking from the rooms, interior pipes start behaving like exposed ones within hours.

Why freezing bursts a pipe, and where it splits

Water is unusual. It expands about 9 percent as it freezes, while nearly everything else contracts.

Most people assume the expanding ice splits the pipe from inside. That is not what usually happens.

The ice forms a plug. Water keeps freezing behind it, and the pressure builds in the trapped water between that plug and the closed faucet downstream.

Pressure in a sealed section can climb from normal household levels into the thousands of pounds per square inch, and something has to give.

So the split usually appears in a section with little or no ice in it, sometimes several feet from the frozen part. That is why the leak is rarely where you expect.

It also explains why a dripping faucet works. An open outlet means the pressure has somewhere to escape, so no sealed section forms.

A split inside a wall or ceiling soaks the cavity long before it shows on the paint, which is why drying out the wall becomes its own job after a freeze.

Copper splits along a seam, PEX stretches and often survives, and older galvanized steel fails at the threads. No material is freeze-proof.

What to keep the house set at

Keep the thermostat at the same temperature day and night during a hard freeze. The overnight setback is what usually causes the problem.

If you are leaving the house, do not turn the heat off. Set it no lower than 55 degrees, which is the standard advice for exactly this reason.

A higher fuel bill for two nights is far cheaper than one burst line, and the numbers are not close.

Open interior doors so heat reaches every room, and open the cabinets under sinks on outside walls.

Below 20 degrees, let a faucet on the coldest run in the house drip, ideally the one furthest from where the water enters.

The signs a pipe is already frozen

The first sign is usually the obvious one. A tap opens and nothing comes out, or only a trickle.

Look for frost on an exposed pipe, a section that bulges slightly, or a strange smell from a drain, which can mean the line is blocked with ice.

If one fixture is dead and the rest are fine, the ice is between the main and that fixture. That gives you the rough location to warm.

Act now rather than waiting for a thaw. A frozen pipe is a burst that has not happened yet, and most bursts are discovered when the ice melts.

What to do if a pipe is frozen right now

Open the faucet that pipe feeds, both hot and cold. Running water helps melt the ice and gives the pressure an escape.

Warm the pipe from the faucet end back toward the frozen section, so meltwater can drain out rather than pool behind more ice.

Use a hair dryer, a heating pad, warm towels, or a space heater kept clear of anything flammable. Take your time.

Never use a blowtorch, propane heater, or open flame. It is a leading cause of house fires during cold snaps, and it can also burst the pipe by boiling trapped water.

Know where your main shutoff is before you start. If the pipe already split, shutting off the main water supply is the first move, not the last.

If you cannot find the frozen section, cannot reach it, or the whole house has no water, call a plumber rather than opening walls in the dark.

If it already burst, what happens next

Shut the water off at the main, cut power to the area if water is near anything electrical, then photograph everything before you clean.

A burst line releases a lot of water fast, and most of it ends up somewhere you cannot see. The ordered steps are here: what to do first when a pipe bursts.

Frozen pipe damage is one of the cases insurers usually do pay, because the burst is sudden and accidental.

The exception matters. Carriers can deny a claim if the house was left unheated and unattended, so a documented thermostat setting is worth having. Read how burst pipe coverage works before you call it in.

Then start drying immediately. The EPA gives you 24 to 48 hours before mold begins, and that clock starts the moment the water arrives.

Keep reading

PreventionHow to prevent frozen pipes from burstingWater Damage by CauseBurst pipe? What to do first, step by stepWater Damage by CauseHow to shut off the main water supply fastInsuranceDoes homeowners insurance cover a burst pipe?CostsWhat does water damage restoration cost?Drying & RemovalHow long does it take to dry out walls after a leak?