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Frozen Pipe Calculator

Work out how long a pipe has before it freezes, how much time insulation buys, and how small a drip actually has to be to stop it.

How to use this calculator

  1. 1Enter only the length that is actually in the cold. The rest of the system is being kept warm by the house.
  2. 2Be honest about wind. A draughty crawl space or a vented soffit above a pipe run multiplies the heat loss several times over.
  3. 3Use the drip figure as a target flow, not as a number of drops. A thin continuous stream is the right picture.
  4. 4Open the tap furthest from where the water enters the house, so the flow passes the whole vulnerable run.

How the calculation works

R per foot = ln(r2 / r1) / (2 pi k) + 1 / (h x 2 pi r2) Heat loss per foot = (T_water - T_ambient) / R Time to 32 F = water mass x 1.0 x (T_water - 32) / heat loss Time to freeze solid = that + water mass x 143.4 / heat loss at 32 F Drip needed (lb/hr) = total heat loss / (T_supply - 33)
r1, r2
Outside radius of the pipe and of the insulation over it, in feet
k
Conductivity of the insulation - about 0.02 BTU per hour per foot per degree F for closed-cell foam
h
Outside air film coefficient, which rises sharply with wind - the reason a draught matters so much
143.4
Latent heat of fusion of water in BTU per pound. Freezing takes far more energy out than cooling does

The latent term dominates. Cooling a pipe of water from 52 F to freezing takes about 20 BTU per pound; freezing that same pound takes 143. This is why the time to start freezing and the time to freeze solid are so different, and why a pipe that has begun to freeze still has hours in hand.

Insulation appears only in the resistance term. There is no heat input anywhere in these equations, which is the mathematical statement of why insulation alone cannot prevent freezing.

The drip calculation is a steady-state heat balance: flowing water brings in sensible heat, and the flow needed is whatever carries in as much as the run loses.

Worked example

A bare 1/2 in copper run in an exterior wall at 15 F

  1. 1.A pipe in an exterior wall sees about 25 F when it is 15 F outside - the cavity is barely buffered, because it is on the cold side of the insulation.
  2. 2.Bare 1/2 in copper in a 5 mph draught loses 0.66 BTU per hour per foot per degree, so 20 ft across a 27 degree difference loses 356 BTU an hour.
  3. 3.The run holds 2 lb of water. Cooling it from 52 F to freezing takes 40 BTU, which at that rate is about seven minutes.
  4. 4.Freezing it then takes 2 x 143.4 = 287 BTU, and at the smaller temperature difference that is three hours more.

Result: Ice within ten minutes; solid in about three hours

The same run with a foam sleeve

  1. 1.Half an inch of closed-cell foam adds a cylindrical resistance of ln(0.078/0.026) / (2 pi x 0.02), which is about 8.7 - against an air film resistance of roughly 1.4.
  2. 2.That is the whole story: total resistance goes from about 1.5 to 8.2, so heat loss falls from 356 to 66 BTU an hour - 81% less.
  3. 3.The freeze time stretches with it, from 3.3 hours to 17.6 - a factor of 5.4.
  4. 4.It is a large improvement, and it is still only time. There is no heat source anywhere in this pipe, so a long enough cold spell freezes it regardless.

Result: 3.3 hours becomes 17.6 - and still no heat source

What the drip actually has to be

  1. 1.A fully exposed 20 ft run at 10 F in a 12 mph wind - about the worst ordinary case.
  2. 2.Wind lifts the air film coefficient from 1.5 to around 7, so the bare pipe loses several times what it would in still air.
  3. 3.To hold the whole run above freezing, incoming water at 48 F leaving at 33 F has to carry in all 756 BTU an hour: mass flow = heat loss divided by the 15 degree drop.
  4. 4.It comes out at 6 gallons an hour - a running tap, not a drip. On a run this exposed the folk advice is simply not enough to keep the water liquid, and what an open tap is really buying is pressure relief.

Result: 6 gal/hr to stay liquid - far more than a drip

Why insulation is not protection

Wrapping a pipe in foam feels like protecting it, and in a sense it is - but only against the clock. Insulation is a resistance to heat flow. It slows the rate at which the water gives up its warmth to the cold air around it. It contributes no warmth of its own.

The consequence is unavoidable: given a long enough period below freezing, an insulated pipe reaches the same temperature as an uninsulated one. All the foam has done is delay the arrival. For an overnight cold snap that delay is exactly what is needed, and half-inch foam sleeves are among the cheapest and most worthwhile things anyone can do to an exposed run. For a week of hard frost, they buy hours out of a hundred and sixty and the pipe freezes anyway.

This is why the guidance that actually prevents burst pipes is about heat rather than insulation: keep the space above freezing, open cabinet doors so room air reaches pipes in exterior walls, or run water so that the main supplies the heat continuously. Insulation belongs alongside those, not instead of them.

What actually bursts the pipe

The intuitive picture is that ice expands and splits the pipe from the inside where it forms. That is not what happens, and the difference matters for how you respond.

Water expands about nine percent on freezing. In a pipe with an open path to anywhere, that expansion simply pushes water along - there is no pressure build-up at all. Bursts happen when an ice plug forms somewhere in a run and freezing continues behind it, compressing the trapped water between the plug and a closed tap. Water is nearly incompressible, so the pressure climbs very fast, and the pipe fails at whatever its weakest point is.

That point is frequently nowhere near the ice. Plumbers routinely find the split in a warm section of the house, downstream of a frozen run in a crawl space, which puzzles homeowners considerably. It also explains why opening a tap protects a pipe even at a trickle too small to keep it liquid: it gives the pressure somewhere to go.

  • Open a tap — Relieves the pressure that actually bursts pipes, even if the flow is not enough to prevent ice.
  • Furthest fixture — Open the one furthest from the incoming main so flow passes the whole vulnerable run.
  • Hot and cold both — If both lines run through the cold space, both need to move. The hot line freezes too.
  • Cabinet doors — Under a sink on an exterior wall, opening the doors lets room air at the pipes. Free and effective.
  • Shut off and drain — For an unoccupied house, the only reliable answer. No water, no freeze.

The 20 degree threshold and where it comes from

The figure quoted everywhere - that pipes are at risk below 20 degrees Fahrenheit - is not folklore. It comes from field testing by the Building Research Council at the University of Illinois, which instrumented residential water systems through real winters and found that uninsulated pipes in unconditioned attics began to freeze once the outdoor temperature fell to about 20 degrees.

It is worth reading that carefully, because it is a threshold for a specific and particularly vulnerable case, not a universal safe temperature. A pipe in a windy crawl space, or an exposed hose bib, or a run against an uninsulated north wall, will freeze above 20. And a survey of plumbers in southern states found burst call-outs beginning when temperatures fell into the teens - which reflects both milder construction standards and the fact that pipes routinely run through unheated spaces in warm climates.

The practical reading is that 20 degrees is when to start acting in a well-built northern house, and rather warmer than that in a house that was never built for a hard freeze. Duration matters as much as depth: a night at 18 degrees is survivable where three days at 25 is not.

What this assumes, and where it stops

Assumptions

  • Still water in the pipe, with no flow and no heat input other than what the water already carries.
  • The pipe is at a uniform temperature along the exposed run, and the surrounding space is at a steady temperature.
  • Insulation is closed-cell foam at about 0.02 BTU per hour per foot per degree F, fitted tightly with no gaps.
  • The temperature a pipe sees in a given location is estimated from the outdoor temperature by a shelter factor, which is a broad approximation for a very variable situation.

Limitations

  • Real spaces are not at a steady temperature. A crawl space lags outdoor conditions by hours, which is why pipes there often freeze on the second cold night rather than the first.
  • Gaps in insulation dominate. A sleeve that stops short of a fitting, or is split open at an elbow, loses most of the benefit at that point - and that point is where the ice plug forms.
  • It does not predict bursting. Whether a pipe splits depends on where the plug forms, whether a tap is open, and the pipe material, and the calculation here stops at freezing.
  • Buried and below-grade pipes are a different problem governed by frost depth and soil conditions.
  • The location shelter factors are broad estimates. A thermometer in the actual space beats them.

Common questions

At what temperature do pipes freeze?

The commonly cited threshold is 20 F outdoors, from field research on uninsulated pipes in unconditioned attics. It is a threshold for the most vulnerable case rather than a safe limit: exposed runs, windy crawl spaces and hose bibs freeze above it, and duration matters as much as depth. Three days at 25 F is worse than one night at 18.

Does letting the tap drip really work?

Yes, for two separate reasons. Moving water brings in heat from the main continuously, and the flow needed is genuinely small - a fraction of a gallon an hour on a typical run. And an open tap relieves the pressure that actually bursts pipes, which builds between an ice plug and a closed valve. Even a flow too small to prevent ice is worth having for the second reason alone.

Will insulating my pipes stop them freezing?

No, it will delay it - typically by a factor of two to four. Insulation slows heat loss and adds no heat, so with no warmth reaching the pipe it eventually reaches the surrounding temperature regardless. That delay is often all that is needed for an overnight snap. For a sustained freeze you need heat: keep the space above freezing, open cabinet doors, or run water.

Why did my pipe burst somewhere warm?

Because the burst is caused by pressure rather than by ice pressing outward. An ice plug in a cold section traps water between itself and a closed tap, and continued freezing pressurises that trapped water until the pipe fails at its weakest point - which is often well downstream, in a heated part of the house. It is one of the more counter-intuitive things about frozen pipes and one of the best arguments for opening a tap.

Is PEX freeze-proof?

No, but it is considerably more forgiving than copper. PEX stretches as ice expands and often survives a freeze that would split a copper line, which is a real argument for using it in exposed runs. It is not immune - repeated freezes fatigue it, and the fittings do not stretch at all. Treat it as a better bet in a bad situation rather than a solution.

Sources

Formula and content last reviewed on .

Results are estimates for information only, not professional advice.

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