At what temperature do pipes freeze?

Pipes can freeze and burst when the outdoor temperature drops to 20°F or below, according to Texas A&M AgriLife Extension. That's the working answer to "at what temperature do pipes freeze," but a pipe in a drafty attic or along a thin exterior wall can get into trouble before the thermometer reaches 20.

So treat 20°F as an alarm bell, not a guarantee. Water itself freezes at 32°F. The gap between those two numbers is the insulation, the wall and the heated air around most of your plumbing. Take away that protection and the safety margin shrinks fast.

Where the 20-degree number comes from

AgriLife's freeze guide for Texas homeowners says water pipes "have the potential to freeze and burst when the outside temperature reaches 20°F or below." The extension service adapted that guide from American Red Cross material and from freezing-pipe guidance by the Institute for Business and Home Safety, known today as IBHS, so it isn't a number someone pulled from a plumbing forum.

Notice what the figure measures. It's the outside air temperature, not the temperature of the water in your pipe. A pipe inside a heated wall sees warmer air than the weather report. A pipe in an uninsulated attic or crawl space has far less between it and the cold, and an outdoor hose bib has nothing at all.

Our view: the 20-degree rule is most useful for homes in the South, where hard freezes are occasional and nobody built the house with them in mind. AgriLife makes the same point. It notes that pipes in warmer climates often run through uninsulated or underinsulated attics and crawl spaces, and that "we only have a few cold snaps per season."

Why some pipes freeze before it hits 20

Location decides more than the forecast does, because a pipe only feels the air right around it. The American Red Cross lists the pipes that freeze most often:

• Anything outdoors and exposed: hose bibs, pool supply lines, sprinkler lines.
• Supply lines inside the house that sit in unheated space, which the Red Cross says includes basements, crawl spaces, attics, garages and even kitchen cabinets.
• Pipes against poorly insulated exterior walls.

That last group is common in warm-weather building. IBHS says plumbing along exterior walls is prevalent in typically warmer areas and tells homeowners there to "pay close attention to areas of the home with exposed pipes." University of Illinois Extension adds a sneaky one: the short indoor run of pipe that feeds an outside faucet can freeze too.

And don't assume the hot side is safe. AgriLife says a hot water supply line can freeze just like a cold one if water isn't moving through it and the water in the pipe has gone cold, which describes most hot lines in the house by 3 a.m.

The indoor numbers that matter just as much

You can't control the low outside. You can control the thermostat, and the agencies don't all pick the same setting.

The Red Cross is blunt about the trade-off. Skipping the usual nighttime setback may raise your heating bill, it says, but it can prevent "a much more costly repair job if pipes freeze and burst." We agree. Paying for an extra night of heat is cheap next to drying out a kitchen.

Moving water helps too. The Red Cross says to let cold water drip from a faucet served by exposed pipes during hard cold, because running water through the pipe, even at a trickle, helps prevent freezing.

Do pipes burst when they freeze or when they thaw?

Both, in a sense. The pipe usually breaks while it's frozen, but you don't find out until the ice melts and the water finally has somewhere to go.

The San Antonio Water System (SAWS) explains the mechanism: ice that forms during a freeze can cause a blockage and excessive water pressure on your pipes, and that pressure bursts them. The Red Cross adds that water expands as it freezes, and "no matter the strength of a container, expanding water can cause pipes to break."

While the line is still packed with ice, though, nothing flows through the crack. That's why AgriLife warns that "the frozen pipe may already be broken, and it will leak when the water is thawed." The Texas Department of Insurance (TDI) gives the matching advice: if your pipes freeze, turn off the water at the shutoff valve, which keeps broken pipes from leaking into the house after they thaw.

The damage often starts on the first warm afternoon, not on the coldest night. If you're away at work when the ice melts, a split line can run for hours.

Quick answers to the follow-up questions

Will my pipes freeze at 32 degrees?

Probably not the ones inside heated walls, where room air keeps them warmer than the outdoor reading. Pipes with no protection are another story: an outdoor faucet or a sprinkler line sits in the same air the thermometer reads, and the Red Cross puts those at the top of its list of pipes that freeze most often. Treat 32°F as the point to finish your outdoor prep and 20°F as the point to run the whole checklist.

Does pipe insulation actually change the number?

It buys margin. The Red Cross says insulating attics, basements and crawl spaces keeps those areas warmer, and it suggests pipe sleeves or UL-listed heat tape or heat cable on exposed pipes. It even says a quarter inch of newspaper "can provide significant protection" in places that don't usually see frequent or prolonged freezing. That describes a lot of Texas.

Does my insurance care where the thermostat sat?

It can. TDI says home and renters policies might not pay for damage caused by frozen pipes if you didn't take steps to protect them, such as keeping the heat on. Your policy and your carrier decide coverage, so read yours or ask your agent before the cold season. The claim and deductible are always the homeowner's, and our insurance claims page explains how we help document water damage for your carrier.

What to do when the forecast hits 20 or lower

Use the number as your trigger. When the forecast low reaches 20°F, the Red Cross and AgriLife checklists boil down to a short evening routine:

1. Disconnect and drain garden hoses, and close the inside valves that feed outdoor faucets if your house has them.
2. Keep the garage door shut if any water lines run through the garage.
3. Open the cabinet doors under kitchen and bathroom sinks so warm room air reaches the pipes, and move cleaning products out of children's reach while they're exposed.
4. Let a faucet served by exposed pipes drip, unless your utility has asked people to limit water use.
5. Find your main shutoff valve now, while it's dry and daylight.

Then watch for trouble in the morning. A faucet that only trickles is the classic first clue, and our guide to the signs of frozen pipes covers the rest. If the power goes out during the freeze, the rules change, and keeping pipes from freezing during an outage covers what to do. For local history and utility guidance, see our post on frozen pipes in San Antonio.

If the temperature wins and a pipe splits

Close the shutoff valve first, then split the job between two trades. A licensed plumber repairs the pipe, because that's plumbing work and we don't do it; what we handle is the water the break left behind in floors, drywall, cabinets and framing.

Texas Damage Response, a veteran-owned and operated San Antonio company, takes calls 24/7 for emergency restoration. Our crews extract standing water, dry the structure and take moisture readings until it's actually dry, as part of our water damage restoration work. If a freeze has already left water inside, request burst pipe water damage cleanup.