Frozen Pipes in the Basement: How to Prevent Them and What to Do If One Freezes

A frozen pipe is a slow problem until it isn't. Water expands as it freezes, pressure builds between the ice and the closed faucet, and the pipe splits. You usually don't find out until it thaws and starts spraying. In a basement, that means water on the floor, in the walls, and under any finished flooring before anyone notices.

The good news: basement pipes are some of the easiest in the house to protect, because you can usually see and reach them.

The Reach-and-Risk Rule

Match the fix to how exposed the pipe is and whether you can get to it:

The rule of thumb: the closer a pipe is to the outside, the more protection it needs.

5-Minute Diagnostic: Which of Your Pipes Are at Risk?

Do this on a cold day with a flashlight and a cheap digital thermometer.

1. Find your main shutoff. It's usually on the foundation wall where the water line enters the house. Make sure the valve turns. If it's stuck, call a plumber now, not during an emergency.
2. Trace every pipe near the rim joist. Look where the floor framing meets the top of the foundation. Pipes running through or along this area are the most likely to freeze.
3. Check the outdoor faucet lines. Follow each line from the outdoor spigot back into the basement. Look for an interior shutoff valve on that line.
4. Feel for drafts. Run your hand along the rim joist near the pipes. Cold air hitting a pipe is how most basement pipes freeze.
5. Take a temperature reading next to the pipes. If the air around a pipe reads under 45°F while it's cold outside, that pipe is at risk.

Note every at-risk pipe and what it's made of. That's your project list.

PEX vs. Copper: How Worried Should You Be?

What your pipes are made of changes the risk.

PEX is freeze-resistant, not freeze-proof. Repeated freezing weakens it, and the metal fittings between sections are the weak point. If your basement is copper or galvanized and a pipe has frozen more than once, rerouting that section to the warm side of the basement or replacing it with PEX is worth a plumber's quote.

Prevention: Protect Pipes Before the First Freeze

Seal the Rim Joist First

Cold air leaking through the rim joist is the main reason basement pipes freeze. Insulating a pipe while a draft blows on it only slows things down. Seal gaps with canned spray foam like Great Stuff Gaps & Cracks, and cover the full cavity with rigid foam board. Our spray foam vs. rigid foam guide covers which goes where.

Insulate Exposed Pipes

Foam pipe sleeves like HEATIT Pipe Insulation are the cheapest protection you can buy. Measure the pipe's outside diameter (most supply lines are 1/2 or 3/4 inch), buy matching sleeves, and tape the seams and joints. Cover elbows and valves too, since they freeze first.

Insulation slows heat loss. It doesn't create heat. On a pipe that sits in a truly cold space, insulation alone may only buy you time.

Add Heat Cable to Problem Pipes

For a pipe that has frozen before, heat cable is the fix. Buy self-regulating cable, which adjusts its output to the temperature and is safer than older constant-wattage tape. Run it along the pipe per the instructions, plug it into a GFCI outlet, and insulate over it only if the manufacturer says that's allowed.

Shut Off and Drain Outdoor Faucets

Every outdoor faucet should have a shutoff valve on the line inside the basement. Before the first freeze, close it, open the outdoor faucet to drain the line, and disconnect the hose. Leave the outdoor faucet open all winter, and add an insulated cover outside for extra protection. If there's no interior shutoff, a plumber can install one or swap the faucet for a frost-free sillcock, which shuts the water off inside the warm wall.

Keep the Basement Warm Enough

A basement that stays above 55°F protects most pipes on its own. If yours runs colder, a heater with an anti-freeze setting like the De'Longhi oil-filled radiator switches on automatically near 41°F. See our full guide on how to heat a basement.

Get an Alert Before It Freezes

A Wi-Fi temperature sensor placed next to your most exposed pipe will alert your phone when the temperature drops, which matters most when you're traveling. Set the alert at 40°F so you have time to act. For leak alerts too, see our picks for the best water leak detectors.

On the Coldest Nights

When temperatures head toward the single digits, let the faucet farthest from the main shutoff drip slowly, hot and cold. Moving water relieves pressure even if ice starts to form. Open any cabinet or access panels in front of pipes so warm air reaches them.

What to Buy

Everything here costs less than a plumber's service call.

If you only buy two things: foam pipe insulation and the Govee sensor. Insulation handles most basements, and the sensor tells you when it isn't enough.

Frozen Pipe in a Finished Basement Ceiling or Wall

Pipes hidden behind drywall are harder to protect and harder to thaw. They also freeze more often than exposed pipes, because finishing a basement often traps them against the cold rim joist or exterior wall.

How to find the frozen section:

• Turn on each faucet and note which ones don't run. The frozen section is somewhere between the last working fixture and the first dead one.
• Feel the ceiling and walls along that route. The coldest spot, usually near an exterior wall, is the likely location.
• Look for frost, a damp patch, or a bulge in the drywall.

How to thaw it without opening the wall:

1. Open the faucet the pipe feeds.
2. Raise the temperature in the room. Turn up the heat and point a space heater at the wall or ceiling section, keeping 3 feet of clearance.
3. If there's a recessed light, register, or access panel near the frozen spot, remove the cover so warm air can reach inside.
4. Give it time. Thawing through drywall can take hours.

When to cut an access panel: if the pipe doesn't thaw within a few hours, if you see water staining, or if the same spot freezes every winter. A small square cut between joists can be finished with a removable access panel, which lets you insulate the pipe properly and check it in the future. If the pipe sits against the rim joist with insulation on the room side, that's the root cause. The pipe needs to be on the warm side of the insulation, so plan on fixing it once the weather allows.

Leaving the House for the Winter

For a trip of a few days, keep the heat at 55°F or higher, open the cabinet doors in front of pipes, and leave the Wi-Fi sensor running.

For a long absence or a vacant house:

1. Shut off the main water valve.
2. Turn off the water heater, or set it to vacation mode.
3. Open every faucet, starting from the top floor, and let them drain. Open the lowest faucet or drain valve in the basement last.
4. Flush toilets to empty the tanks.
5. Leave the faucets open.
6. Keep the heat on at a low setting if you can. Draining protects the pipes, but heat protects the rest of the house.

A home with a well, a boiler, or a sprinkler system needs more steps. For those, have a plumber winterize it.

Don't Forget the Sump Pump Discharge Line

The pipe that carries water from your sump pump to the yard can freeze at its outdoor end, especially if it drains into a buried line. When it freezes, the pump runs against a blocked line and can burn out. Keep the outdoor end clear of snow, make sure it slopes away from the house so water doesn't sit in it, and consider an anti-freeze discharge fitting that lets water escape if the line blocks. See our sump pump buying guide for more.

What to Do If a Pipe Freezes

Signs of a frozen pipe: a faucet that trickles or won't run, frost on the outside of a pipe, or a section of pipe that's bulging.

1. Open the faucet the pipe feeds. Moving water helps melt the ice and relieves pressure.
2. Find the frozen section. It's usually where the pipe runs closest to an exterior wall or the rim joist.
3. Apply heat from the faucet side first. Work from the faucet end back toward the frozen spot so melting water has somewhere to go. Use a hair dryer, a heating pad, towels soaked in hot water, or a space heater pointed at the area.
4. Keep going until full water pressure returns. Then check every faucet in the house for other frozen spots.
5. Inspect for cracks. Look closely at the thawed section. Pipes often split while frozen and leak only once they thaw.

Never use a torch, propane heater, or any open flame. Besides the fire risk, a torch can boil water inside the pipe and make it burst.

If the Pipe Has Burst

1. Shut off the main water valve immediately.
2. Turn off power to the basement if water is near outlets, cords, or appliances.
3. Open faucets to drain the remaining water out of the lines.
4. Call a plumber.
5. Photograph everything for your insurance claim, then start drying out right away. Our guides on drying out a basement after flooding and basement flooding insurance cover the next steps.

Cost Breakdown and 10-Year Total Cost of Ownership

Estimates for protecting a typical basement with a handful of at-risk pipes. Electricity at about 18 cents per kWh.

The math isn't close. Everything above the last row together typically costs less than a single burst pipe.

Common Mistakes

• Insulating the pipe but not the draft. Cold air through the rim joist will freeze an insulated pipe. Seal first.
• Insulating the wrong side of a pipe. When finishing a basement, putting wall insulation between the pipe and the heated room traps the pipe on the cold side. Pipes should sit on the warm side of the insulation.
• Leaving hoses connected. A connected hose traps water in the outdoor faucet, even a frost-free one, and can split it.
• Using constant-wattage heat tape wrong. Overlapping it or covering it with insulation it isn't rated for is a fire risk. Self-regulating cable avoids most of this.
• Thawing with a torch. It's a fire hazard and can burst the pipe.
• Not knowing where the main shutoff is. Every minute you spend looking for it during a burst is more water on the floor.
• Turning the heat way down when traveling. Keep the house at 55°F or higher, and use a sensor that alerts you if it drops.

When to Call a Pro

• The pipe has burst or you see a crack
• You can't find or reach the frozen section
• The frozen pipe runs behind a finished wall or ceiling and doesn't thaw within a few hours
• The main shutoff is stuck or missing
• You're winterizing a house with a well, boiler, or sprinkler system
• You want an interior shutoff or frost-free sillcock installed
• Pipes keep freezing even after insulation and sealing, which can mean they need to be rerouted

DIY-friendly: air sealing, foam pipe insulation, heat cable on exposed pipe, draining outdoor faucets, and thawing an exposed pipe with a hair dryer. For more on what to take on yourself, see DIY vs. hiring a pro.

Frequently Asked Questions

Glossary

Rim joist

The wood framing that sits on top of the foundation wall. The most common spot for drafts that freeze pipes.

Main shutoff valve

The valve that stops all water entering the house, usually on the foundation wall where the line comes in.

Frost-free sillcock

An outdoor faucet with its valve set inside the wall, where it stays warm enough not to freeze.

PEX

Flexible plastic water supply tubing that resists splitting when frozen.

Self-regulating heat cable

Heat cable that automatically puts out more heat as temperatures drop and less as they rise.

Constant-wattage heat tape

Older heat tape that puts out the same heat at all times and can overheat if overlapped.

Pipe sleeve

A tube of foam insulation that slips over a pipe.

GFCI outlet

An outlet that cuts power when it detects a ground fault. Required for heat cable.

Sump pump discharge line

The pipe that carries water from the sump pump to the outside.

Thermal envelope

The boundary of insulation and air sealing that separates heated space from the outdoors. Pipes should stay inside it.

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