Water expands by roughly 9 percent when it freezes, and the resulting pressure inside a closed pipe can hit 40,000 PSI — more than enough to split copper, PEX, or PVC. What happens when your pipes freeze depends on three factors: how cold it gets, how long the freeze lasts, and where in the system the ice forms. Some freezes thaw quietly with no damage. Others rupture pipes inside walls and dump 200 gallons per hour into the house once thawed. The Insurance Institute estimates frozen-pipe claims average $20,000 per incident. This guide explains what is actually happening when pipes freeze, what the warning signs look like, and the prevention and emergency-response steps every homeowner needs to know.
The Physics of a Freeze
Water inside a pipe freezes from the wall inward. Once a continuous ice plug forms, it creates a closed pressure system between the plug and any closed valve or fixture downstream. The remaining liquid water gets compressed by the expanding ice and pressure climbs rapidly. The pipe ruptures at its weakest point — which is often several feet away from the actual frozen section, between the ice plug and the closest closed valve.
This is why a frozen pipe in a crawlspace can split a tubing joint above a finished ceiling on the floor above. The damage is not always at the cold spot.
Temperature Thresholds
Pipes typically begin freezing once the surrounding air drops below 20 degrees F for at least 6 hours, but pipes in poorly insulated walls or unheated spaces can freeze at higher ambient temperatures if exposed to wind. Vulnerable locations:
- Pipes running through unheated attics, garages, basements, and crawlspaces.
- Pipes in exterior walls, especially on north and west facades.
- Outdoor hose bibbs and irrigation lines if not properly winterized.
- Pipes near drafty foundation vents or rim joists.
Warning Signs Pipes Are Freezing
Catch a freeze before it bursts and you avoid the cleanup. Watch for:
- Reduced flow at faucets, especially the ones served by the most exposed runs.
- Faucets that drip or trickle but won’t open fully.
- Visible frost on accessible pipe sections.
- Strange knocking or popping inside walls during a deep freeze.
If you notice reduced flow when temperatures are below 20 degrees, treat it as an in-progress freeze and start thawing immediately.
Thawing Safely
Open the affected faucet first — this gives expanding ice somewhere to go and reduces pressure as you thaw. Apply heat to the coldest section, working from the faucet back toward the source:
- Hair dryer: safest first option for accessible pipes.
- Heating pad or electric heat tape: wrap around the pipe.
- Hot towels: soak in hot water and wrap around the pipe.
- Space heater: directed at the wall section in front of a frozen pipe inside the wall.
Never use an open flame, blowtorch, or charcoal heater. Pipe insulation is often combustible, and torch-thawed pipes have started thousands of house fires. If you cannot reach the frozen section, shut off the main water valve and call a plumber. Most plumbers charge $200 to $500 for a thaw-and-inspect call.
What Bursting Looks Like
A burst pipe rarely sprays immediately. The split stays plugged with ice while the freeze continues. Once temperatures rise and the ice melts, water pours from the rupture. Homeowners often discover the leak hours after the actual cold snap, when ceilings sag and walls saturate.
If you suspect a freeze and cannot get water flowing, shut off the main water valve before going to bed or leaving the house. The shutoff is usually in the basement near the front foundation or in an outdoor meter pit. Test the valve before you need it; corroded valves seize.
Prevention: Insulation
The first line of defense is foam pipe insulation. Closed-cell polyethylene foam tubing slips over copper and PEX in 6-foot lengths for $1.50 to $3 each at home centers. Cover all exposed pipes in unheated spaces and seal seams with foil-faced tape. Skip felt or fiberglass wraps — they trap moisture and lose insulating value when wet.
For pipes inside exterior walls, the better approach is to seal air infiltration. Caulk and foam any gaps in sheathing, around outlets, and through the rim joist. Air movement, not ambient temperature, drives most wall-cavity freezing.
Heat Tape and Heat Cable
For pipes that freeze repeatedly, install self-regulating heat cable like Easy Heat Freeze Free or Frost King ($25 to $80 per kit). The cable wraps around the pipe and adjusts its heat output based on ambient temperature. Cover the cable with foam pipe insulation to multiply effectiveness. Plug it in by Thanksgiving and unplug it in March.
Drip-Faucet Strategy
During severe cold, opening one or two faucets to a slow drip keeps water moving and lowers freeze risk on those runs. Run a pencil-thin stream from both hot and cold sides of one faucet on each exterior wall plumbing run. The added water cost is trivial — typically a few cents per night. This is a temporary tactic, not a substitute for proper insulation.
Hose Bibbs and Outdoor Faucets
Standard hose bibbs are vulnerable because the valve is on the outside of the wall. Frost-free sillcocks like the Woodford 17 series move the valve 6 to 12 inches inside the heated envelope and self-drain when closed. Replace any non-frost-free hose bibb the next time you have a wall opened. In the meantime, disconnect hoses every fall and install an insulated faucet cover ($5).
If a Pipe Bursts
Steps in order, the moment you find water:
- Shut off the main water valve.
- Open faucets to drain remaining water out of the system.
- Cut power to wet electrical circuits at the breaker panel.
- Call a plumber and your insurance carrier.
- Move belongings off wet flooring; document everything with photos.
Homeowner insurance policies generally cover sudden pipe breaks but not gradual leaks or freezes that occur because the heat was off. Keep heat at 55 degrees minimum any time you leave the house in winter.
Frequently Asked Questions
Can PEX pipes burst when frozen?
PEX is more freeze-resistant than copper but is not freeze-proof. PEX usually expands enough to survive one freeze cycle but fails after repeated freezes or when frozen in a long straight run.
How cold does it have to be to freeze pipes?
Below 20 degrees F for 6+ hours is the typical threshold for exposed pipes. Well-insulated interior plumbing rarely freezes above 0 degrees F.
Should I leave the heat on if I am traveling?
Yes. Set thermostats no lower than 55 degrees and have a neighbor check the house if you are gone more than a few days. Open kitchen and bathroom cabinets so warm air reaches pipes against exterior walls.
Will running my furnace harder prevent freezing?
It helps, but pipes in unheated spaces (attic, crawlspace) get little benefit. Insulation and air sealing matter more than furnace output.
How long does a thaw take?
Anywhere from 30 minutes for a short ice plug to 6 hours for a long frozen run inside a wall. Patience matters; high-heat shortcuts crack pipes.
