Where do pipes actually freeze?
Freezing is about location, not overall house temperature. A house at a comfortable 70°F can still have a pipe at 20°F if that pipe runs through the wrong space.
The vulnerable locations in a typical Iowa home
- Supply lines in exterior walls, especially on the north and west sides
- Pipes in unheated crawlspaces and unfinished basement rim joists
- Lines running through an attached but unheated garage
- Outdoor hose bibs and their supply stubs, if not frost-free or not drained
- Kitchen and bath sinks on exterior walls, where the cabinet blocks warm air
- Any pipe near a foundation crack or an open crawlspace vent letting cold in
Why does a frozen pipe burst?
Water expands roughly 9% when it turns to ice, but that expansion is not what splits the pipe. What happens is that an ice plug forms and seals the line. Water continues to freeze and expand behind it, and the pressure between that plug and a closed faucet downstream has nowhere to go.
Pressure climbs until the pipe wall fails — often at a point some distance from the ice itself. Frequently the split does not reveal itself until the ice thaws and water begins flowing through the rupture, which is why so many burst-pipe calls come in on the warming day after the cold snap rather than during it.
What prevention actually works?
The most effective step during an extreme cold warning is also the simplest: open the faucet served by a vulnerable line to a slow, steady drip. Moving water resists freezing, and more importantly the open faucet gives expanding ice somewhere to push, which prevents the pressure buildup that bursts pipes.
Beyond that, open cabinet doors under sinks on exterior walls so household heat reaches the plumbing, keep the thermostat no lower than about 55°F even when away, disconnect and drain garden hoses before the first hard freeze, and add pipe insulation or heat tape to accessible runs in crawlspaces and garages.
What if a pipe is already frozen?
If a faucet produces only a trickle on a cold morning, treat it as frozen. Leave that faucet open so melting water has an escape path, and warm the suspect section gently — a hair dryer, a heat lamp, or towels soaked in hot water, working from the faucet end back toward the frozen section.
Never use an open flame or a propane torch. It is a genuine fire risk, and it can damage the pipe. If you cannot locate or reach the frozen section, or you suspect a break, shut off the main water supply and call for help. Knowing where your main shutoff is before you need it is worth doing today.
Frequently asked questions
At what temperature do pipes start to freeze?
Vulnerable pipes generally start to be at risk when outdoor temperatures drop to around 20°F or below, particularly with wind and sustained cold overnight. Well-protected interior pipes tolerate much colder. The variable that matters is the pipe's own exposure, not the reading on your porch thermometer.
How much water should I let drip?
A steady, thin stream — a bit more than an occasional drop. It uses a modest amount of water over a cold night, which is trivial compared to the cost of repairing a burst line and the water damage that follows. Prioritize faucets served by pipes in exterior walls or unheated spaces.
Should I leave the heat on if I travel in winter?
Yes, always. Set it no lower than about 55°F. Shutting the heat off entirely to save on a trip is one of the more expensive mistakes we see in Iowa. If you will be away during a cold stretch, also ask someone to check the house, and consider shutting off the main water supply as added insurance.
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