How long does it take for pipes to freeze?
Hours, not days, when the conditions are bad. Utah State University Extension puts it plainly: when unprotected water lines sit in freezing temperatures for several hours, the water freezes. Nobody can give you an exact clock, because the same cold night can freeze one pipe and leave another fine.
These are the things that set the pace:
- How cold it gets. The colder the air around the pipe, the faster the water loses heat.
- How long the cold lasts. A short dip below freezing before sunrise is different from a full night and day in the teens.
- Where the pipe runs. A pipe in a heated kitchen has the whole house keeping it warm. A pipe in a vented crawlspace or a garage ceiling has almost nothing.
- Drafts and wind. Cold air leaking through a rim joist, a dryer vent or a gap around a hose bib can freeze a pipe that sits inside an otherwise warm wall.
- Insulation. Foam sleeves slow heat loss, but they do not add heat. Utah State University Extension notes that insulation will not protect a pipe from a long freeze without some source of heat or water flow.
- Moving water. Water sitting still in a pipe freezes sooner than water that is flowing, which is why a trickle from a faucet helps.
At what temperature do pipes freeze?
Water freezes at 32°F, but the air outside your house has to get a good deal colder before the pipes inside it are in real trouble. The number most sources use is 20°F. Texas A&M AgriLife Extension says pipes can freeze and burst when the outside temperature reaches 20°F or below. The Insurance Institute for Business and Home Safety, as cited by the International Code Council, says freezing of unprotected pipes begins at about 20°F. State Farm says pipes in uninsulated areas are most at risk below 20°F.
Water itself still freezes at 32°F, so a line fully exposed to outside air, like a hose left on a spigot, can freeze before the thermometer reaches 20. The National Weather Service uses the term hard freeze for air at 28°F or lower for an extended time. When you see a Hard Freeze Warning in the forecast, treat it as your cue to check the pipes in the cold corners of the house.
| Outdoor temperature | What it means for pipes | Risk |
|---|---|---|
| Above 32°F | Water in the pipe stays liquid | No freeze risk |
| 32°F to 28°F | Water left in hoses and lines fully exposed to outside air can freeze | Low inside the house, higher outdoors |
| 28°F or lower for a long stretch | The National Weather Service calls this a hard freeze | Rising for pipes in crawlspaces, garages and outside walls |
| About 20°F or lower | The danger line extension services and insurers use for exposed and uninsulated pipes | High, and higher the longer it lasts |
The table is a guide, not a promise. A pipe with a cold draft blowing across it can freeze at a warmer temperature than the table suggests, and a pipe tucked inside a heated wall can ride out colder nights.
Which pipes freeze first in Northern Virginia homes?
The pipes that freeze first are the ones that sit closest to outside air. Texas A&M AgriLife Extension lists hose bibs, sprinkler lines, and pipes in basements, crawlspaces, attics, garages and outside walls with little or no insulation. The University of Illinois Extension points to pipes near outer walls, in crawlspaces and in attics.
Here is where that shows up in the houses around here:
- Crawlspaces in 1950s homes. Most of the roughly 1,675 houses in Pimmit Hills, built from 1950 to the late 1950s, sit on a slab or a crawlspace. A vented crawlspace lets winter air reach the supply lines. See our water damage page for Falls Church.
- Ramblers and split-levels from the 1960s and 1970s. Neighborhoods like Mantua, Mosby Woods and Kings Park West in Fairfax, and the Sterling Park ramblers in Sterling, are worth a close look. Check the sink on an outside wall, any pipe along the rim of the lower level, and the line that feeds each hose bib.
- Rooms and baths over a garage. In townhomes and houses with living space over the garage, from Leesburg to Ashburn, water lines can run through the garage ceiling. If that ceiling is not well insulated, those lines sit in some of the coldest air in the house.
- Hose bibs and their supply pipe. A hose left connected traps water in the spigot and the short pipe that feeds it through the wall.
- The water meter. Loudoun Water notes that meters can freeze during extreme cold snaps and recommends keeping water flowing through the meter. If no faucet in the house works, call your utility.
How fast can a frozen pipe burst?
A pipe can split while it is still frozen, so there is no safe amount of time to wait. The International Code Council explains that pipes usually do not burst from ice pushing on the pipe wall. Ice blocks the pipe, and pressure builds in the water trapped between the ice and a closed faucet. That pressure is what breaks the pipe, often in a section where little or no ice has formed.
The ice can hold a crack shut. When the pipe thaws, the water comes out at full house pressure. The University of Illinois Extension says an eighth-of-an-inch crack can release up to 250 gallons of water a day. That is why so many burst pipes show up the morning after the coldest night, or when a family comes home from a winter trip. If the pipe is above a ceiling, the first sign may be a stain or a drip in the room below. If that happens, read water leaking from the ceiling: what to do.
How do I know if my pipes are freezing right now?
The clearest sign is a faucet that gives you only a trickle, or nothing, on a very cold morning while other faucets still run. Other signs include frost on an exposed pipe, a toilet that will not refill, or no water at a sink on an outside wall. If one pipe has frozen, check the rest. Texas A&M AgriLife Extension notes that if one pipe freezes, others may freeze too.
For step-by-step thawing, read What to do if your pipes freeze. The short version: open the faucet, look for cracks, and use gentle heat. Never use a torch or open flame.
How can I keep my pipes from freezing on a cold night?
You cannot change the forecast, but you can buy your pipes a lot more time.
- Keep the heat on and steady. Loudoun Water, State Farm and the University of Illinois Extension all say to keep the thermostat at 55°F or higher, even when you are away. Texas A&M AgriLife Extension adds to keep the thermostat at a steady setting day and night.
- Let a faucet trickle. Loudoun Water suggests a trickle of cold water from the faucet farthest from your main shutoff, ideally at a high point in the house. It helps, but Fairfax County Fire and Rescue warns it may not stop a pipe from freezing on its own.
- Open cabinet doors. Let warm air reach the pipes under kitchen and bathroom sinks on outside walls. Move cleaning products out of reach of kids and pets first.
- Close the garage door if water lines run through the garage or the rooms above it.
- Shut off and drain outdoor faucets. Fairfax County Fire and Rescue recommends shutting off water to unheated areas and outdoor faucets and draining those pipes.
- Seal drafts and insulate exposed pipes. The University of Illinois Extension recommends sealing leaks that let cold air in where pipes run, and using pipe insulation or UL-approved heat tape. Follow heat tape directions exactly. Fairfax County Fire and Rescue warns a small mistake can start a fire.
For the full before-winter checklist, see How to prevent frozen pipes in Northern Virginia.
What should I do if a frozen pipe bursts?
Shut off the main water valve first, then keep people away from water near outlets, cords and the electrical panel. Fairfax County Fire and Rescue says to call the fire department if water reaches electrical outlets or panels, or threatens a furnace or water heater. Take photos of the pipe and every wet area before you clean up.
Then call for help. A plumber fixes the pipe. We take care of the water damage: we extract the standing water, dry the structure until moisture readings say dry, photograph the damage, write the estimate, send that documentation to your insurer and meet your adjuster on site. The same crew does the rebuild. See how we handle burst and frozen pipe damage, or read about water extraction and drying.
Freeze damage is expensive. State Farm says its average frozen pipe claim payment was more than $30,000. Your policy and your insurer decide what is covered, and freeze damage in a house left unheated can be treated differently. Read Does homeowners insurance cover burst pipes? before you call your agent.
Sources
- Utah State University Extension: Advice to prevent frozen pipes
- Texas A&M AgriLife Extension: Preventing and thawing frozen pipes
- University of Illinois Extension: Cold weather, frozen pipes
- International Code Council: Build it to code to minimize damage from frozen water pipes
- National Weather Service glossary: Hard freeze
- State Farm: Frozen pipes, beat winter water woes before they strike
- Loudoun Water: Frozen pipes
- Fairfax County Fire and Rescue: Frozen water pipes