At what temperature do pipes freeze? When to drip your faucets in Boise
Why 20°F and not 32°F
The University of Illinois Building Research Council tested residential water systems in winter. Uninsulated pipes in an unconditioned attic began to freeze when the outdoor temperature fell to about 20°F. Unheated parts of a house stay a few degrees warmer than outside, so the pipe needs colder air than 32°F to reach freezing. A survey of plumbers backed this up: burst-pipe calls began when temperatures fell into the teens.
But wind and drafts change it. A pipe next to a gap where a cable enters the wall, or in a crawl space with open vents on a windy night, can freeze at temperatures well above 20°F.
Boise's freeze season in numbers
| Month | Average low, Boise airport |
|---|---|
| November | 32.3°F |
| December | 24.5°F |
| January | 24.9°F |
| February | 29.3°F |
Average December and January lows sit in the mid-20s. So on the colder nights of a normal Boise winter the temperature drops past the 20°F alert line, and arctic cold snaps go far below it. Treat any forecast low near 20°F as "drip tonight".
Tonight is going to be cold: checklist
- Drip faucets fed by pipes on exterior walls, over a crawl space or in the garage. A slow, steady drip of cold water is enough; on single-handle faucets set it to the middle.
- Open cabinet doors under kitchen and bathroom sinks on outside walls so house heat reaches the pipes. Move cleaning chemicals out of reach of kids and pets.
- Keep the thermostat at the same temperature day and night. No night setback during a cold snap.
- Close the garage door if water lines run through the garage.
- Hose bibs: garden hose disconnected, outdoor faucet shut off from inside and drained.
- Know where your main water valve is, and make sure it turns, before you need it.
Which pipes freeze first in Treasure Valley homes
- Outdoor hose bibs and irrigation lines that were not drained.
- Pipes in crawl spaces, especially with open foundation vents.
- Pipes in garages and attics.
- Supply lines to sinks on exterior walls, often kitchen sinks under a window.
- Lines near gaps where cables, vents or the water service enter the house.
Our 24/7 local pro line for Boise and the Treasure Valley is opening soon. Until then: stop the water using the steps on this page, then contact a licensed water damage restoration company in your area. Check that they are licensed and insured before work begins.
Signs a pipe is already frozen
- A faucet gives only a trickle or nothing, while others work.
- Frost on an exposed pipe, or a bulge in it.
- A toilet that will not refill.
What to do: open that faucet, warm the pipe gently with a hair dryer, heating pad or a space heater kept at a safe distance, starting near the faucet. Never use an open flame. Keep a hand on the main water valve in case a split shows up as it thaws. Step-by-step: burst or frozen pipe: the first 60 minutes.
Is dripping worth the water bill?
A slow drip uses a small amount of water compared with the cost of a burst pipe. Insurance industry data shows the average paid water damage and freezing claim was $15,400 for 2019–2023, and that is before your deductible and the disruption. Dripping is cheap insurance on the nights that need it.
Quick answers
At what temperature do pipes freeze?
Water freezes at 32°F, but pipes inside a house are protected by the building. Research by the University of Illinois Building Research Council found uninsulated pipes in unheated spaces start freezing when the outdoor temperature drops to about 20°F. Pipes exposed to drafts or wind can freeze above that.
When should I drip my faucets in Boise?
When the forecast low is around 20°F or colder, let cold water drip from faucets fed by pipes on exterior walls, in crawl spaces or in garages. The American Red Cross notes that even a trickle of running water helps prevent pipes from freezing.
Should I drip hot or cold water?
The Red Cross advice is to let cold water drip from faucets served by exposed pipes. On a single-handle faucet, set the handle to the middle so both lines move a little. Both hot and cold supply lines can freeze if water is not moving.
How long does it take for pipes to freeze?
It depends on how cold it is, how exposed the pipe is and whether there is wind or a draft. An uninsulated pipe in a cold crawl space or exterior wall can freeze within hours on a hard-freeze night, which is why the warning time is the evening forecast, not the morning after.
What temperature should I keep my house at to prevent frozen pipes?
The Red Cross recommends leaving the heat at no lower than 55°F if you go away in cold weather, and keeping the thermostat at the same temperature day and night during a cold snap.