Winter Plumbing Cold Snap: What to Do When Temperatures Drop Fast

A cold snap with forecast temperatures below 20°F in Seattle means pipes in crawl spaces, garages, and exterior walls face real freeze risk. This is an action guide — what to do right now to protect your plumbing before temperatures drop tonight.

How to Protect Pipes During a Sudden Cold Snap

When the forecast shows temperatures dropping below 20°F within the next 24 hours and you haven’t done pre-winter pipe preparation:

Immediate actions (do these now):

1. Let vulnerable faucets drip. Identify the faucets most exposed to cold:
– Kitchen sink on an exterior wall
– Bathroom sink in a bathroom with exterior walls
– Garage faucets
– Any faucet that has had pressure issues in cold weather before

Set these to a slow, steady drip — not a stream, just a trickle. Both hot and cold at each vulnerable fixture.

2. Open cabinet doors. Under sinks on exterior walls, open the cabinet doors and push aside any stored items that block warm air. This allows heated room air to circulate around the supply pipes.

3. Disconnect outdoor hoses. A connected garden hose traps water in the hose bib body, preventing drainage and causing freezing even in frost-free bibs. Disconnect all hoses now.

4. Close foundation vents. If your crawl space has operable vents (louvered openings in the foundation wall), close them. This keeps cold air from circulating through the crawl space around the supply lines.

5. Set the thermostat. Keep interior temperature at 60°F or above. If anyone else in the household controls the thermostat, tell them not to turn it below 60°F during the cold event.

6. Locate the main shut-off valve. Before anything happens, find the main water shut-off (typically near the water meter or at the house entry point). Know how to close it quickly — this is the first step if a pipe bursts.

What to Do With Plumbing Before a Cold Snap

If you have 24+ hours notice:

• Insulate accessible pipes in the crawl space with foam pipe insulation (available at any hardware store, $0.50–$2 per linear foot). Focus on any pipe that runs near foundation vents or in exposed corners.
• Seal drafts and gaps around pipes where they penetrate the foundation or floor with spray foam or caulk. Cold air flowing past pipes is more dangerous than cold air in a still space.
• Check that the water heater area is adequately heated — if the utility room is unheated, consider a small space heater set to run during the coldest hours.

What to check first:
– Are there any already-dripping or slow-running faucets? If you have known pressure or flow issues in cold weather, those pipes are your priority.
– Do you have a crawl space with supply lines? Crawl space pipes are the highest-risk location in Seattle homes.
– Are any supply lines in the garage — to a utility sink, washing machine connection, or hose bib? Garages can get as cold as outside.

How Cold Does It Have to Be for Pipes to Freeze?

The threshold: Pipes generally freeze when they’ve been at or below 32°F for a sustained period. But several factors determine whether a specific pipe freezes:

Insulation: A pipe with 3/4-inch foam insulation has dramatically more protection than a bare pipe. Insulated pipes in a moderately cold space (30°F) may not freeze even over several hours.

Water flow: Flowing water requires more heat extraction to freeze than still water. Pipes with slow flow (drip open) are more resistant than pipes with completely still water.

Pipe location: A pipe inside the house envelope (in a heated wall) is protected. A pipe in a crawl space that reaches 20°F is not.

Duration: A brief dip to 25°F for 3 hours is very different from 15°F for 18 hours. The heat stored in the water and pipe walls provides a buffer — sustained cold is what freezes pipes.

Seattle practical threshold: Treat any forecast that includes temperatures below 20°F for more than 4–6 consecutive hours as a pipe freeze risk, particularly for crawl space and exterior wall pipes.

Should I Let My Faucets Drip During a Cold Snap?

Yes — for any faucet served by a pipe in a cold-exposed location.

When to drip:
– Forecast temperatures below 20°F
– Pipes in crawl spaces that haven’t been insulated
– Any history of freeze problems in the home during cold weather
– Pipes in exterior walls without adequate insulation

How much to drip: A slow, steady drip — about the width of a pencil lead. This keeps water moving without wasting significant volume.

Which faucets: Faucets on exterior walls, in garages, or at the ends of supply lines (far from the main entry). Also drip outdoor hose bibs if the interior shut-off valve is closed but the bib line still has water in it.

Cost: A dripping faucet at roughly 1 gallon/hour uses about 24 gallons during a 24-hour cold snap. At Seattle water rates, this is approximately $0.10–$0.20. Cheap insurance.

Cold Snap Overnight — What to Do With Pipes

If you’re going to sleep during a cold snap:

• Confirm the thermostat is set to 60°F or above and note the setting — thermostat programming sometimes overrides manual settings at night
• Start the drip at vulnerable faucets before bed — don’t wait to see if it gets cold
• Set a reminder to check the faucets in the morning — confirm they’re still running and no ice has formed

If you wake up to no water:

1. Check other faucets — if some have water and one doesn’t, the frozen pipe is localized to the pipe serving that fixture
2. If no fixtures have water, the freeze is on the main supply line (or you’ve lost city supply — check if neighbors have water)
3. Do not use an open flame to thaw pipes. Use a hair dryer on low heat, warm towels, or a space heater directed at the exposed area

How to Protect Outdoor Faucets During a Cold Snap

Disconnect hoses immediately. This is the single most important step for outdoor faucets. Even frost-free bibs can freeze if a hose is connected — the hose prevents drainage of the stem.

Frost-free bibs: These bibs have the valve seat 8–10 inches inside the wall where temperatures are warmer. When fully closed, they drain by gravity — but only if no hose is attached. With the hose disconnected, a properly functioning frost-free bib requires no further protection in Seattle’s typical cold snaps.

Standard (non-frost-free) bibs: Shut off the interior valve serving the bib (usually in the basement, crawl space, or utility room), then open the bib to drain the line between the valve and the bib. This is essential — a standard bib with water in the pipe will freeze during a cold snap.

Insulating faucet covers: Foam or fabric outdoor faucet covers ($3–$8 at hardware stores) add some insulation protection. Useful for frost-free bibs that are in very exposed locations or where the stem seal has aged.

Unexpected Cold Snap — Quick Pipe Protection Tips

Checklist for protecting your plumbing in the next 2 hours before temperatures drop:

Action	Time Required	Cost
Let vulnerable faucets drip	5 minutes	$0.10–$0.20/night
Open cabinet doors under sinks	2 minutes	Free
Disconnect outdoor hoses	5 minutes	Free
Close crawl space foundation vents	10 minutes	Free
Raise thermostat to 60°F+	1 minute	Minimal
Locate main shut-off valve	5 minutes	Free

If you have more time:

Action	Time Required	Cost
Insulate crawl space pipes with foam	1–4 hours	$30–$150
Seal drafts around pipe penetrations	30 minutes	$10–$20
Add space heater to utility room	15 minutes	$30–$60 for the heater

How to Prepare Pipes for a Winter Storm Warning

A winter storm warning gives you 24–48 hours notice. Use that time:

Day before:
– Buy foam pipe insulation for any crawl space pipes not yet insulated
– Pick up a space heater for utility room or garage if needed
– Test the main shut-off valve — if it hasn’t been used in years, make sure it operates
– Confirm you know how to operate the valve and that it’s not seized

Evening before:
– Set all vulnerable faucets to a slow drip
– Open cabinet doors
– Disconnect hoses
– Close foundation vents
– Set thermostat to 60°F minimum

During the storm:
– Check water flow at dripping faucets periodically — if flow stops, a freeze is starting
– Monitor the most vulnerable areas (crawl space, garage)
– Keep a phone number for a plumber handy — if a pipe bursts, immediate response matters

What Happens to Pipes During a Sudden Temperature Drop?

The physics: When water temperature drops below 32°F and remains there, ice crystals form and grow. As ice grows, it expands — ice occupies about 9% more volume than liquid water. In a closed pipe, this expansion creates pressure. Pipe material determines what happens next:

Copper pipe: Relatively rigid. Ice expansion builds pressure rapidly. The pipe or a fitting may crack at the point of highest stress.

PEX pipe: Flexible — PEX can absorb some expansion without cracking. PEX pipe is significantly more freeze-resistant than copper or galvanized steel.

Galvanized steel: Rigid. Similar to copper — ice expansion creates pressure that may crack a fitting or the pipe wall.

The burst location: Pipes often don’t burst at the freeze point — they burst wherever pressure is highest when the ice plug blocks flow. This can be some distance from where the ice actually is.

After the thaw: Frozen pipes sometimes don’t leak until they thaw. Ice can seal a crack temporarily; when it melts, water pressure pushes through. Always inspect for leaks after a freeze event — even if water appears to flow normally immediately after thawing.

Plumbing Checklist Before Cold Weather Hits

Pre-winter (September–October):
– [ ] Install foam pipe insulation on all crawl space supply lines
– [ ] Replace standard hose bibs with frost-free models if not already done
– [ ] Close and test main shut-off valve
– [ ] Check and close foundation vents
– [ ] Insulate and seal around pipe penetrations in exterior walls

Before each cold event (when forecast shows below 20°F):
– [ ] Disconnect and drain all outdoor hoses
– [ ] Let vulnerable faucets drip (both hot and cold)
– [ ] Open cabinet doors under sinks on exterior walls
– [ ] Raise thermostat to 60°F minimum
– [ ] Check space heater in any unheated utility room

After a cold event:
– [ ] Check all water lines for proper flow
– [ ] Inspect under sinks and in crawl space for any signs of leaks
– [ ] Look for water stains on ceilings below crawl space areas

FAQ

Q: How do I protect pipes during a sudden cold snap?
A: Let vulnerable faucets drip slowly, open cabinet doors under sinks on exterior walls, disconnect outdoor hoses, close crawl space foundation vents, and keep the thermostat above 60°F. Know where the main shut-off valve is in case a pipe freezes or bursts.

Q: How cold does it need to be for pipes to freeze?
A: Pipes in unheated spaces can freeze when ambient temperature drops below 20°F for sustained periods (4–8+ hours). Factors that reduce freeze risk: insulation, pipe location inside the heated envelope, slow water flow (dripping faucet).

Q: Should I let faucets drip during a cold snap?
A: Yes. A slow drip at faucets on exterior walls or in vulnerable areas keeps water moving and relieves pressure if ice begins to form. Drip both hot and cold lines at each vulnerable faucet.

Q: How do I protect outdoor faucets during a cold snap?
A: Disconnect all garden hoses immediately. Frost-free hose bibs handle Seattle cold snaps when hoses are disconnected. For older standard bibs, close the interior shut-off valve and open the bib to drain the line.

Q: What is the plumbing checklist before cold weather?
A: Install foam insulation on crawl space pipes before November. Before each cold event: disconnect hoses, drip vulnerable faucets, open cabinet doors, close foundation vents, raise thermostat to 60°F.