Search this question and you'll find "6 to 10 hours" on one heating company's blog, "8 to 12" on another, and "12 to 24" on a third. None of them asks how cold it is outside or how well your house is insulated — the two things that actually decide it.
The honest range is under 2 hours to more than 28. Which end you're on matters a great deal, because the advice for one is wrong for the other.
Why It Varies So Much
Houses cool the way a hot drink does: quickly at first while the inside-outside gap is large, then progressively slower. Two things set the pace.
The outdoor temperature. Heat loss is proportional to the temperature gap, so a house at 40°F outside loses heat roughly half as fast as the same house at 0°F.
The envelope. A tight, well-insulated house holds heat three to four times longer than a draughty pre-war one. This is the variable people underestimate.
Combine them and you get a spread that no single number can represent. Time to fall from 68°F to 55°F — the point where plumbing starts being at risk:
- Pre-1950, uninsulated, 0°F outside: about 2 hours
- 1980s standard insulation, 20°F outside: about 7 hours
- Well insulated, 20°F outside: about 10 hours
- Well insulated, 40°F outside: about 20 hours
- Very tight modern envelope, 40°F outside: about 28 hours
The 7-hour case is what the "6 to 10 hours" articles are describing. If your house is older and it's genuinely cold out, you have a fraction of that. Our power outage cooling calculator works out the timeline for your own house and outdoor temperature.
The Temperatures Worth Knowing
- 60°F — comfort is gone, condensation starts forming on cold surfaces.
- 55°F — the standard floor for protecting plumbing. Plan around this one.
- 50°F — hypothermia risk becomes real for infants, elderly people, anyone unwell, and pets.
- 40°F — treat the house as unsafe to sleep in.
Your Pipes Don't Care What the Thermostat Says
This is the caveat that matters most, and it's why "when does the house hit 55°F" isn't the whole question.
The pipes that burst are almost never in the heated space. They're in exterior walls, crawl spaces, attics and unheated garages — places that follow the outdoor temperature much more closely than your living room does. Exposed, uninsulated pipes can begin freezing within roughly four to six hours of sub-freezing exposure, potentially while the house still just feels chilly.
So use the indoor timeline as your cue to act, not as a countdown to damage. The protective steps cost nothing:
- Open cabinet doors under kitchen and bathroom sinks so household air reaches the plumbing.
- Set faucets to a slow drip. Moving water is much harder to freeze, and it relieves the pressure that actually bursts pipes.
- Know where your main shutoff is before you need it in the dark.
- If a long outage in deep cold looks likely, draining the system is the definitive answer — but that's a decision to make early.
One detail that surprises people: pipes don't usually burst because ice expands against the pipe wall. They burst because ice forms a plug and the water trapped between that plug and a closed faucet is squeezed to enormous pressure. That's precisely why a dripping tap helps — it gives the pressure somewhere to go.
What to Do While the House Is Still Warm
Every one of these works far better before the house cools than after:
- Close off unused rooms. Shrinking the heated volume concentrates the remaining heat where people are.
- Cover the windows. Blankets or heavy curtains over glass cut a large share of the loss — glass is the weakest part of almost any envelope.
- Block draughts at doors and unused fireplaces. An open damper is a chimney-shaped hole in your house.
- Keep exterior doors shut. Each opening dumps warm air you have no way to replace.
- Gather everyone into one room at night. Body heat is not trivial in a small closed space.
The Safety Part We're Not Going to Soften
Cold itself is slow. The things people do to fight it are what kill quickly.
- Never run a generator indoors — not in a garage, not with the door open, not in a shed. Keep it well away from doors, windows and vents. Carbon monoxide is odourless and outages are exactly when it kills.
- Never heat with an oven, stove, grill or camp stove. Same reason.
- Never backfeed a generator into a wall outlet — it can electrocute the utility crews restoring your power.
- Combustion heaters need ventilation and a working CO alarm. A battery-powered CO alarm is the single most useful thing to own before an outage happens.
- If the house is heading below 40°F and you have somewhere to go, go. A warming centre or a friend's spare room is not a failure.
The Longer-Term Answer
Insulation and air sealing extend this timeline permanently — a benefit that never shows up in a payback calculation but is worth real money the one winter you need it. Our insulation savings calculator covers the bill savings side.
If you're considering backup power, one point often missed: most gas and oil heating systems need only a modest amount of electricity for controls and the blower. Backing up just the heating is dramatically cheaper than backing up a whole house. Our solar vs generator comparison and battery storage calculator cover the options.
And if your heating runs but your bills spike in cold snaps, that's a different problem — see why heat pump bills spike in winter.