Why "6 to 24 Hours" Is a Useless Answer
Houses cool the way a cup of coffee does — fast at first while the gap between inside and outside is large, then progressively slower. That's exponential decay, and it's governed by one property of your building: how long it takes to lose most of its stored heat, which engineers call the time constant.
Two things dominate the result, and neither appears in the usual advice:
- How cold it is outside. The rate of heat loss is proportional to the indoor-outdoor gap. At 40°F outside you have hours of slack; at −10°F the same house drops through the same range several times faster.
- How well the envelope holds heat. A tight, well-insulated house can have a time constant three to four times that of a draughty pre-war one.
Put those together and the honest range for "how long until pipes are at risk" is roughly under 2 hours to over 28 hours — not 6 to 24. Some worked examples, all starting at 68°F and measuring the time to reach 55°F:
- Pre-1950 uninsulated, 0°F out: about 2 hours
- 1980s standard, 20°F out: about 7 hours (this is the case the "6–10 hours" articles describe)
- Well insulated, 20°F out: about 10 hours
- Well insulated, 40°F out: about 20 hours
- Very tight envelope, 40°F out: about 28 hours
The reason this matters isn't academic. If you're in the two-hour case and you're reading advice written for the seven-hour case, you'll make the wrong call about whether to stay.
The Temperatures That Matter
- 60°F — comfort is gone and condensation risk rises on cold surfaces.
- 55°F — the standard floor for protecting plumbing. This is the number to plan around.
- 50°F — hypothermia risk becomes real for infants, elderly people, anyone unwell, and pets.
- 40°F — treat the house as unsafe to sleep in.
These are the same thresholds we use in our personal heating savings calculator, where they set the floor on how far you should turn the thermostat down deliberately.
Your Pipes Are Colder Than Your Thermostat
This is the single most important caveat, and the calculator can't model it for you.
The pipes that actually burst are almost never in the heated space. They're in exterior walls, crawl spaces, attics, and unheated garages — places that track outdoor temperature far more closely than your living room does. Common guidance is that exposed, uninsulated pipes can begin freezing within about four to six hours of sub-freezing exposure, which can be well before your thermostat reads 55°F.
So treat the indoor timeline above as when to act, not when damage starts. The standard protective steps — opening cabinet doors under sinks so household air reaches the plumbing, letting faucets drip so water keeps moving, and knowing where your main shutoff is — cost nothing and buy real margin.
If an extended outage in deep cold is likely, draining the system is the definitive answer. That's a decision to make before you lose the ability to make it comfortably.
What Extends the Timeline
- Close off unused rooms. Shrinking the heated volume concentrates what heat remains where people are.
- Cover windows. Blankets or heavy curtains over glass cut a large share of radiant loss. Glass is the weakest part of almost every envelope.
- Block draughts at doors and unused fireplaces. Air leakage matters more as the house cools, because you can't feel it once everything is cold.
- Don't open exterior doors more than you must. Each opening dumps warm air you can't replace.
- Insulation and air sealing extend this permanently, not just during outages. Our insulation savings calculator covers the bill savings; the outage resilience is a benefit that never shows up in a payback calculation.
A Safety Note We're Not Going to Soften
This calculator is about heat loss, not about how to generate heat. But the failure modes in outages kill people every winter, so:
- Never run a generator indoors — not in a garage, not in a shed, not near an open window. Carbon monoxide is odourless and outages are when it kills. Keep it well away from doors, windows and vents.
- Never heat a home with a stove, oven, grill or camp stove. Same reason.
- Never backfeed a generator into a wall outlet. It can electrocute utility workers restoring your power.
- Combustion heaters need ventilation and a working CO alarm. Battery-powered CO alarms are the one purchase that matters most here.
- If the house is heading below 40°F and you have somewhere else to go, go. A warming centre is not a failure.
If you're weighing backup power for future outages, our solar vs generator comparison covers the trade-offs, and our battery storage calculator covers sizing. Note that most gas and oil heating systems need only a modest amount of electricity to run their controls and blower — backing up the heating alone is far cheaper than backing up a whole house.