What Happens to Your Home When the Power Goes Out

The ice storm hit on a Thursday evening. By Friday morning, 180,000 homes across southern Maine had lost power. Temperatures dropped into the single digits. For some households, the power came back within hours. For others, it was three days.

We heard from dozens of homeowners in the weeks that followed. Some described houses that stayed remarkably warm for 24 hours or more without any heat running. Others described homes that dropped below 50 degrees in six hours and reached freezing by midnight. Same storm. Same outdoor temperatures. Same duration of outage. Completely different experiences.

The difference was the building envelope.

The Physics of a Cooling House

When your heating system stops - whether from a power outage, a fuel delivery problem, or a mechanical failure - your house becomes a slowly cooling box. The rate at which it cools is governed by straightforward physics.

Heat Stored in Your Home

Your home contains stored thermal energy in every material within it - walls, floors, furniture, water in pipes, the air itself. The total amount of stored heat depends on the mass and temperature of these materials. A 2,000 square foot home at 68 degrees contains a significant amount of thermal energy above the freezing point.

This stored heat is your reserve. It is what keeps your home warm after the heating system stops. The question is how fast that reserve drains away.

The Rate of Heat Loss

Heat flows from warm to cold. The rate of flow depends on:

Temperature differential. The larger the gap between inside and outside temperature, the faster heat flows out. If it is 10 degrees outside and 68 degrees inside, heat flows faster than if it is 30 degrees outside and 68 degrees inside.

Insulation resistance. Insulation slows the rate of heat flow. An R-49 attic loses heat roughly three times more slowly than an R-15 attic. Walls with dense-pack cellulose insulation (R-13 to R-15 for a 2x4 wall) lose heat far more slowly than walls with empty cavities.

Air leakage. Air leaks allow warm air to physically leave the building and be replaced by cold outdoor air. This is often the dominant heat loss mechanism in older homes, especially during windy conditions. Air sealing reduces this loss dramatically.

Surface area. Larger homes have more exterior surface area and lose heat faster, all else being equal. However, compact building shapes (like a simple rectangular plan) are more efficient than complex shapes (like homes with lots of bump-outs, dormers, and additions).

The Math

Building scientists model heat loss using a metric called the building load coefficient (BLC), which combines insulation, air leakage, and surface area into a single number representing the rate of heat loss per degree of temperature difference.

Without getting into the specific calculations, here is what the numbers look like for two typical Maine homes:

House A - Poorly insulated (common pre-1960's condition)

• Attic: R-10 fiberglass batts
• Walls: Empty cavities
• Basement: Uninsulated
• Air leakage: High (equivalent to leaving a window open)
• Starting temperature: 68 degrees
• Outside temperature: 10 degrees
• Time to reach 50 degrees: 5-7 hours
• Time to reach 40 degrees: 10-14 hours
• Time to reach 32 degrees: 14-20 hours

House B - Well insulated (post-weatherization condition)

• Attic: R-49 blown cellulose
• Walls: Dense-pack cellulose
• Basement: Insulated and air sealed
• Air leakage: Low (professionally air sealed)
• Starting temperature: 68 degrees
• Outside temperature: 10 degrees
• Time to reach 50 degrees: 16-22 hours
• Time to reach 40 degrees: 28-40 hours
• Time to reach 32 degrees: 40-60+ hours

The well-insulated home retains heat two to three times longer. During a 48-hour power outage at 10 degrees, House A reaches freezing and puts plumbing at serious risk. House B stays above 40 degrees and remains livable, if uncomfortable.

These are estimates based on typical construction and simplified modeling. Actual performance depends on many variables. But the magnitude of the difference is consistent across the homes we assess and the data we collect.

What Happens as Your Home Cools

Understanding the stages of cooling helps you know what to expect and when to take action during an extended outage.

Stage 1 - The First Few Hours (68 to 60 degrees)

The initial temperature drop is the slowest because the temperature differential between inside and outside is at its smallest (relatively speaking) and the thermal mass of the house is fully charged. During this stage, the house feels cool but not uncomfortable. This is the window where most short outages resolve before conditions become difficult.

In a well-insulated home, this stage lasts 8-12 hours. In a poorly insulated home, it may last only 3-5 hours.

Stage 2 - Getting Cold (60 to 50 degrees)

At 60 degrees, the house feels noticeably cold. By 50, it is uncomfortable for most people, especially for children, elderly residents, and anyone with health conditions. This is the temperature range where you should be implementing your backup heat plan if you have one - lighting the wood stove, starting the generator, or making arrangements to stay elsewhere.

In a well-insulated home, this stage may take another 6-10 hours. In a poorly insulated home, it passes in 3-5 hours.

Stage 3 - Risk Zone (50 to 40 degrees)

Below 50 degrees, the house is cold enough that prolonged exposure becomes a health concern for vulnerable occupants. Hypothermia risk increases, especially for people who are not active and generating body heat. Pets are also at risk.

At 40 degrees, you should be seriously considering leaving the house if you do not have backup heat. Pipes in exterior walls and unheated spaces may be approaching freezing.

Stage 4 - Critical (Below 40 degrees)

Below 40 degrees, the risk of frozen pipes increases rapidly, especially for pipes in exterior walls, near the sill plate, and in unheated spaces. Water damage from a burst pipe can cost $5,000 to $20,000 or more to repair.

At 32 degrees interior temperature, any unprotected water line is at risk of freezing. Leaving faucets dripping helps but does not eliminate the risk for pipes in poorly insulated locations.

What You Can Do During an Outage

Concentrate in One Room

Close doors to rooms you are not using. Move the family into one room - ideally a centrally located room on the main floor. Body heat from four people in one room is measurable - it can maintain the room 5-8 degrees warmer than an unoccupied room.

Cover Windows

Windows are typically the weakest thermal point in any home. Hang blankets over windows at night to add insulation and reduce radiant heat loss. Even a single blanket provides meaningful benefit.

Use Passive Solar

During daylight hours, open curtains and blinds on south-facing windows to let solar heat in. The sun provides free heat that can slow or temporarily reverse the cooling trend. Close them again at sunset.

Protect Plumbing

If temperatures are dropping toward freezing, open cabinet doors under sinks on exterior walls to let room heat reach the pipes. If you have access to the main water shutoff and temperatures are approaching 32 degrees, consider shutting off the water and draining the lines.

Do Not Use the Oven or Grill for Heat

Every winter, people in Maine are hospitalized or killed by carbon monoxide from using gas ovens, charcoal grills, or portable propane heaters indoors. Do not do this. Carbon monoxide is invisible, odorless, and lethal.

The Permanent Solution

Every power outage is temporary. But the building envelope condition that determines whether your home stays livable during that outage is permanent - until you improve it.

The upgrades that extend your home's heat retention during an emergency are the same ones that reduce your heating bills every day:

• Air sealing the attic floor, sill plate, rim joist, and penetrations
• Attic insulation to R-49 or R-60 with blown-in cellulose
• Wall insulation with dense-pack cellulose, especially if wall cavities are currently empty
• Basement insulation to protect the lower building envelope and keep pipes above freezing

These improvements typically reduce annual heating costs by 20-40%. The emergency resilience they provide is a critical secondary benefit that most homeowners do not think about until they experience an outage.

Efficiency Maine rebates can reduce the cost of insulation and air sealing projects. Rebate amounts are income-dependent, and we handle the entire application process. As a Top Efficiency Maine Contractor for 10+ years, we know exactly how to structure projects to maximize available incentives.

Get Your Home Assessed

If you want to know how your home would perform during a multi-day winter power outage - or if you already know the answer and did not like it - a free energy assessment is the right first step.

We evaluate your building envelope with thermal imaging and blower door testing, identify the specific improvements that would make the biggest difference, and give you a clear plan with costs, rebates, and projected benefits.

Schedule your free assessment or call (207) 221-3221. We have been making Maine homes more comfortable, efficient, and resilient since 2006. The next ice storm is not a question of if - it is a question of when. How your home performs during that storm is something you can control.