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How Cold Will an Unheated House Get?

Enduring extreme cold in an unheated house is neither comfortable nor safe, but this situation may arise when you have a power outage or blackout. You may also be wondering if it’s safe to leave your house unheated if you’re away during the winter.

An unheated house can get as cold as the outside temperature, depending on the insulation, number of occupants, and certain variables. Solar houses tend to stay a few degrees warmer than the nighttime lows while poorly-insulated properties may be much colder.  

Numerous factors such as climate, location, exposure to sunlight, wind conditions, and weatherstripping determine how cold an unheated house gets. In this article, I will explain how cold an unheated house can get and the associated risk to both health and property. 

How Cold Affects an Unheated House

An unheated house starts to cool instantly after the heat source is turned off, and the initial drop in temperature is typically quick and sharp. 

Well-insulated properties tend to retain some heat, but gradually all houses will get colder without a heat source. Houses with poor insulation lose heat faster and may stay freezing cold throughout the winter.

Here are a few ways in which the cold affects an unheated house:

The Temperature Starts to Mirrors Outside Temperatures

An unheated house and the temperatures outside exchange heat energy in a few ways, all while trying to sustain thermal equilibrium

Whether heated or unheated, a house loses heat through:

  • Conduction: heat loss through contact (through the walls, doors, and windows)
  • Convection: heat loss through a medium (the air in cavity walls, water in the pipes)
  • Radiation: heat loss from a warm to a cold surface (your body transferring heat to a window’s cold glass)

A house may have an excellent thermal envelope, which typically includes insulation and weatherstripping. However, the house will still lose heat gradually through conduction, convection, and radiation. Of course, the loss of heat is much slower and somewhat restricted when compared to a house with poor insulation.

Here’s a table to help you better understand the typical thermal envelope of a home, where it loses heat, and how to minimize heat loss.

Thermal EnvelopePercentage of Heat LossHow to Minimize Heat Loss
The roof25%Insulate the roof 
The walls, doors, and windows35%Install energy-efficient windows and doors and insulate the walls
The floors (except underground foundation)10%Insulate the floors 

Additionally, you also want to install weatherstripping throughout the house to seal any cracks or gaps.

Despite these measures, an unheated house will lose heat until it attains thermodynamic equilibrium with the cold outside. As such, a house with poor insulation may lose all its heat and be as cold as the temperature outside in a few hours. 

However, a house with a properly insulated thermal envelope may not be as cold as the ambient temperature for days, or even weeks in some cases. Of course, this heat retention depends on the indoor temperature at the time when the heating was shut off. 

The House May Still Be Warmer Than Nighttime Lows

Let’s consider a scenario where the indoor temperature of a reasonably-insulated house is 68°F (20°C) when the heating stops, and the minimum or low on a cold night is 32°F (0°C). 

In this case, the indoor temperature of the unheated house may drop below 50°F (10°C) during the night. If the house in this example has poor insulation, the indoor temperature may drop as low as 41°F (5°C) and get closer to 32°F (0°C). 

Most houses will not lose all the indoor heat in one night unless windows and doors are open. Besides, the unheated house may have people, pets, working lights, and other sources of heat, such as appliances. The presence of these factors will prevent a complete loss of heat.  

Additionally, the house gets warmer during the day and will lose that heat the following night. This rise in temperature during the day will depend on its location, available sunlight, and other factors.

For example, a solar house with large energy-efficient windows oriented towards the winter sun will get sufficiently warm during the day. In contrast, a typical house with limited direct exposure to sunlight will remain cold even in the daytime. 

A house with a passive solar design can retain the heat gained throughout the day. Thus, such a property will remain warmer than the nighttime lows even without a heat source. 

It May Get Colder Than the Average Daytime Highs

An unheated house cannot get colder than the nighttime lows within a short time. However, it can have a colder indoor temperature than the daytime highs. Not all houses have sufficient exposure to sunlight, and some properties may have drapes or curtains drawn, so some parts of the house will have no direct sunlight in winter.:

  • An unheated house with standard insulation may be less than 10°F (5°C) warmer than the minimum temperature at night. However, poor insulation will neutralize this effect, and an unheated house can be almost as cold as the average low at night.
  • An unheated house with standard insulation may be around 10°F (5°C) colder than the maximum temperature during the day. However, a house with passive solar design principles can be just as warm as the average high, even warmer if there is little or no cold wind.

Many local factors and attributes are at play here, from climate to the construction of the house. 

For instance, an unheated house without any direct exposure to sunlight may become a cold spot, the exact opposite of an urban heat island in summer. Thus, the house may feel colder than the ambient outdoor temperature.

Additionally, a solar design does not work identically in every region and climate. Frigid zones with overcast skies are of little help to the solar setup in an unheated house during the coldest days. 

How Cold Damages an Unheated House

Depending on how cold an unheated house gets, the property inside is vulnerable to different kinds of damage. However, the severity of damage depends on many factors.

For instance, an exceptional thermal envelope or insulation can prevent a house from losing all its heat, which will reduce the damage done. Likewise, an unheated house that is set up for the extreme cold will suffer less damage.

Additionally, some unchangeable factors like the kind of flooring will also determine the type and extent of damage. It’s worth remembering that even the most advanced insulation can’t keep an unheated house warm indefinitely. So while you may mitigate some damage, a few issues are unpreventable.

Here are some manageable and some inevitable effects of cold on an unheated house: 

1. Damaged Plumbing 

Subfreezing temperatures will damage the plumbing of an unheated house unless all the lines are drained, and the pipes are free of water. Even if you live in a place where the minimum hovers near 32°F (0°C) but doesn’t dip below, your pipes can freeze and crack.

The water in your plumbing system has significant pressure, and this pressure raises the freezing point of water. Thanks to the increased pressure, the water in your pipes doesn’t need sustained exposure to 32°F (0°C) to freeze. 

The location of each pipe and the way it is insulated will have a significant impact on plumbing. All pipes closer to the outer walls and those connected to outdoor fixtures will probably freeze overnight. 

A few pipes may crack, with some leaking water as they thaw. As such, you may return to find your unheated house flooded. Also, the water in the toilet bowls, water heater tanks, and any reservoirs can freeze if the temperature in an unheated house drops close to 32°F (0°C). 

2. Cracked or Warped Floors, Walls, and Other Fixtures

Cold temperatures in an unheated house cause wood to contract or shrink. Hence, wooden fixtures in the house are vulnerable to damage, including hardwood floors and even the studs in the walls. 

This problem gets worse if you factor in condensation or moisture in an unheated house that gets cold. High humidity or moisture causes wood to expand. Thus, an unheated house has to deal with expanding and shrinking floors, joists, rafters, studs, and more. This rapid expansion and contraction can cause significant damage to the wooden structures.  

Generally, the wooden installations in residential properties are flush, and there’s no space for any shrinkage or expansion. Therefore, an unheated house may cause significant damage to the floors, walls, and all the woodwork. 

Aside from the wood, metallic fixtures are also prone to rust and corrosion when left exposed for too long in an unheated house. This damage is due to the settling of moisture on these metallic parts.

As the air inside an unheated house gets cold, it can no longer retain the existing moisture. The relative humidity inside the house leads to extensive condensation, causing moisture to settle on surfaces. While the condensation is evident on windows, the dampness on other fixtures isn’t that obvious. 

3. Ruined Appliances 

Like your plumbing, all appliances connected to a water line are vulnerable to damage unless the hoses are drained and disconnected. This list includes your washing machine, water filter, and refrigerator. 

Additionally, extreme cold may shut down the freezer and refrigerator. All residential refrigerators with or without freezers have a thermostat with a heat sensor that detects the room temperature. 

If the temperature inside an unheated house drops below the coldest setting of a fridge, the appliance will shut down automatically. The freezer tends to stop working first after which your refrigerator may also stop functioning. Additionally, a refrigerator’s condensate line or drain pan will freeze up in frigid conditions.

Therefore, you must drain all water lines and shut down these appliances if you have to leave your house unheated in the cold. 

Health Hazards of Extreme Cold in an Unheated House

Many winter cabins have what is known as a floating floor. These floors allow the installation to contract and expand as a single unit. With this kind of setup, you can prevent structural damage and the serious implications of a compromised floor. However, regular residential houses usually don’t have such flooring.

Most modern constructions have not factored in natural winter-proofing for ages. As such, everything from the typical flooring to the appliances is set up to endure the winters in a heated house. 

And just like structural damage, an unheated house also poses a few health hazards to its occupants.

Buildup of Mold and Mildew

The colder the air in a closed unheated house, the greater is the relative humidity. Much of this vapor will condense and settle in various places all over the house as the air cools further. As such, you may have a buildup of mold and mildew if your house continues to stay unheated.

Mold does not die even if the temperature in an unheated house is below the freezing point. The mold spores stay dormant in extreme cold and spread vigorously when the environment is conducive. Breathing in this space can be harmful to the human body, and as such, a mold and mildew outbreak is a ticking bomb in an unheated house.

Lack of Ventilation Leads To Stale Air

The windows in an unheated house are typically closed to retain heat. However, without ventilation, there’s only stale air circulating in the living space.  Mold and mildew would worsen the problem with other allergens and microbial volatile organic compounds. 

Stale air is a health hazard, especially for those with respiratory and cardiovascular problems. Additionally, stale air can have severely adverse effects on those with a history of allergies.

Conclusion

An unheated house can get as cold as the outside temperature or remain a few degrees warmer if it has excellent insulation and there are many living occupants in the house, like people and pets.

Additionally, an unheated house can suffer extensive damage and become a health hazard if it hasn’t been set up to endure the extreme cold. Some damages and health risks can be avoided by ensuring that the house has sufficient heating and ventilation.

Author

  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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