Balance point temperature is the outdoor temperature at which a building needs no heating or cooling to stay comfortable inside. At this specific temperature, the heat a building gains from people, lights, appliances, and sunlight exactly offsets the heat it loses through walls, windows, and air leaks. The conventional default is 18 °C (about 65 °F), but every building has its own unique balance point depending on how well it’s insulated, how many heat sources are inside, and how much sun it receives.
How It Works
Every building constantly loses heat to the outdoors through its walls, roof, windows, and any gaps in the structure. At the same time, it constantly gains heat from the people inside, the lights they turn on, the appliances they run, and the sunlight streaming through windows. The balance point is the outdoor temperature where those two forces cancel each other out perfectly. When the temperature outside drops below the balance point, the building starts losing heat faster than internal sources can replace it, so the heating system kicks on. When the outdoor temperature climbs above it, the building accumulates more heat than it can shed, and cooling becomes necessary.
The formula is straightforward: start with your thermostat setting, then subtract the total internal and solar heat gains divided by the building’s overall heat loss rate. If your thermostat is set to 21 °C and your internal gains effectively “cover” 5 degrees worth of heat loss, your balance point sits at 16 °C. That means your furnace stays off until the outdoor temperature drops below 16 °C.
Why 18 °C Is the Default (and Why It’s Often Wrong)
Energy professionals and government agencies around the world have long used 18 °C (65 °F) as a standard balance point for calculating heating and cooling needs. China uses 18 °C for heating calculations and 26 °C for cooling. ASHRAE, the main U.S. engineering body for heating and cooling standards, also bases many of its guidelines on this figure.
The problem is that 18 °C is a rough average, not a universal truth. Research consistently shows that buildings with stronger insulation standards have lower balance point temperatures than the standard assumes. In one Korean study, the balance point varied by 2 °C just between the coldest and warmest regions of the country, and insulation quality had an even bigger effect. Using the wrong balance point to estimate energy use can lead to significantly misleading results, because it changes the count of heating and cooling degree days, the metric that drives most energy cost projections.
What Heating and Cooling Degree Days Have to Do With It
Heating degree days (HDD) and cooling degree days (CDD) are the standard way to estimate how much energy a building will need over a season. On any day the average outdoor temperature falls below your balance point, the difference between the two counts as heating degree days. If your balance point is 16 °C and the average outdoor temperature is 5 °C, that day adds 11 heating degree days. Sum those up over a winter and you get a picture of how hard the heating system has to work.
The accuracy of this calculation hinges entirely on choosing the right balance point. If you plug in the standard 18 °C for a well-insulated home that actually has a balance point of 14 °C, you’ll overestimate heating needs by counting many mild days as “heating days” when the building was perfectly comfortable on its own. For energy auditors, utility planners, and homeowners trying to size equipment or forecast energy bills, this distinction matters.
What Pushes the Balance Point Up or Down
Three main factors determine where a building’s balance point lands: how fast the building loses heat, how much free heat it generates internally, and how much solar energy it captures.
Heat Loss Rate
This depends on insulation, window quality, and air tightness. A conventional home might have wall insulation with a heat transfer rate of 0.3 to 0.7 W/m²K and windows rated around 1.8 to 2.2 W/m²K. A passive house, by comparison, targets 0.15 W/m²K for walls (requiring 30 to 40 cm of insulation) and 0.80 W/m²K or better for windows. The passive house also limits air leakage to 0.6 air changes per hour at a standard pressure test, compared to mainstream buildings that assume at least 0.5 air changes per hour just from natural ventilation. All of that means the passive house bleeds heat far more slowly, so its internal gains cover a much larger share of the heating load, pushing the balance point well below 18 °C.
Internal Heat Gains
People, lights, computers, cooking, and appliances all generate heat. A typical home produces roughly 18 W per square meter of floor area from these sources. A small office building generates about 38 W/m², a large office around 78 W/m², and a packed department store can hit 101 W/m². Artificial lighting alone ranges from 6 to over 20 W/m² depending on the type and density of fixtures. More internal heat gain means the building needs less help from the heating system, which lowers the balance point.
Solar Gains
Sunlight entering through windows adds free heat that also shifts the balance point down. This is why the balance point changes throughout the day and across seasons. A south-facing building with large windows will have a noticeably lower balance point on a sunny winter afternoon than on a cloudy morning. Passive house windows are specifically designed to balance insulation with a solar heat gain coefficient around 50%, letting in substantial warmth while minimizing heat escape.
Building Balance Point vs. Heat Pump Balance Point
If you’re researching heat pumps, you’ll encounter a related but different concept: the heat pump balance point. This is the outdoor temperature at which the heat pump’s maximum output exactly matches the building’s heat loss. Above that temperature, the heat pump handles everything. Below it, supplemental heating from electric resistance coils or a backup furnace kicks in.
The building balance point tells you when the building needs any heating at all. The heat pump balance point tells you when that specific piece of equipment can no longer keep up on its own. In a well-designed system, the heat pump balance point sits well below the building balance point, meaning there’s only a narrow band of very cold weather where backup heat is needed.
How to Lower Your Building’s Balance Point
A lower balance point means your building stays comfortable without mechanical heating across a wider range of outdoor temperatures, which directly translates to lower energy bills and smaller heating equipment. The most effective strategies target the heat loss side of the equation. Adding insulation to walls, attics, and foundations reduces the rate at which warmth escapes. Upgrading to double or triple-pane windows with low heat transfer ratings makes a significant difference, since windows are typically the weakest point in any building envelope. Sealing air leaks around doors, windows, ductwork, and penetrations stops warm air from escaping through gaps.
On the heat gain side, passive solar design captures more sunlight during heating months. Even choosing energy-efficient appliances can paradoxically raise your cooling balance point in summer, since they dump less waste heat into your living space. Taken together, these upgrades can shift a building’s balance point several degrees lower, potentially eliminating weeks of furnace runtime each year.