Heat guide
How Buildings Lose Heat
Buildings lose heat through their envelope and through air exchange. The rate depends on temperature difference, area, thermal transmittance and airflow.
Transmission through the envelope
A simple steady heat-loss model uses Q̇ = UAΔT for each wall, roof, floor, door or window. U is the assembly transmittance, A is area and ΔT is the indoor-outdoor temperature difference.
Air exchange carries heat too
Outdoor air entering through intentional ventilation or uncontrolled leakage must be heated to indoor temperature. This sensible heat load can be important in leaky buildings or cold weather.
Thermal bridges increase local and total heat flow
Framing, edges and structural penetrations create paths with higher transmittance than surrounding insulated areas. Whole-envelope calculations should account for them when their effect is significant.
Solar and internal gains reduce net heating demand at times
People, appliances, lighting and sunlight can add heat. These gains vary with time, so a peak design heat-loss calculation may treat them conservatively or separately.
Heat-loss estimate is not complete equipment sizing
Selecting heating equipment can require design weather, ventilation requirements, zoning, distribution losses, recovery systems, warm-up loads and local standards. A simplified calculator should therefore label its result as an estimate.
Peak load and annual energy respond differently to weather
A heating system may be sized for a relatively rare cold design condition, while annual energy use depends on the many hours spent at milder temperatures. Reducing U-values and leakage lowers both, but equipment capacity and seasonal consumption should not be treated as the same metric.
Thermal storage, solar gains and internal gains can shift loads through the day without changing the underlying envelope properties. Dynamic simulation becomes valuable when timing, intermittent occupancy or large thermal mass materially affects the result.
Infiltration is variable rather than a fixed material property
Uncontrolled air leakage depends on wind, stack effect, pressure differences and the size and distribution of leakage paths. A single air-change estimate can support preliminary sizing, but measured airtightness and an appropriate infiltration model give a better basis when leakage materially affects the load.
Ventilation heat loss depends on air flow and temperature difference
When outdoor air enters a heated building, energy is required to raise that air toward indoor temperature. A simplified sensible ventilation load is proportional to air mass flow, air specific heat and the indoor-outdoor temperature difference.
Heat-recovery ventilation can transfer part of the outgoing air’s energy to the incoming stream, reducing this load without eliminating the need for ventilation.
Ground and adjacent-space boundaries need their own treatment
Floors over soil, walls against earth and surfaces adjoining unconditioned spaces do not always experience the same temperature difference as an exposed exterior wall. Ground temperature changes more slowly than outdoor air and heat flow can be multidimensional near edges.
Whole-building calculations therefore use methods suited to each boundary rather than applying one exterior-air ΔT to every surface.
A simple transmission example
Suppose a wall has an area of 30 m², a U-value of 0.30 W/(m²·K) and an indoor-outdoor temperature difference of 20 K. The steady transmission loss is Q̇ = UAΔT = 0.30 × 30 × 20 = 180 W.
That number covers only the chosen wall at that condition. Windows, roof, floor, doors, thermal bridges, infiltration and ventilation must be added separately for a room or building heat-loss estimate.
Peak heat loss and annual energy are different questions
A design heat-loss calculation estimates a rate, usually in watts or BTU/h, at a selected cold-weather condition. Annual heating energy accumulates changing loads over thousands of hours and is measured in kWh, MJ or similar energy units.
A building can therefore have a moderate peak load but high annual energy use, or a high design peak that occurs only rarely. Equipment sizing and energy-cost analysis should not be treated as the same calculation.
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Sources and further reading
Use the linked primary or authoritative resources for additional detail, standards and source-specific conditions.