BUILDING PHYSICS / ENGINEERING GUIDE

Building Wall Heat Loss: Formula, Example and Calculator

Reviewed by WattCostLab Editorial Team · Updated September 2, 2026

For a steady-state plane wall, heat flux is q = UΔT and total wall heat flow is Q = U·A·ΔT. The result is an instantaneous thermal load under the selected conditions.

The basic wall heat-loss equations

Once the overall U-value of a wall is known, steady-state heat transfer is simple to calculate. Heat flux is the heat-transfer rate per unit wall area:

Total wall heat flow is:

Here q is in W/m², Q is in W, U is in W/m²K, A is wall area in m² and ΔT is the indoor-outdoor temperature difference in kelvins or degrees Celsius difference.

The same calculation using thermal resistance

Because U = 1/R_total, the equations can also be written as:

This form is useful when you are building up a wall from material layers because the thermal resistances can be added directly.

Worked example

Using the default WattCostLab wall model, R_total = 1.61920 m²K/W and U = 0.61759 W/m²K. For indoor air at 20°C and outdoor air at −10°C, ΔT = 30 K.

• Heat flux: 0.61759 × 30 = 18.53 W/m².
• For a 10 m² wall: 18.53 × 10 = 185.28 W.
• If the modeled area doubled to 20 m² under the same U and temperatures, total heat flow would double.

What the result means

The result is a heat-transfer rate at one set of boundary conditions. If indoor temperature is higher than outdoor temperature, positive Q represents heat flowing outward in the WattCostLab convention. If outdoor temperature is higher, the sign reverses and heat flows inward. When indoor and outdoor temperatures are equal, this idealized conductive model produces zero net heat flow.

Why wall area matters but U-value does not depend on area

U-value describes the modeled thermal performance per square metre per kelvin. It does not change merely because the wall is larger. Total heat flow Q does depend on area, so a large wall with a good U-value can still transfer more total heat than a small wall with the same U-value.

Instantaneous heat loss versus seasonal energy

Watts are a rate of energy transfer. To convert a constant heat-transfer rate to energy, multiply by time. For example, a perfectly constant 185.28 W sustained for one hour corresponds to 0.18528 kWh of heat transfer. Real weather and indoor conditions vary, so seasonal heating demand cannot be obtained accurately by multiplying one design-point result by an entire season.

A seasonal model would need changing outdoor temperatures, solar effects, ventilation/infiltration, internal gains, heating-system efficiency and the other parts of the building envelope. The wall calculator deliberately stays transparent and limited to its stated one-dimensional steady-state problem.

How insulation changes Q

Adding thermal resistance lowers U. At a fixed area and temperature difference, lowering U reduces q and Q proportionally. This is why the target-U feature is useful: it lets you compare the current wall and an insulated wall using the same boundary conditions.

Common heat-loss mistakes

• Using wall U-value to represent windows, roof and air leakage at the same time.
• Forgetting to use wall area in square metres when using SI units.
• Treating a single outdoor design temperature as if it were the entire year.
• Using a layer-only R-value instead of the complete wall resistance intended by the model.

Scientific basis

The WattCostLab wall calculation is based on the multilayer steady-state model described in Paraschiv et al., Energy Reports 6 (2020), 343–353. DOI: 10.1016/j.egyr.2020.08.055.

Temperature difference is as important as U-value

A low U-value reduces heat transfer, but the driving force is ΔT. The same wall can have very different instantaneous heat flow on a mild day and a cold day. For example, if U and area are fixed and ΔT doubles from 10 K to 20 K, q and Q double. This proportionality is a useful check on any steady-state wall calculation.

It also explains why a single Q result should always be reported with its indoor and outdoor temperatures. A statement such as “this wall loses 185 W” is incomplete unless the modeled area, U-value and temperature difference are known.

From heat flow in watts to energy in kilowatt-hours

Heat flow Q is power. If conditions were constant, energy would be Q multiplied by time. Divide watts by 1000 to obtain kilowatts, then multiply by hours. A 185 W steady heat flow maintained for 10 hours would correspond to 1.85 kWh of heat transfer through that modeled wall. In reality, outdoor temperature and other boundary conditions change, so this simple time multiplication is mainly useful for illustrating the units.

Heat loss through one wall versus whole-building heat loss

A whole building contains many parallel heat-flow paths: walls, windows, roof, floor, doors and thermal bridges, plus ventilation and uncontrolled infiltration. The wall formula applies to the wall area you enter. To build a simplified whole-envelope transmission model, each component would need its own U·A·ΔT term and the results would be summed. Air-exchange losses would be handled separately.

Using the calculator for sensitivity analysis

Start with a baseline assembly and note U, q and Q. Then change one input at a time. Increasing insulation thickness should reduce U and Q. Increasing wall area should leave U unchanged but increase Q. Increasing ΔT should leave U unchanged but increase q and Q. Changing material order while keeping the same thicknesses and conductivities should leave total R and U unchanged in this simple series model, while the internal temperature profile changes.

These predictable relationships make the calculator useful for teaching and for checking spreadsheet calculations. If a result violates one of these basic sensitivities, inspect the units and input definitions before drawing conclusions.