Heat Loss Through a Wall

How to use the calculator

1. Enter the Wall area (A) — the total length × height of the wall section — and select m² or ft² as the unit. Include window and door areas only if you are not calculating them separately.
2. Enter the Overall U-value (U) of the complete wall assembly in W/(m²·K) or BTU/(hr·ft²·°F). If you only know the R-value, use \(U = 1/R\) (converting R to m²·K/W if it is given in imperial units).
3. Enter the Indoor temperature (T in) — the design temperature you want to maintain inside, typically 20–22 °C (68–72 °F).
4. Enter the Outdoor temperature (T out) — the coldest expected temperature for your location, also called the heating design temperature.
5. Read the Rate of heat loss (Q) in watts, kW and BTU/hr, and the Heat lost per day in kWh, MJ and BTU.

What is wall heat loss?

Whenever the air inside a building is warmer than the air outside, heat flows through the walls from the warm side to the cold side. This conductive heat flow is the largest single component of a building's heating load in cold climates, alongside ventilation losses and losses through windows, the roof and the floor. The rate of that flow depends on just three things: how big the wall is, how well insulated it is, and how large the temperature difference across it is.

The U-value (thermal transmittance) is the standard measure of how easily heat passes through a complete wall assembly — brick or siding, insulation, plasterboard, and the thin films of air on both surfaces. A lower U-value means a better-insulated wall. Modern building codes in cold regions typically require walls around 0.15–0.35 W/(m²·K), while an uninsulated solid brick or concrete wall can easily be 1.0–2.0 W/(m²·K) and lose three to five times more heat through the same area.

The result \(Q\) tells you how much heating power (in watts or BTU/hr) is constantly escaping through that one wall at the given temperatures. The per-day figure converts this into energy, which you can multiply by your energy prices to estimate the running cost. If you enter an outdoor temperature higher than the indoor temperature, the calculator reports a net heat gain instead — useful for estimating cooling loads in hot climates.

Assumptions, limitations and practical tips

• Steady state: the formula assumes constant temperatures on both sides. In reality, outdoor temperature fluctuates, and walls store heat (thermal mass), so daily results are approximations — accurate enough for sizing and cost estimates, not for hour-by-hour simulation.
• Use the overall U-value of the assembly, not just the insulation layer. If you have layer-by-layer R-values, add all the thermal resistances together and take the reciprocal: \(U = 1 / \sum R\).
• Thermal bridging: studs, mortar joints and metal ties conduct more heat than the insulation between them. Published U-values for the whole assembly usually account for this; a U-value computed from insulation alone will underestimate the loss.
• Design temperatures: for heating-system sizing, use the outdoor temperature that is exceeded on all but the coldest ~1% of hours in your climate (check local design conditions), not the all-time record low.
• This is one surface only. To size a boiler or heat pump, repeat the calculation for every wall, window, roof and floor, and add an allowance for air infiltration. To check how much of the total could be covered by solar, you can then compare the load with our Solar Panel Size calculator.
• Air leakage matters: in leaky older buildings, drafts can account for a heat loss comparable to the walls themselves, and this calculator does not include them.

If the heating load you compute seems large, note that halving the U-value (doubling the insulation) halves the loss through that wall — insulation upgrades scale linearly, which is what makes them such an effective first step in energy retrofitting. When planning any added electric heating, make sure the circuit is protected correctly — our Circuit Breaker Size calculator can help you check the wiring side.