Heat loss calculator

How do I calculate heat loss through a wall?

Heat loss = area × temperature difference ÷ R-value. 1,200 ft² of R-13 wall with 70 °F inside and 20 °F outside loses 1,200 × 50 ÷ 13 ≈ 4,615 BTU per hour.

What is an R-value?

Resistance to heat flow, in ft²·°F·h/BTU — the higher, the better the insulation. Its inverse, U = 1/R, is the heat that flows through each square foot per degree.

Why do windows lose so much heat?

Their R-value is far lower than an insulated wall’s: 200 ft² of R-3 windows lose about 3,333 BTU/h at a 50 °F difference — nearly three-quarters as much as 1,200 ft² of R-13 wall.

Does this include drafts?

No — it covers conduction through surfaces only. Air leaking in and out adds to the loss; for sizing a furnace, use a full load calculation.

How do I convert BTU/h to watts?

1 BTU/h = 0.293 W, so 8,000 BTU/h is about 2.3 kW.

Do I subtract the window area from the wall area?

Yes. Enter the wall area without the windows and doors in it, and enter the windows and doors as their own surfaces. Otherwise the window area is counted twice, once as a well-insulated wall and again as a window with a much lower R-value.

How do I add up several layers in one wall?

Layers in series add their R-values: a wall that is R-13 in the cavity plus a continuous layer of R-1.5 behaves as about R-14.5. Parallel paths, such as studs next to insulation, do not simply average, so framing makes the whole-wall R-value lower than the insulation label alone; use the assembly value when you know it.

How do I convert metric RSI values to R-values?

Multiply the RSI (m²·K/W) by 5.678 to get the US R-value (ft²·°F·h/BTU). An RSI of 2.3 is about R-13. Do not enter an RSI value in the R-value field directly: the result would be off by a factor of nearly six.

How the calculation works

For each surface, heat flows from the warm side to the cold side at a rate proportional to the area and the temperature difference, and inversely to the resistance of the surface: Q = A × ΔT ÷ R, in BTU per hour when A is in square feet, ΔT is in degrees Fahrenheit and R is in ft²·°F·h/BTU. The loss through the whole envelope is the sum over all the surfaces. Using ΔT as the difference between the indoor temperature and the outdoor design temperature for your location gives the loss at the coldest expected conditions.

The model treats each surface as a single uniform R-value in steady conditions. It leaves out air leakage, the heat needed to warm ventilation air, the effect of sunlight and thermal mass, and heat transfer through the ground under a slab or around a basement wall.

Worked example

Take a small house with 1,000 ft² of R-13 walls, 200 ft² of R-3 windows, a 1,000 ft² R-38 ceiling and a 1,000 ft² R-19 floor. These R-values are only illustrative inputs. At 70 °F inside and 20 °F outside (a difference of 50 °F), the losses are about 3,846 BTU/h through the walls, 3,333 through the windows, 1,316 through the ceiling and 2,632 through the floor: 11,127 BTU/h in all, or about 3.3 kW. The windows are only 6% of the area but cause 30% of the loss, and the walls account for 35%.

Mistakes to avoid

Mixing up R-value and U-factor is the first. A window label usually gives a U-factor (the heat flow per square foot per degree), and the R-value is its inverse: U = 0.33 means R-3. Using the nominal R of the insulation instead of the whole assembly is the second. Using the average winter temperature instead of the design temperature is the third: the average understates the peak load, and a furnace sized from it would not keep up on the coldest days. For equipment sizing, a complete load calculation that includes air leakage is the proper tool.

Sources: the conduction formula Q = A × ΔT ÷ R and the relation U = 1 ÷ R are standard building-physics relations; the RSI factor 5.678 is the standard unit conversion. The R-values in the example are illustrative. Related tools: BTU, insulation, furnace size and heating cost calculators.