| No. | Material | Temp. outside | Temp. inside | Length, mm | Height, mm | Thickness, mm |
|---|---|---|---|---|---|---|
| Wall No.1 | ||||||
| 1 | ||||||
| Wall No.2 | ||||||
| 1 | ||||||
| Wall No.3 | ||||||
| 1 | ||||||
| Wall No.4 | ||||||
| 1 | ||||||
| Floor slab No.1 | ||||||
| 1 | ||||||
| Floor slab No.2 | ||||||
| 1 | ||||||
About Heat Loss Calculation
The calculator estimates heat losses through a room’s building envelope: walls, slabs, windows, and doors. It also adds an approximate component for air exchange (infiltration). The result helps you see where the main heat losses occur and estimate the heating power required.
Guidelines and recommendations
Converting dimensions to calculation units
Area A is computed from the entered dimensions in millimetres, converted to square metres. For a rectangular part of a construction, the formula is A = (L·H)/1 000 000, where L and H are in mm, and A is in m2.
Temperature difference ΔT for each construction is taken as ΔT = Tin − Tout in °C (numerically equal to K).
Thermal resistance of a layered assembly
Layer resistance Ri is calculated from the layer thickness and the material thermal conductivity: Ri = di/λi, where di is in metres and λi is in W/(m·K), giving Ri in m2·K/W.
Total resistance R for multiple layers is the sum: R = Σ(di/λi). Thickness is entered in mm, so the conversion is di(m) = di(mm)/1000.
Key assumption: only the materials’ contribution is included in the layer-by-layer resistance. Surface resistances to heat transfer on the inside and outside (Rsi, Rse) and corrections for thermal bridges are not added here. For a standards-based envelope calculation, EN ISO 6946 is commonly used with surface resistances and allowances for non-uniformity.
Heat losses through walls and slabs
Heat loss through an element Q is computed using “area × temperature difference ÷ resistance”: Q = A·ΔT/R, where Q is in W, A in m2, ΔT in K, and R in m2·K/W.
Net wall area is reduced by the area of openings in that wall. The calculator uses Anet = Awall − Aopenings, then Qwall = Anet·ΔT/R.
Heat losses through windows and doors
Windows and doors are calculated using a tabulated coefficient related to heat transfer. For the selected opening type, the calculator uses Ropen in m2·K/W, equivalent to R = 1/U, where U is in W/(m2·K).
Opening heat loss Qopen is calculated as Qopen = Aopen·ΔT/Ropen. This is the same as Qopen = U·Aopen·ΔT. The opening area is computed from its dimensions in mm and converted to m2.
Air exchange component (infiltration)
Air exchange heat loss Qinf is added as an approximate estimate using the common sensible heat formula: Qinf = 0.33·n·V·ΔT, where V is in m3, n in 1/h, ΔT in K, and Q in W.
Numbers used in this calculation: n = 1.0 1/h and V = Afloor·3.0, meaning volume is taken as floor area (m2) multiplied by a fixed height of 3.0 m. The factor 0.33 W/(m3·K) corresponds to an approximation of air heat capacity under normal conditions. This part is intended as a guideline. For a standards-based heating load that includes ventilation, EN 12831-1 is commonly used.
Final result and summation principle
Total heat loss Qtotal is the sum of all components: Qtotal = ΣQwalls + ΣQslabs + ΣQopenings + Qinf. If for any element ΔT ≤ 0 or R = 0, its contribution is taken as 0 W to avoid inflating the result.
European standards commonly used for this type of calculation
- EN ISO 6946 - calculation of thermal resistance and thermal transmittance (U-value) for multi-layer building components.
- EN ISO 10077-1 - calculation of U-values for windows, doors, and shutters (frame-based components).
- EN 12831-1 - calculation of the design heat load (required heating power), including envelope and ventilation.
- EN ISO 13789 - heat transmission through the building envelope and ventilation at building level (heat loss balance).
FAQs
Why are wall heat losses calculated as A·ΔT/R?
This follows directly from steady-state heat transfer through a flat layered element. The resistance R indicates how strongly the assembly reduces heat flow. The higher R is, the lower the heat loss Q for the same area A and temperature difference ΔT.
How is a window U-value related to the calculation?
Windows and doors are often specified by the thermal transmittance U in W/(m2·K). The calculator uses the equivalent R = 1/U, so Q = A·ΔT/R is identical to Q = U·A·ΔT. This keeps openings consistent with the same calculation structure as layered elements.
Why is the opening area subtracted from the wall area?
Walls and openings are calculated with different characteristics and methods. If you do not subtract openings, their area would be counted twice, once as part of the wall and once as a window or door. That is why the calculator uses Anet = Awall − Aopenings.
How accurate is the infiltration and air exchange part?
It is a guideline add-on so the total heat loss is not underestimated. The calculator uses Q = 0.33·n·V·ΔT with fixed n = 1.0 1/h and V = A·3.0. For a more exact ventilation-related heating load, EN 12831-1 is typically used with actual ventilation rates and airtightness data.
Why can my result differ from a “standards-based” heat loss calculation?
Standards-based methods typically include inside and outside surface resistances, account for thermal bridges, include non-uniformity factors, and use realistic ventilation conditions. Here the result is a clear engineering estimate based on layers, openings, and a typical infiltration allowance. For design decisions, compare against calculations per EN ISO 6946 and EN 12831-1.