MasterTool

Building Heat Loss Calculator

Calculate a building's conduction heat loss from labelled typical U-values, surface areas and the design temperature difference.

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What this tool does

Calculate a building's conduction heat loss from labelled typical U-values, surface areas and the design temperature difference. U-values are typical published values for common constructions; use tested assembly U-values or 1/R for your actual build-up when you have them. Infiltration is excluded.

How to use the Building Heat Loss Calculator

  1. Enter or select exterior wall area in sqft.
  2. Enter or select wall u-value (btu/h·ft²·°f) — typical insulated wall ≈ 0.07.
  3. Enter or select ceiling/roof area in sqft.
  4. Enter or select roof u-value — typical insulated ceiling ≈ 0.03.
  5. Enter or select window area in sqft.
  6. Enter or select window u-value — typical double glazing ≈ 0.30.
  7. Enter or select door area in sqft.
  8. Enter or select door u-value — typical insulated door ≈ 0.20.
  9. Enter or select floor area (over unheated space) in sqft.
  10. Enter or select floor u-value — typical insulated floor ≈ 0.05.
  11. Enter or select design temperature difference δt (°f).
  12. Read the calculated result; change any measurement to compare alternatives.

Formula

heat loss (BTU/h) = Σ (U × A) × ΔT over walls, roof, windows, doors and floor
wallArea
Exterior wall area (sqft, before metric conversion)
wallU
Wall U-value (BTU/h·ft²·°F) — typical insulated wall ≈ 0.07
roofArea
Ceiling/roof area (sqft, before metric conversion)
roofU
Roof U-value — typical insulated ceiling ≈ 0.03
windowArea
Window area (sqft, before metric conversion)
windowU
Window U-value — typical double glazing ≈ 0.30
doorArea
Door area (sqft, before metric conversion)
doorU
Door U-value — typical insulated door ≈ 0.20
floorArea
Floor area (over unheated space) (sqft, before metric conversion)
floorU
Floor U-value — typical insulated floor ≈ 0.05
deltaT
Design temperature difference ΔT (°F)

U-values are typical published values for common constructions; use tested assembly U-values or 1/R for your actual build-up when you have them. Infiltration is excluded.

Worked example

For building heat loss calculator, the following measurements illustrate the exact method: Exterior wall area: 1200; Wall U-value (BTU/h·ft²·°F) — typical insulated wall ≈ 0.07: 0.07; Ceiling/roof area: 1500; Roof U-value — typical insulated ceiling ≈ 0.03: 0.03; Window area: 200; Window U-value — typical double glazing ≈ 0.30: 0.3; Door area: 40; Door U-value — typical insulated door ≈ 0.20: 0.2; Floor area (over unheated space): 1500; Floor U-value — typical insulated floor ≈ 0.05: 0.05; Design temperature difference ΔT (°F): 60.

Inputs

  • Exterior wall area1200 sq ft
  • Wall U-value (BTU/h·ft²·°F) — typical insulated wall ≈ 0.070.07
  • Ceiling/roof area1500 sq ft
  • Roof U-value — typical insulated ceiling ≈ 0.030.03
  • Window area200 sq ft
  • Window U-value — typical double glazing ≈ 0.300.3
  • Door area40 sq ft
  • Door U-value — typical insulated door ≈ 0.200.2
  • Floor area (over unheated space)1500 sq ft
  • Floor U-value — typical insulated floor ≈ 0.050.05
  • Design temperature difference ΔT (°F)60

Result

  • Design heat loss (BTU/h)16,320
  • Total conductance ΣUA (BTU/h per °F)272
  • Design heat loss (kW)4.78

Results explained

Design heat loss (BTU/h)
Design heat loss (BTU/h) from the formula above. U-values are typical published values for common constructions; use tested assembly U-values or 1/R for your actual build-up when you have them. Infiltration is excluded.
Total conductance ΣUA (BTU/h per °F)
Total conductance ΣUA (BTU/h per °F) from the formula above. U-values are typical published values for common constructions; use tested assembly U-values or 1/R for your actual build-up when you have them. Infiltration is excluded.
Design heat loss (kW)
Design heat loss (kW) from the formula above. U-values are typical published values for common constructions; use tested assembly U-values or 1/R for your actual build-up when you have them. Infiltration is excluded.

Frequently asked questions

Element by element: Q = Σ U × A × ΔT. Each surface contributes its U-value × area × indoor–outdoor temperature difference, and the contributions add up to the design heat loss in BTU/h.

Labelled typical values used as defaults: insulated stud wall ≈0.07, insulated ceiling/roof ≈0.03, double-glazed window ≈0.30, insulated door ≈0.20, floor over an unheated space ≈0.05 (BTU/h·ft²·°F). U is 1 ÷ R-value, so an R-13 wall is about U 0.077 before framing effects.

The local design temperature difference: indoor temperature minus the winter design outdoor temperature for your area (for example 70 °F indoors and 10 °F design outdoors gives ΔT 60). Using the coldest night of the year instead oversizes the result.

It is the starting point for furnace or heat-pump capacity — but add infiltration separately and do not oversize aggressively; the Manual J tool in this category includes an infiltration term and solar/internal gains for cooling.

Air leakage (infiltration), thermal bridging beyond the entered U-values, and ground-coupled slab losses with their different temperature difference. Treat the total as the conduction component of heat loss.