Home & Construction
Building Design Heat Loss Calculator
Estimate steady design heat loss from component areas and U-values plus an air-change ventilation term, then report transmission, ventilation, total kW, and floor-area intensity.
Wall transmission loss (kW)-
Window and door transmission loss (kW)-
Roof transmission loss (kW)-
Floor transmission loss (kW)-
Ventilation and infiltration heat loss (kW)-
Total envelope transmission loss (kW)-
Total modeled design heat loss (kW)-
Heat loss per entered floor area (W/m²)-
Decision view
Building-envelope heat-loss section and composition
Building-envelope heat-loss section and compositionHeat arrows leave walls, glazing, roof, floor, and ventilation in proportion to their calculated share of the total design load.
Exact scenario comparisonAir changes per hour changes while all other entered assumptions remain constant.
| Air changes per hour | Wall transmission loss (kW) | Window and door transmission loss (kW) | Roof transmission loss (kW) | Floor transmission loss (kW) | Ventilation and infiltration heat loss (kW) | Total envelope transmission loss (kW) | Total modeled design heat loss (kW) | Heat loss per entered floor area (W/m²) |
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How to use Building Design Heat Loss Calculator
- Enter areas and U-values for the four envelope groups.
- Enter heated volume and air changes per hour.
- Enter the design temperature difference.
- Inspect the building section and component-loss composition before using the total.
Calculator guide
Understanding Building Design Heat Loss Calculator
Building heat loss is a sum of envelope transmission and air-change loads. This calculator keeps walls, glazing, roof, floor, and ventilation visible so the dominant design component is not hidden inside a single total.
Calculation method
How the calculation works
Calculate U-area-temperature transmission losses by envelope component and add an explicit 0.33 air-change ventilation approximation. Calculate U A ΔT for each envelope component, calculate 0.33 n V ΔT for air changes, convert watts to kilowatts, and add every component.
Detailed calculation process
Build the design load from envelope and ventilation components
The defaults use a 30 K design temperature difference, 300 m³ indoor volume, and 0.50 air changes per hour.
General formula: Q_trans = Σ(U_i A_i ΔT)/1000; Q_vent = 0.33 n V ΔT/1000; Q_total = Q_trans + Q_vent; q_A = 1000 Q_total/A_floor Each surface loses heat in proportion to its U-value, area, and temperature difference. The 0.33 coefficient approximates the volumetric heat capacity of air when n is in h⁻¹ and volume is in m³.
What each symbol means
Worked substitution with the default inputs
The default building loses 6.285 kW: 4.800 kW through the envelope and 1.485 kW through modeled air changes, or 57.136 W/m² of floor.
Heat-loss anatomy
Trace heat through each building boundary
The envelope diagram sizes outward arrows by each calculated component load.
Worked situations
Practical examples
- Default glazing contributes 1.680 kW, the largest single component.
- Envelope transmission totals 4.800 kW.
- Ventilation raises the design total to 6.285 kW.
Better inputs
Useful tips
- Use net areas so openings are not counted twice.
- Use a design air-change rate appropriate to infiltration and ventilation strategy.
- Keep internal and solar gains outside this pure heat-loss audit.
Before relying on the result
Limitations and common mistakes
- The model is steady-state and uses a simplified 0.33 air heat-capacity coefficient.
- Thermal bridges, intermittent heating, heat recovery, solar gains, internal gains, and weather dynamics are excluded.
- Equipment sizing requires code, climate, ventilation, and professional design checks.
Reference
Key terms
- Transmission loss
- Heat conducted through envelope components.
- Air change
- Replacement of one building volume of air.
- Design ΔT
- Indoor-outdoor temperature difference used for sizing.
Important note
Calculated from the entered measurements and stated coverage or quantity rules. Confirm field dimensions, waste, product requirements, structural conditions, and local codes before purchasing or building.
Frequently asked questions
Why can windows exceed wall loss?
A high glazing U-value can outweigh its smaller area.
Does the model include heat recovery?
No. Enter an appropriately reduced effective ACH only if that assumption is justified.
What does 0.33 represent?
It is an approximate air heat-capacity factor for W from m³, h⁻¹, and K.
Is this heating equipment size?
It is a simplified design heat-loss estimate, not a complete equipment selection.