Estimate the total heat loss of a building through walls, windows, roof, floor, and ventilation. Compute annual heating energy, fuel costs, and carbon emissions based on ASHRAE and ISO 13790 methodologies.
Heat loss is the rate at which thermal energy escapes from a building to the outside environment. It is the primary driver of heating demand and energy costs in residential and commercial buildings. Accurately estimating heat loss is essential for HVAC system sizing, energy performance certification (EPC, ENERGY STAR), retrofit planning, and carbon footprint reduction.
Total Heat Loss = Transmission Loss + Ventilation Loss
Qtotal = Σ (Ui · Ai · ΔT) + 0.33 · n · V · ΔT
where U = thermal transmittance (W/m²K), A = area (m²), ΔT = indoor–outdoor temperature difference (K), n = air changes per hour (ach), V = building volume (m³).
The transmission loss component accounts for heat escaping through the building envelope — walls, windows, roof, and floor. Each element has a characteristic U-value (thermal transmittance) that quantifies its insulating performance: lower U-values mean better insulation. The ventilation loss component accounts for heat carried away by air infiltration and mechanical ventilation, which depends on the building's airtightness and ventilation strategy.
The calculator follows the steady-state heat loss method defined in ISO 13790 and ASHRAE Fundamentals. For each building element, the heat loss is computed as:
Qi = Ui · Ai · (Tindoor − Toutdoor)
The total transmission loss is the sum over all envelope components. Ventilation loss is calculated using the air change rate (n) and building volume (V):
Qvent = 0.33 · n · V · (Tindoor − Toutdoor)
The constant 0.33 is the volumetric heat capacity of air (Wh/m³K) at typical indoor conditions. The total heat loss (in watts) is then multiplied by the heating season hours to obtain annual energy consumption (kWh), which is multiplied by the energy price and CO₂ factor to give annual cost and emissions.
This is a simplified steady-state model that assumes constant indoor and outdoor temperatures and uniform building properties. For more detailed dynamic simulations (e.g., using EnergyPlus or IES VE), time-varying weather data and thermal inertia effects are considered. However, the steady-state method remains the industry standard for quick assessments, regulatory compliance, and preliminary design.
| Parameter | Symbol | Unit | Typical Range | Description |
|---|---|---|---|---|
| U-value (Walls) | Uw | W/m²K | 0.15 – 1.50 | Thermal transmittance of wall construction; lower = better insulation. |
| U-value (Windows) | Ug | W/m²K | 0.80 – 3.00 | Includes glass and frame; triple glazing offers the lowest values. |
| U-value (Roof) | Ur | W/m²K | 0.10 – 0.60 | Heat loss through ceiling/roof; insulation thickness is key. |
| U-value (Floor) | Uf | W/m²K | 0.15 – 0.70 | Ground floor heat loss; depends on insulation and ground conditions. |
| Air Changes per Hour | n | ach | 0.15 – 1.50 | Rate of air exchange; lower = more airtight building. |
| Temperature difference | ΔT | K | 15 – 40 | Indoor minus outdoor design temperature. |
| Heating season hours | h | h/year | 1500 – 3500 | Number of hours per year when heating is required. |
A 1980s detached house in the UK (floor area 80 m², volume 224 m³) with solid brick walls (U = 1.0), single-glazed windows (U = 2.8), uninsulated loft (U = 0.45), and concrete floor (U = 0.50) was evaluated. The heating season is 2400 hours, indoor temperature 21°C, outdoor design −5°C, air changes 0.8 ach.
Initial total heat loss: ~8,200 W → annual energy ~19,700 kWh → cost ~$2,360/year (at $0.12/kWh) → CO₂ ~8,900 kg/year.
After a deep retrofit — cavity wall insulation (U = 0.35), triple-glazed windows (U = 0.8), 300 mm loft insulation (U = 0.15), floor insulation (U = 0.20), and improved airtightness (0.3 ach) — the heat loss dropped to ~2,800 W, annual energy to ~6,700 kWh, cost to ~$800/year, and CO₂ to ~3,000 kg/year.
Annual savings: ~$1,560 and ~5,900 kg CO₂. Payback period on the retrofit investment was estimated at 6–8 years.