Building Energy Efficiency
Estimate heat loss, annual heating demand, and EPC-style rating from U-values, air tightness, and heating degree days. Compare your building to Part L and Passive House standards.
About This Calculator: Building Energy Efficiency
?Why It Matters
Energy efficiency cuts bills, reduces carbon, and improves comfort. EPC ratings affect property value; Part L compliance is mandatory for new builds.
⚙How It Works
We use steady-state heat loss: fabric (U×A×ΔT) plus ventilation (0.33×n×V×ΔT). Degree days convert to annual demand. EPC bands follow kWh/m² thresholds.
📖What You'll Learn
- Part L requires walls ≤0.18, roofs ≤0.11
- Passive House targets ≤15 kWh/m² and ≤0.6 ACH
📋 Quick Examples — Click to Load
🏠 EPC-Style Energy Rating
318 kWh/m²/yr → Rating G
- • Improve wall insulation (target U ≤ 0.18)
- • Upgrade roof insulation (target U ≤ 0.11)
- • Consider triple glazing (U ≤ 1.2)
- • Reduce air leakage; consider MVHR
- • Target EPC C or better for resale value
📊 Heat Loss Breakdown
📊 Rating Comparison
📈 Cumulative Cost Over Time
📊 Improvement Savings Potential
For educational and informational purposes only. Verify with a qualified professional.
✏️ Design Facts & Insights
UK Part L 2022: walls ≤0.18 W/m²K for new builds.
Passive House heating demand ≤15 kWh/m²/year.
EPC A requires <50 kWh/m² primary energy.
Building energy efficiency is measured by U-values (heat loss per m² per degree), air tightness, and annual heating demand. Fabric heat loss Q = U × A × ΔT; ventilation adds 0.33 × n × V × ΔT. EPC ratings A–G reflect kWh/m²/yr. UK Part L and Passive House set strict targets for new and retrofitted buildings.
Sources: UK EPC methodology, CIBSE, Passive House Institute, Part L.
Key Takeaways
- • U-value = heat loss (W) per m² per K; R-value = 1/U. Lower U = better insulation.
- • Fabric + ventilation heat loss × HDD × 24 gives annual heating demand in kWh.
- • EPC A (<50 kWh/m²) requires super-insulation and efficient heating.
- • Part L 2022: walls ≤0.18, roofs ≤0.11; Passive House targets ≤15 kWh/m² and ≤0.6 ACH.
Did You Know?
How Does Heat Loss Work?
Fabric Heat Loss
Q = U × A × ΔT. Each building element (walls, roof, floor, windows) loses heat proportional to its U-value and area. A 100 m² wall at U=0.5 with 20°C difference loses 1000 W continuously.
Ventilation Heat Loss
0.33 × n × V × ΔT. Each air change replaces the volume with cold air. At 5 ACH, a 200 m³ house loses ~330 W per degree. Airtightness and MVHR dramatically reduce this.
Degree Days
HDD = Σ(max(0, T_base − T_outdoor)) over the heating season. UK averages 2000–2500. Multiply total heat loss coefficient by HDD × 24 to get annual kWh.
Expert Tips
EPC Band Thresholds (kWh/m²/yr)
| Rating | Max kWh/m² | Typical Building |
|---|---|---|
| A | 0–50 | Passive House, new premium |
| B | 50–75 | Part L compliant new build |
| C | 75–100 | Well-insulated retrofit |
| D | 100–150 | UK average |
| E | 150–200 | Older housing |
| F | 200–300 | Poor insulation |
| G | >300 | Leaky, uninsulated |
Frequently Asked Questions
What are U-values and R-values?
U-value (W/m²K) measures heat loss through a building element: lower is better. R-value (m²K/W) is thermal resistance, the inverse of U-value. A wall with U=0.25 has R=4. UK Part L requires walls ≤0.18 for new builds; Passive House targets ≤0.15.
How is fabric heat loss calculated?
Fabric heat loss Q = U × A × ΔT, where U is U-value (W/m²K), A is area (m²), and ΔT is temperature difference. Total fabric loss sums all elements (walls, roof, floor, windows). Ventilation loss adds 0.33 × air changes × volume × ΔT.
What do EPC ratings A–G mean?
EPC (Energy Performance Certificate) rates buildings A (best) to G (worst) by primary energy use per m². A: <50 kWh/m²/yr, B: 50–75, C: 75–100, D: 100–150, E: 150–200, F: 200–300, G: >300. UK homes average D; new builds target B or better.
What is Part L compliance?
Part L of UK Building Regulations sets maximum U-values and carbon emissions for new and refurbished buildings. 2022 updates require walls ≤0.18, roofs ≤0.11, floors ≤0.13. Non-compliance blocks building control sign-off.
What are Passive House standards?
Passive House (Passivhaus) targets ultra-low energy: heating demand ≤15 kWh/m²/yr, airtightness ≤0.6 ACH, and U-values ≤0.15 for walls. Achieved via super-insulation, triple glazing, and mechanical ventilation with heat recovery (MVHR).
How do air changes per hour affect heat loss?
Each air change per hour (ACH) replaces the building volume with outdoor air. Ventilation heat loss = 0.33 × n × V × ΔT (W). A leaky 1970s home may have 10+ ACH; a Passive House <0.6. Reducing ACH via airtightness and MVHR cuts heating demand significantly.
Key Statistics
Official Data Sources
⚠️ Disclaimer: This calculator uses simplified heat loss models and area assumptions. For compliance, EPC assessment, or retrofit design, consult a qualified energy assessor or building engineer. Results are for guidance only.
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