Energy Performance Certificates (EPCs) are more than just coloured graphs and a letter grade. Behind each score lies a data-rich assessment driven by building physics, especially two key concepts: U-values and air leakage.
These technical measurements are the silent influencers of your EPC rating—often more important than your heating system or solar panels.
Let’s break down the science behind EPCs and explain how U-values and air leakage impact your home’s energy performance certificate.
1. What Are U-Values—and Why Do They Matter?
A U-value measures how easily heat flows through a building element (walls, roofs, floors, windows).
Lower U-values mean better insulation.
| Building Element | Typical Good U-Value (W/m²·K) |
|---|---|
| Wall (insulated cavity) | 0.3 or lower |
| Roof (loft insulation) | 0.16 or lower |
| Floor (insulated slab) | 0.22 or lower |
| Double Glazing | 1.6 or lower |
| Triple Glazing | 1.0 or lower |
In EPC assessments, the U-values of each element are used to calculate heat loss rates across the home. The higher the combined U-value score, the worse your EPC rating.
👉 EPC Tip: Improve U-values with external wall insulation, floor insulation, better glazing, and loft upgrades.
2. Air Leakage: The Invisible EPC Killer
Even if your U-values are excellent, poor air tightness can ruin your EPC score.
Air leakage (also called infiltration or draughts) occurs when warm air escapes through:
Gaps around windows and doors
Cracks in the wall structure
Unsealed loft hatches
Poorly fitted floorboards
Ventilation voids
Air leakage is measured in m³/h·m² at a pressure of 50 Pascals (if an air test is done). Without a test, the EPC software assumes default (often poor) air leakage rates.
👉 EPC Tip: Sealing gaps, using airtight membranes, and getting an air permeability test can significantly boost your score.
3. How These Two Factors Combine in EPC Software
EPC software (SAP or RdSAP) uses U-values and air leakage rates to calculate your home’s:
Heat demand (kWh/year)
Primary energy consumption
Carbon emissions
Heating and cooling costs
It then compares this data to a reference building to assign your EPC band (A–G).
Even small improvements in thermal envelope design can shift your property up one or two EPC bands.
4. Why You Might Be Losing Points Without Realising
If you haven’t had an air tightness test, your EPC may assume an air permeability of 10 m³/h·m² or worse.
If insulation levels aren’t documented, the software defaults to outdated or uninsulated values.
If U-values aren’t specified for extensions or retrofits, they may be penalised.
👉 EPC Tip: Always give your assessor U-value calculations or insulation specs, and ask about default assumptions being used.
5. Can You Improve U-Values and Airtightness in an Existing Home?
Absolutely. Consider:
Cavity wall insulation or external wall systems
Floorboard sealing or underfloor insulation
Upgrading to triple glazing
Installing a mechanical ventilation system with heat recovery (MVHR)
Professional draught proofing and blower door testing
Even targeted improvements to weak areas can raise your EPC score and cut your energy bills.
Final Word: EPCs Are Engineered, Not Estimated
Understanding U-values and air leakage reveals the true science behind EPCs. These aren’t just checklists—they’re complex calculations based on how heat behaves in your building.
The more control you have over these technical elements, the more accurate—and favourable—your EPC will be.
Need help improving your EPC score through better building science?
Speak to a qualified EPC assessor who can model your property’s thermal performance with precision.
📍 Final Takeaway
The EPC graph is just the beginning. When you know how to read between the lines, your report becomes more than a score—it becomes a blueprint for saving energy, reducing emissions, and increasing property value.
Ready to unlock the full story of your EPC?
📅 Book your expert EPC assessment today at EPCrate.co.uk.
