Air Permeability and Its Measured Impact on EPC Efficiency

Air leakage—also known as air permeability—is one of the lesser-understood but critical components in an EPC (Energy Performance Certificate) assessment. While insulation and heating systems often steal the spotlight, uncontrolled air movement through gaps and cracks can significantly drag down your EPC rating.

In this guide, we’ll unpack what air permeability means, how it’s measured, how it affects your EPC score, and what you can do to reduce it.

🌀 What Is Air Permeability?

Air permeability refers to the rate at which air leaks in and out of a building through gaps, cracks, and poorly sealed joints—not through intentional ventilation.

It’s influenced by:

• Gaps around windows and doors

• Unsealed pipework penetrations

• Loft hatches, electrical outlets, and floorboards

• Poorly constructed wall junctions

The lower the air permeability, the more airtight your building is—meaning less heat loss and better energy efficiency.

📏 How Is Air Permeability Measured?

In new builds or major refurbishments, air permeability is often tested using a blower door test, as part of SAP assessments (Standard Assessment Procedure).

Key Metrics:

• Measured in m³/h/m² (cubic metres of air per hour per square metre of envelope area)

• UK Building Regulations for new builds require ≤10 m³/h/m² (Part L compliance)

⚠️ Existing homes (assessed via RdSAP) don’t usually get tested. Instead, assessors use default assumptions based on the age, type, and construction of the building—which can penalise older properties.

📊 How Air Permeability Affects EPC Ratings

Air permeability directly influences heat loss and primary energy demand—both of which feed into the final EPC score.

The leakier your home, the more heating energy is needed, which increases both the cost and carbon emissions—resulting in a lower EPC score.

🏚️ Common Causes of Poor Air Permeability

• Gaps around doors and windows

• Poorly sealed loft hatches

• Cracks in walls, floorboards, or skirting

• Open chimney flues or unused fireplaces

• Unsealed cables and pipe penetrations

Even minor issues like unsealed electrical outlets on external walls can have a cumulative impact.

🛠️ How to Improve Air Tightness (and EPC Efficiency)

✅ 1. Seal Drafts Around Doors and Windows

• Use draught-proofing strips, seals, and letterbox brushes

• Upgrade single glazing or poorly fitted uPVC frames

✅ 2. Insulate and Seal Loft Spaces

• Add loft insulation and properly seal around loft hatches

• Ensure pipework and recessed lights are also sealed

✅ 3. Block Unused Chimneys

• Use chimney balloons or fit a chimney cap and draught excluder

✅ 4. Fill Gaps in Floors and Skirting

• Use flexible sealants or foam fillers

• Consider breathable membranes for older properties to balance ventilation

✅ 5. Install Airtightness Tapes or Membranes

• Common in new-builds or extensions

• Can drastically lower air leakage if done properly

⚖️ EPC Upgrade Potential

If your home is borderline between EPC bands, reducing air permeability can add several points to your rating—especially when combined with insulation or heating system improvements.

📈 Example:
A detached 1980s property with visible air gaps and no draught-proofing might score a low D.
After adding door seals, blocking chimney drafts, and sealing loft hatches, the EPC could move to a high D or low C, especially if combined with LED lighting or thermostat upgrades.

🧪 Do You Need a Blower Door Test?

Only new builds or major refurbishments are required to conduct a blower door test under SAP methodology.
For most existing homes (RdSAP), air permeability is estimated. However, if you’re undergoing a retrofit or aiming for high EPC scores (B or A), it may be worth commissioning a test voluntarily.

🏘️ EPC Compliance and Landlord Obligations

If you’re a landlord, ensuring airtightness can help you:

• Meet or exceed the minimum EPC Band E for rental properties

• Avoid expensive heating costs or complaints from tenants

• Futureproof against stricter MEES (Minimum Energy Efficiency Standards)

🧩 Final Thoughts

Air permeability may be invisible—but its impact on EPC efficiency is tangible.

🔑 Key Takeaways:

• It affects heat loss, energy bills, and comfort levels

• Even small changes—like sealing gaps or draught-proofing—can improve your EPC score

• Especially important in older homes where default assumptions work against you

Need Help Sealing the Gaps?

At EPCrate.co.uk, we don’t just assess—we advise on improvements to help you get the most out of your EPC. Whether you’re a landlord preparing a property or a homeowner planning renovations, we’ve got you covered.

✅ Book a certified assessor
✅ Get tailored advice on airtightness improvements
✅ Boost your EPC without costly overhauls

Seal the gaps, score the gains.
Air tightness could be the quiet upgrade your home truly needs.