Energy Performance Certificates (EPCs) measure a building’s energy efficiency and potential running costs. One of the key factors influencing EPC ratings is wall insulation. While both internal wall insulation (IWI) and external wall insulation (EWI) improve thermal efficiency, they affect EPC assessments differently. Understanding these differences helps property owners, landlords, and developers make informed decisions when upgrading energy efficiency.
Internal vs. External Wall Insulation: The Basics
Internal Wall Insulation (IWI)
Applied to the inside of external walls.
Often used in listed buildings, older properties, or where external appearance must be preserved.
Can slightly reduce floor space but is easier and cheaper to install in certain scenarios.
External Wall Insulation (EWI)
Applied to the outside of external walls, typically followed by a render or cladding finish.
Improves thermal bridging more effectively and reduces heat loss across the building envelope.
Can enhance exterior aesthetics and property value but may be more expensive.
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How EPC Assessments Treat IWI vs. EWI
| Feature | Internal Wall Insulation | External Wall Insulation |
|---|---|---|
| Thermal Performance | Reduces heat loss but may leave minor thermal bridges at junctions | Typically more comprehensive, reduces thermal bridging more effectively |
| Impact on EPC Rating | Moderate improvement, sometimes limited if thermal bridging persists | Higher EPC improvement due to complete envelope coverage |
| Installation Considerations | Less disruptive to external appearance, may reduce internal floor area | More costly, requires scaffolding, affects external look |
| Maintenance & Longevity | Can be affected by moisture or condensation if not properly installed | Generally durable, protects underlying structure from weather |
Practical Implications for Property Owners
Older Buildings
Internal wall insulation is often the only viable solution for heritage or listed properties.
Cost vs. EPC Impact
IWI can deliver a good EPC uplift at lower cost, but EWI may provide the maximum rating improvement.
Thermal Comfort
Both IWI and EWI improve comfort by reducing drafts and cold spots, but EWI often gives a more consistent temperature profile throughout the property.
Moisture Management
IWI can lead to condensation issues if the wall is not properly treated. EWI helps protect the structural wall and minimizes moisture problems.
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Tips to Maximise EPC Gains
Combine Insulation with Efficient Heating
Upgrading boilers, radiators, or heat pumps alongside wall insulation amplifies EPC improvements.
Seal Thermal Bridges
Pay attention to floor-to-wall and wall-to-ceiling junctions, especially with IWI.
Provide Accurate Installation Details to Assessors
Specify insulation type, thickness, and installation method to ensure EPC accurately reflects energy savings.
Consider Long-Term Benefits
EWI may have higher upfront cost but often results in better energy performance, property value, and durability.
Conclusion
Internal and external wall insulation both improve EPC ratings and energy efficiency, but they do so differently. IWI is practical for older or listed buildings, while EWI often provides the best overall EPC improvement. By understanding these differences and working with experienced assessors, property owners can make the right choice for both energy performance and long-term sustainability.
Get Expert EPC Guidance
At Urgent EPC, we provide professional EPC assessments that account for all insulation strategies. Explore our pricing plans or contact us to schedule your assessment today.
