When improving energy performance, property owners often face a key decision: internal wall insulation (IWI) or external wall insulation (EWI). Both approaches can significantly affect a property’s Energy Performance Certificate (EPC) rating, but the impact is not identical. Understanding these differences is crucial for making the right investment choice.

Internal Wall Insulation (IWI)

Internal wall insulation involves fixing insulation boards or stud systems to the inside of external walls.

Advantages:

  • Lower upfront cost compared to external systems.

  • Less disruptive to the building’s exterior appearance.

  • Can be applied room by room, allowing phased upgrades.

Drawbacks:

  • Reduces internal floor space.

  • May cause thermal bridging if not installed carefully.

  • Disrupts interior finishes and fittings.

Impact on EPC:
EPC ratings improve because IWI reduces heat loss, but the gains may be less significant if installation introduces cold bridges or ventilation issues.

External Wall Insulation (EWI)

External systems involve fixing insulation materials to the outside of the property, finished with render or cladding.

Advantages:

  • Provides a continuous thermal layer, minimizing cold bridges.

  • Protects external fabric from weathering.

  • Retains full internal space.

Drawbacks:

  • Higher cost than internal systems.

  • May alter the property’s external appearance (planning permission may be required).

  • Can be disruptive to neighbors during installation.

Impact on EPC:
EWI generally produces greater improvements in EPC ratings because it offers more consistent insulation performance and reduces whole-building heat loss.

EPC Considerations: Internal vs. External

  • Older Properties: IWI may be the only option if external changes are restricted (e.g., listed buildings).

  • Maximizing EPC Gains: EWI often leads to higher ratings, especially when combined with roof and floor insulation.

  • Cost vs. Benefit: IWI is more affordable, but EWI often delivers a stronger improvement in both energy savings and EPC outcomes.

Why This Matters for Homeowners

  • Property Value: Better EPC ratings can enhance resale and rental value.

  • Energy Bills: Improved insulation reduces heating demand, lowering costs.

  • Compliance: Meeting EPC minimum standards (particularly for landlords) often requires substantial insulation upgrades.

Why Choose EPC Rate?

EPC Rate offers expert EPC assessments in London, helping property owners understand the best route to improve ratings:

For advice on insulation strategies and EPC improvements, contact EPC Rate today.