Internal vs External Wall Insulation: Making the Right Choice

Wall insulation is fundamental to retrofit projects under PAS2035, yet the choice between internal (IWI) and external wall insulation (EWI) systems remains one of the most significant decisions retrofit coordinators face. Both approaches offer distinct advantages and limitations, and selecting the right solution requires understanding property-specific constraints, performance outcomes, and practical delivery considerations.

Understanding the Fundamental Differences

External wall insulation involves fixing insulation boards to the outer surface of masonry or brick walls, then covering with a finish coat. The insulation layer sits outside the structural wall, creating a continuous thermal envelope around the building.

Internal wall insulation is installed on the inner surface of external walls, typically using rigid boards or batts fixed to the existing wall structure. The finished surface then receives plasterboard and decoration.

This seemingly simple distinction creates significant differences in performance, installation complexity, and suitability across different property types.

External Wall Insulation: Advantages and Constraints

Key Advantages

• Thermal performance: Eliminates thermal bridging at wall junctions, floors and corners when properly specified
• Preserved internal space: No reduction in floor area or room dimensions
• Moisture management: Keeps the structural wall warm and dry, reducing condensation risk within the building envelope
• Extended building life: Protects the external wall from weather exposure and temperature cycling
• Aesthetic control: Opportunity to improve external appearance with modern finishes

Practical Constraints

• Listed building restrictions or conservation area limitations
• High-rise buildings requiring specialist access equipment and fall prevention measures
• Complex junctions with windows, doors and roof lines requiring careful detailing
• Potential neighbour disputes in terraced properties
• Higher upfront material and labour costs
• Weather dependency during installation
• Requirement for specialist contractors with appropriate accreditation

Internal Wall Insulation: Advantages and Constraints

Key Advantages

• Faster deployment: Installation can proceed regardless of external weather conditions
• Lower cost: Generally 20-30% cheaper than equivalent EWI specifications
• Simpler site logistics: Work primarily from inside the building, minimising disruption to neighbours
• Fewer planning constraints: Suitable for listed buildings where external appearance must remain unchanged
• Flexibility: Can be applied room-by-room or phased across the property

Significant Limitations

• Reduced floor area: Typical installations reduce room dimensions by 75-150mm
• Thermal bridging: Uninsulated elements (floor slabs, ceiling joists, structural columns) create cold bridges, reducing effective performance
• Condensation risk: Structural wall remains cold; poorly specified systems trap moisture within the building envelope
• Service integration: Existing wiring, plumbing and heating systems must be relocated or worked around
• Window and door reveals: Thickened wall construction affects reveals, sill depths and architectural detailing
• Building durability: Structural wall loses passive solar gain and heating benefit, potentially shortening service life

Critical Technical Considerations

Moisture and Condensation

The most significant difference between approaches lies in moisture management. External insulation keeps the structural wall warm, minimising interstitial condensation risk. Internal insulation leaves the structural wall at external air temperature; any moisture must be managed through vapour control or hygroscopic buffering. In properties with poor ventilation or high occupancy, IWI carries elevated condensation risk unless properly specified with effective air barriers and ventilation strategies.

Thermal Bridging Calculation

EWI systems achieve y-values typically between 0.05-0.15 W/m²K when properly detailed. IWI systems commonly deliver y-values of 0.25-0.40 W/m²K due to floor slab and structural element bridging. This difference is not trivial; across a semi-detached house, it equates to 3-4kW additional heat loss in winter conditions.

Compatibility with Existing Systems

EWI installations can work around existing boilers, extractors and building services with minimal disruption. IWI projects frequently require relocation of electrical outlets, thermostats and heating controls, increasing labour costs and programme duration.

Making Your Decision

Select external wall insulation when:

1. Building height permits safe access and working platforms
2. Planning and conservation constraints permit external alterations
3. Budget allows for specialist installation costs
4. Property has no significant moisture or damp history
5. Long-term performance and durability are prioritised

Select internal wall insulation when:

1. Listed building or conservation area status prevents external work
2. Budget is constrained and phased delivery is acceptable
3. Site access or neighbour relationships preclude external working
4. Rapid deployment timeline is essential
5. Room dimensions and architectural detailing can accommodate thickened construction

Many retrofit programmes employ mixed approaches, combining EWI on accessible elevations with IWI on constrained facades. This pragmatic solution optimises performance within practical and financial limits.

Conclusion

Neither approach is universally superior. EWI delivers better thermal performance and building durability but demands greater investment and planning compliance. IWI offers cost and programme advantages but requires meticulous specification to manage condensation and thermal bridging. Your decision should reflect building-specific constraints, client priorities, and long-term performance expectations established during the retrofit assessment process.