Wall insulation is fundamental to reducing heat loss and improving energy efficiency in buildings. However, choosing between internal wall insulation (IWI) and external wall insulation (EWI) requires careful consideration of property characteristics, budget constraints and long-term performance goals. This guide outlines the key factors to evaluate when making this decision.

Understanding the Fundamental Differences

Internal and external wall insulation achieve the same objective—reducing thermal transmittance—but operate in distinctly different ways.

External Wall Insulation is applied to the outside of the building envelope. The insulation layer, typically 100-150mm thick, is fixed to the external wall and then finished with a weather-resistant render or cladding system. This approach keeps the thermal mass of the building structure on the warm side of the insulation.

Internal Wall Insulation is installed on the inner surface of external walls, usually 40-100mm thick, and finished with plasterboard or other internal finishes. The building's thermal mass remains on the cold side of the insulation layer.

Key Performance Considerations

Thermal Performance

Both systems can achieve excellent U-values when properly specified. Modern external systems typically achieve U-values of 0.15-0.20 W/m²K, whilst internal systems can reach 0.20-0.30 W/m²K depending on thickness. External insulation generally offers superior thermal performance in practice because:

• It eliminates thermal bridging through the building structure
• The building's thermal mass is utilised to moderate internal temperatures
• Fewer cold spots develop internally

Moisture Management

This is critical and often determines suitability. External insulation maintains the building envelope at a consistent temperature, keeping the structure warmer and drier. Internal insulation can create a condensation risk if not properly designed, as the external wall becomes colder.

Properties with solid masonry construction or those in damp locations require particularly careful assessment when considering internal insulation.

Practical Installation Factors

Disruption and Occupancy

Internal insulation can be installed whilst buildings remain occupied, with work typically confined to individual rooms. External insulation requires external scaffolding and creates more significant site disturbance, though it avoids internal disruption.

For occupied housing associations with limited vacant stock, internal insulation may offer practical advantages despite performance trade-offs.

Space Implications

Internal insulation reduces internal floor area, typically by 100-150mm per wall. In properties with limited internal space—particularly flats and terraced houses—this can be problematic. External insulation preserves internal dimensions entirely.

Technical Complications

External insulation requires careful detailing around:

• Window reveals and sill heights
• Balconies and projections
• Parapets and roof junctions
• Building services penetrations

Internal insulation presents different challenges, particularly around thermal bridging at party walls, service routes and structural connections.

Cost Analysis

Material costs for external systems are typically 20-30% higher than internal solutions. However, total cost comparison must include:

1. Scaffolding and site management (external only, significant cost)
2. Window and door reveal work (external more intensive)
3. Temporary rehousing costs if occupants must be relocated (internal avoids this)
4. Disruption costs and programme extension (external typically slower)

For retrofit projects, external insulation often costs £120-180 per square metre installed, whilst internal systems typically range from £80-130 per square metre. However, external systems often deliver better value over the building's lifecycle through superior durability and thermal performance.

Suitability by Property Type

External Insulation Preferred For:

• Buildings with significant thermal bridging issues
• Properties in damp or exposed locations
• Listed buildings where internal space must be preserved
• Buildings with complex internal layouts or multiple occupancies
• Properties where occupants need to remain during works

Internal Insulation Preferred For:

• Terraced and semi-detached houses with dry construction
• Budget-constrained projects with minimal occupancy
• Buildings where external access or scaffolding is problematic
• Projects requiring rapid installation with minimal site disturbance

Regulatory and Standards Compliance

Both systems must comply with Building Regulations and relevant British Standards. External insulation systems require BS EN 13141-1 compliance for ventilation design. Internal systems must demonstrate adequate vapour management and resistance to moisture ingress.

Fire performance is critical. External systems require non-combustible materials above 7.5 metres for residential buildings. Internal systems must achieve appropriate fire ratings for materials adjacent to occupied spaces.

Making Your Decision

Evaluate decisions using this framework:

1. Assess building condition, construction type and moisture history
2. Establish available budget and lifecycle cost tolerance
3. Evaluate site constraints and occupancy requirements
4. Determine internal space availability and retention importance
5. Confirm access for external works or occupant disruption tolerance
6. Conduct thermal bridging and moisture risk modelling

Neither solution is universally superior. External insulation typically delivers better performance and durability, whilst internal insulation can offer practical advantages in specific contexts. Thorough assessment of individual property characteristics and project constraints will guide the optimal choice.