Internal wall insulation (IWI) remains a contentious but necessary tool in the retrofit sector. Whilst external wall insulation (EWI) is generally the preferred approach under PAS2035, there are scenarios where internal solutions become the only viable option. Understanding when to specify IWI and how to ensure compliance with the Building Safety Institute's standards is critical for retrofit coordinators.

When Internal Wall Insulation Is Appropriate

IWI should be considered in specific circumstances where external measures present genuine obstacles:

• Listed buildings and conservation areas — where external appearance must be preserved and planning restrictions prevent EWI installation
• Terraced properties with shared walls — where neighbouring properties haven't consented to external work, though this requires careful consideration of condensation risk
• Cavity wall complications — where existing cavity fill prevents EWI, though cavity remediation may be preferable
• Space constraints — urban properties with minimal external clearance or pedestrian access issues
• Rental properties awaiting tenant turnover — where minimal disruption is needed during the transition period

However, these scenarios should trigger thorough investigation before IWI becomes the default solution. Many retrofit professionals find that initial obstacles can be overcome through creative specification or phased approaches.

Key PAS2035 Requirements for Internal Wall Insulation

PAS2035:2019 (soon to be superseded by PAS2035:2024) sets stringent requirements for IWI projects. Retrofit coordinators must ensure compliance across several critical areas:

Moisture Risk Assessment

This is the cornerstone of PAS2035 compliance for IWI. The standard requires:

• Detailed hygrothermal modelling using validated software tools to predict moisture behaviour within the wall build-up
• Assessment of interstitial condensation risk, particularly at the insulation–wall interface
• Consideration of capillary moisture movement in masonry
• Evaluation of surface condensation risk in the occupied space

The assessment must demonstrate that moisture levels remain below critical thresholds that could cause mould growth, structural decay, or insulation degradation. This typically requires a vapour control layer (VCL) specification, though the precise position and permeability varies by wall construction.

Thermal Bridge Management

Internal insulation creates thermal bridges at party walls, floors, and ceiling junctions. PAS2035 requires:

• Detailed thermal bridging calculations using approved methodology (typically ISO 14683 or equivalent)
• Specification of thermal break details to minimise heat loss through structural elements
• Recognition that corner effects and junction details will significantly impact whole-building performance

Retrofit coordinators should budget for these elements explicitly rather than viewing them as optional enhancements.

Air Tightness and Ventilation

IWI projects must account for air leakage pathways:

• The insulation layer should incorporate an air control layer to prevent convective heat loss
• Existing ventilation provisions must be reassessed, particularly in older properties with natural draught
• Background ventilation and moisture removal capacity should be verified before work begins
• Mechanical ventilation with heat recovery (MVHR) is often necessary to maintain adequate air quality without excessive heat loss

Workmanship and Quality Assurance

PAS2035 mandates rigorous on-site quality control:

• Insulation must achieve continuous coverage without voids or compression
• Vapour control layers must be properly sealed at all joints and penetrations
• Services routed through the insulation layer require careful detailing to prevent air leakage and thermal bypass
• Regular inspection checkpoints should be scheduled during installation

Practical Coordination Challenges

Retrofit coordinators implementing IWI face specific challenges beyond external solutions:

Space loss — Typical IWI with insulation, VCL, and finishes consumes 75–125mm of floor area per wall, materially reducing room dimensions and affecting furniture placement. This must be communicated clearly to occupants at the design stage.

Service coordination — Electrical outlets, heating pipes, and drainage require careful rerouting. Specification details must address penetration sealing and thermal performance comprehensively.

Staged delivery — Many retrofit projects using IWI proceed room-by-room to minimise disruption, requiring phased commissioning and quality assurance protocols.

Moving Forward

Internal wall insulation will remain part of the retrofit toolkit, particularly for heritage properties and constrained urban settings. Success depends on thorough PAS2035 compliance, rigorous moisture and thermal assessment, and clear communication about trade-offs with occupants. Retrofit coordinators should view IWI not as a shortcut but as a technically demanding approach requiring the same rigour as any external solution.