Windows and doors represent significant cost items in fabric-first retrofits, yet their replacement isn't always necessary to achieve compliance with PAS2035 requirements. Understanding the assessment criteria that justify replacement versus retention is essential for retrofit coordinators managing both budgets and performance outcomes.

The PAS2035 Assessment Framework

PAS2035:2019 requires that retrofit specifications achieve notional performance standards through a whole-building energy model. Windows and doors feature prominently in this assessment because they represent significant thermal weak points in the building envelope.

The standard doesn't mandate replacement of existing windows and doors. Instead, the justification for replacement emerges from the technical assessment process:

Thermal Performance Requirements

The primary driver for window and door replacement is typically thermal performance. The notional building standard used for comparison assumes:

When existing single-glazed windows or doors with poor U-values create significant thermal bridges that cannot be adequately compensated elsewhere in the retrofit specification, replacement becomes justified. This is particularly relevant in solid-wall retrofit scenarios where window performance directly impacts overall building performance.

When Retention Is Viable

Retrofit coordinators should consider retaining existing windows and doors where:

Many retrofit projects successfully achieve PAS2035 compliance without wholesale window replacement, particularly where investment in cavity wall or external wall insulation is substantial.

Air Tightness and Installation Quality

Beyond thermal U-value, window and door replacement may be justified by air tightness requirements. Poor air tightness around existing frames significantly undermines retrofit performance, particularly in combination with mechanical ventilation systems.

Assessment should determine:

If existing windows cannot reliably achieve air tightness standards without excessive intervention, or if the cost of remedial sealing approaches replacement costs, replacement becomes the more practical option.

Condensation Risk and Moisture Management

Windows and doors with poor thermal performance create condensation risk, particularly when internal moisture levels increase due to occupant behaviour or reduced natural ventilation. This risk increases in retrofit scenarios where:

Where existing windows demonstrate persistent condensation issues or inadequate thermal capacity to manage moisture, replacement justified as a moisture risk management measure becomes defensible within the retrofit specification.

Documentation and Justification

Retrofit coordinators should document the assessment rationale in technical specifications:

This documentation supports both client decision-making and future compliance verification under PAS2035 Quality Assurance processes.

Practical Considerations

Beyond technical justification, retrofit coordinators should recognise practical factors:

The question of whether windows and doors require replacement ultimately stems from the individual building's retrofit strategy. No blanket rule applies—justification emerges from systematic assessment against PAS2035's performance requirements and consideration of alternative retrofit pathways.