Overheating in retrofitted properties is no longer a peripheral concern—it is a fundamental requirement within PAS2035. As the standard matures and more retrofit projects reach completion, the risk of creating homes that are uncomfortable, costly to cool, and potentially unsafe during summer months has become a critical focus for retrofit coordinators, designers, and installers across the UK.

Why Overheating Matters in Retrofit

When existing buildings are retrofitted to improve energy efficiency, the interventions that reduce heating demand in winter—better insulation, airtightness, and reduced air changes—can inadvertently increase overheating risk in summer if not carefully managed. This paradox is central to PAS2035's emphasis on whole-building performance and seasonal resilience.

Vulnerable occupants, including older people, young children, and those with health conditions, face particular risk during prolonged heat events. As climate projections indicate more frequent and intense summer temperatures across the UK, overheating prevention is both a regulatory requirement and a matter of occupant wellbeing.

PAS2035 Requirements for Overheating Assessment

PAS2035 Part 1 requires that retrofit design must be assessed to ensure it does not increase the risk of overheating. This assessment must be carried out at the pre-retrofit evaluation stage and documented within the Energy Performance Certificate scope.

Pre-Retrofit Evaluation

The retrofit coordinator must:

• Identify the baseline overheating risk in the existing property
• Consider the property's thermal mass, window orientation, shading, ventilation capacity, and occupancy patterns
• Evaluate whether proposed retrofit measures will worsen summer conditions
• Specify mitigation strategies within the retrofit scope

Thermal Modelling

Dynamic thermal modelling (such as CIBSE TM59 methodology) is the standard approach for assessing overheating risk. This goes beyond simple steady-state energy calculations to simulate hour-by-hour indoor temperatures across a full year, including summer design conditions.

Key modelling inputs include:

• Building geometry and fabric properties (U-values, thermal mass, air permeability)
• Window orientation, size, and solar shading specifications
• Ventilation rates and control strategies
• Internal heat gains from occupancy, lighting, and appliances
• Local climate data for typical summer weather

Design Strategies to Manage Overheating

Retrofit coordinators should integrate overheating mitigation into the overall retrofit package. Several proven strategies exist:

Passive Cooling

Natural ventilation: Specifying openable windows, vents, and cross-ventilation routes allows occupants to cool the home passively during cooler morning and evening hours.

Thermal mass: In some cases, exposed internal thermal mass (concrete, masonry) can absorb heat during the day and release it at night. This strategy requires careful design to be effective.

Solar Shading

Window shading—whether external shutters, blinds, or vegetation—can reduce solar gains by 50–80% depending on specification. External shading is more effective than internal but must balance summer cooling needs with winter solar gains for passive heating.

Ventilation Strategy

If mechanical ventilation with heat recovery (MVHR) is installed, it should include summer bypass capability to allow fresh air circulation without heat recovery when outdoor temperatures are cool. Automated dampers or user controls are essential.

Reduced Internal Heat

Specification of efficient lighting (LED), reduced plug loads, and careful appliance selection minimises unwanted internal heat generation.

Common Pitfalls and Risk Areas

Overheating risk is most acute in:

• Top-floor flats and loft conversions with limited thermal mass
• Properties with large south- or west-facing glazing
• Homes with poor existing ventilation that cannot be improved
• Urban properties with high external air temperatures due to heat island effects

Retrofit coordinators should flag these early in the evaluation phase so design adaptations can be made before work commences.

Documentation and Sign-Off

PAS2035 requires that overheating risk assessment and mitigation measures are clearly documented and approved before retrofit work begins. This should be recorded in the Energy Performance Certificate report and cross-referenced in technical design specifications.

Where overheating risk cannot be adequately mitigated within the agreed retrofit scope, this limitation must be communicated transparently to the building owner or housing association. User guidance on summer operation (window opening, shading use, ventilation control) should be provided post-retrofit.

Conclusion

Overheating assessment is not a compliance checkbox—it is a professional responsibility to ensure retrofit improves overall comfort and resilience. By integrating overheating evaluation into pre-retrofit assessment and specifying appropriate design and control measures, retrofit coordinators protect occupant wellbeing and deliver homes fit for current and future UK climates.