Overheating Risk Assessment in PAS2035 Design

Overheating has become a critical consideration in retrofit projects under PAS2035. As buildings become more thermally efficient through improved insulation and air tightness, the risk of excessive indoor temperatures—particularly during summer months—increases significantly. This guide outlines practical approaches to assessing and managing overheating risk during the design phase.

Understanding Overheating in Retrofit Context

Overheating occurs when indoor temperatures persistently exceed comfort thresholds, typically above 26°C for living spaces. In retrofit projects, this risk intensifies because:

• Enhanced insulation reduces heat loss, trapping solar gains and internal heat
• Improved air tightness limits natural ventilation opportunities
• Existing buildings may lack adequate shading or thermal mass
• Traditional cooling systems are often absent or insufficient

PAS2035 requires retrofit coordinators to balance energy efficiency improvements with thermal comfort, making overheating assessment essential during design development.

Early Assessment Requirements

Begin overheating risk evaluation during the retrofit design stage, not after construction. This involves:

1. Building orientation and exposure analysis — Document solar orientation, surrounding shading from trees or buildings, and local climate data
2. Existing thermal characteristics — Review building fabric, window types, ventilation systems, and thermal mass capacity
3. Occupancy patterns — Identify vulnerable residents (elderly, young children, those with health conditions) and usage patterns
4. Current cooling provision — Assess existing air conditioning, fans, or passive cooling capabilities

Assessment Methodology

Several approaches support overheating evaluation in PAS2035 projects:

Overheating Risk Screening

Use simplified criteria for initial risk identification:

• Flats in upper storeys with significant glazing on east or west elevations
• Properties with limited natural ventilation options
• Buildings with minimal thermal mass (timber frame construction, for example)
• Locations in urban heat islands or southern regions

Detailed Thermal Modelling

For higher-risk properties, dynamic thermal modelling provides comprehensive analysis. This involves:

• Using recognised software (such as PHPP or dynamic simulation tools) to model hourly temperature variations across a typical summer year
• Testing multiple scenarios, including window opening assumptions and ventilation strategies
• Evaluating performance against accepted comfort criteria, typically the 1% exceedance criteria from CIBSE guidance
• Assessing interventions' effectiveness before finalising the design specification

Dynamic modelling is particularly valuable when retrofit works significantly alter building performance characteristics.

Natural Ventilation Strategy

Effective natural ventilation forms the foundation of overheating mitigation in most retrofit designs:

• Window design and operation — Ensure openable windows on opposite elevations enable cross-ventilation; include clear guidance for occupants on when and how to use them
• Stack effect optimisation — Tall spaces and roof vents encourage warm air stratification and stack-driven ventilation
• Night cooling — Promote cooler night-time air entry to cool thermal mass, with security considerations addressed through window restrictors
• Shading integration — Combine passive ventilation with external shading to reduce solar gains

Passive Cooling Measures

Integrate multiple passive strategies within the retrofit design:

• External shading — Specifying brise-soleil, external blinds, or deciduous planting reduces solar heat gain, particularly on south and west elevations
• Thermal mass — Exposed internal masonry or specialist thermal mass materials absorb heat during the day and release it at night
• Green infrastructure — Green roofs and living walls provide evaporative cooling and reduce solar absorption
• Reflective surfaces — Light-coloured roof finishes and wall colours reduce absorbed solar radiation

Active Cooling Considerations

In instances where passive measures prove insufficient, active cooling may be required:

• Heat pump systems offer dual functionality—heating in winter, cooling in summer
• Mechanical ventilation with heat recovery (MVHR) can include cooling loops in appropriate circumstances
• Localised cooling (fans, ground source systems) may support vulnerable occupants
• Ensure active cooling solutions align with retrofit energy targets and avoid significant energy penalty

Design Documentation and Handover

Robust documentation ensures overheating mitigation translates from design to occupied building:

• Specify clear operational guidance for natural ventilation, including window opening recommendations for different seasons
• Include maintenance schedules for shading, vegetation, and any mechanical systems
• Provide occupant guidance on thermal comfort management and emergency contact procedures
• Document design assumptions and performance predictions for future reference

Monitoring and Adaptation

Post-retrofit monitoring supports evidence-based refinement:

• Consider temperature monitoring during the first summer to validate design assumptions
• Use occupant feedback to adjust operational strategies
• Document lessons learned for future projects in similar properties

Effective overheating risk assessment in PAS2035 retrofits requires integrated design thinking, balancing energy efficiency with thermal comfort. By addressing overheating early, coordinators ensure retrofits deliver genuine improvements to building performance and occupant wellbeing.