Introduction: The Overheating Challenge in Retrofit

As retrofit activity intensifies across the UK housing stock, overheating risk has emerged as a critical consideration in retrofit design and delivery. PAS2035:2019 (Retrofitting dwellings for improved energy efficiency – Specification and guidance) and its companion standard PAS2030:2023 (Retrofitting dwellings for improved energy efficiency – Process specification) establish clear requirements for assessing and managing overheating risk. Yet many retrofit practitioners remain uncertain about what these standards actually demand and how to implement them effectively.

Modern retrofit work—particularly the addition of insulation, air-tightness improvements, and the installation of mechanical ventilation with heat recovery (MVHR)—can inadvertently increase overheating risk if not carefully designed. PAS2035 requires a systematic, evidence-based approach to prevent thermal discomfort and protect vulnerable occupants.

What PAS2035 Requires for Overheating Assessment

Mandatory Risk Evaluation

PAS2035 mandates that retrofit designers and coordinators undertake overheating risk assessment as part of the standard design process. This is not optional. The standard requires that:

• Overheating risk must be evaluated using recognised assessment methods
• Assessment should consider the building's thermal characteristics, orientation, glazing, ventilation strategy, and occupant vulnerability
• Risk evaluation must be documented and form part of the retrofit specification
• Mitigation measures must be specified where risk is identified

Thermal Modelling Requirements

For many retrofit projects, particularly those involving significant fabric improvements, PAS2035 expects thermal modelling to include overheating risk assessment. Dynamic thermal simulation (such as PHPP—Passive House Planning Package—or specialist overheating assessment tools) may be necessary, especially in:

• Flats and apartments with restricted ventilation options
• Properties with large south-facing glazed areas
• Developments in urban heat island zones or south-facing aspects
• Retrofit projects serving vulnerable populations (elderly residents, families with young children)

Even where dynamic modelling is not performed, designers must apply static assessment methods and document their reasoning transparently.

Practical Overheating Mitigation Strategies

Passive Design First

PAS2035 guidance emphasises passive solutions before mechanical intervention. Key passive strategies include:

• External shading: Fixed or operable solar shading on south, east, and west-facing windows significantly reduces solar gain
• Glazing specifications: Lower solar heat gain coefficients (SHGC) help minimise unwanted solar gain whilst maintaining daylighting
• Thermal mass: Exposed internal masonry or phase-change materials can buffer temperature swings, particularly in lightweight constructions
• Ventilation strategy: Night-time purge ventilation removes accumulated heat; window opening provision (manual or automated) is essential

Ventilation System Design

Where MVHR systems are specified, PAS2035 requires careful design to prevent overheating. Considerations include:

• Summer bypass functionality to allow outside air directly into the dwelling without heat recovery when external temperatures are favourable
• Adequate outdoor air intake positioning to minimise warm, polluted air ingestion
• Control strategies allowing occupants to adjust ventilation rates seasonally
• Integration with natural ventilation opportunities (openable windows) rather than full reliance on mechanical systems

Occupant Behaviour and Control

PAS2035 recognises that occupants' ability to manage their own thermal comfort is crucial. Retrofit designs must enable:

• Operable windows positioned and sized to permit effective natural ventilation
• Clear, understandable controls for MVHR systems and any active cooling
• Information provided to residents about managing seasonal thermal comfort
• Consideration of occupant vulnerability and ability to respond to overheating

Common Retrofit Scenarios and Overheating Risk

Certain retrofit interventions create elevated overheating risk:

Improved airtightness reduces uncontrolled air leakage, trapping heat in summer. This demands proportionate ventilation improvements and passive cooling opportunities.

Loft and cavity wall insulation reduces heat loss but can increase summertime temperatures in top-floor flats and south-facing spaces. Thermal mass, external shading, and ventilation become more critical.

Window replacement with high-performance, low-U-value glazing may inadvertently increase solar gains if SHGC is not adequately specified for the building's climate exposure.

Documentation and Compliance

PAS2035 requires that overheating assessment and mitigation measures are clearly documented within retrofit specifications, design reports, and handover information. Retrofit coordinators must ensure:

• Assessment methodology is explicitly stated
• Design decisions addressing overheating risk are justified
• Installers understand how to implement and test mitigation measures
• Occupants receive clear guidance on using systems and controls to manage thermal comfort

Overheating risk assessment, whilst adding complexity to retrofit design, ultimately delivers better occupant outcomes and complies with PAS2035's core aim: delivering genuinely improved, healthy, comfortable homes.