Retrofit projects aim to improve energy efficiency and reduce heating demand, yet paradoxically, poorly designed interventions can increase overheating risk—particularly during summer months and in vulnerable properties. This guide outlines assessment methods and mitigation strategies for retrofit coordinators and installers.
Understanding Overheating in Retrofit
Overheating occurs when internal temperatures rise to uncomfortable or unsafe levels, typically above 26°C sustained over several days. In retrofit projects, overheating risk increases when:
- Improved insulation traps solar gain without adequate ventilation
- Air-tightness measures reduce natural cooling through infiltration
- Thermal mass is insufficient to moderate temperature swings
- Shading and ventilation strategies are overlooked during design
Vulnerable occupants—elderly residents, those with health conditions, and very young children—face greater risk from sustained heat exposure.
Assessment Methods
Dynamic Thermal Modelling
The most reliable approach involves dynamic thermal simulation using recognised software tools. This models hourly temperature variations across a full year, accounting for:
- Building geometry and construction materials
- Internal heat gains from occupancy and appliances
- Solar radiation on all facades
- Ventilation rates under different conditions
- Local weather data for the building's location
Modelling should test multiple scenarios, including high-occupancy periods and extreme summer weather conditions. UK guidance suggests assessing performance against criteria such as CIBSE TM59 for residential buildings.
Simplified Risk Screening
For smaller projects or initial scoping, use simplified risk assessment:
- Identify building orientation and extent of glazing on south and west facades
- Map existing shading from trees, buildings or features
- Review current ventilation provision and window opening potential
- Assess thermal mass (concrete, masonry) within the building
- Consider occupant profile and vulnerability factors
High-risk scenarios include south-facing apartments with extensive glazing, limited natural ventilation, and minimal thermal mass.
Key point: Don't assume retrofit automatically causes overheating. Improved insulation combined with good ventilation and shading often reduces overheating risk compared to poorly performing original buildings.
Mitigation Strategies
Solar Shading
External shading is most effective, reducing solar gain before it enters the building:
- Fixed shading: Overhangs, louvers and brise-soleil suit predictable sun angles but may reduce winter solar gains
- Moveable shading: External blinds and shutters offer flexibility but require maintenance and occupant engagement
- Vegetation: Trees and climbing plants provide summer shading and winter light, though maintenance is necessary
For retrofit projects, external retrofits to windows (rather than internal blinds) provide superior performance.
Natural Ventilation and Air Movement
Effective summer cooling relies on natural ventilation to remove internal heat gains:
- Ensure windows open freely and occupy positions enabling cross-ventilation
- Install or upgrade trickle vents to maintain background ventilation when windows are closed
- Consider window opening restrictions on upper floors for safety, but design these to allow adequate ventilation
- Where mechanical ventilation is installed, programme night-time operation to cool the building mass
Thermal Mass
Internal thermal mass (exposed concrete, brick or tile) absorbs daytime heat and releases it at night when temperatures cool:
- Avoid over-boarding or carpeting exposed concrete floors
- Specify internal finishes that enhance rather than conceal thermal properties
- In retrofit, consider adding phase-change materials or thermal storage products where mass is limited
Window Specification
High-performance windows reduce winter heat loss but can increase solar gain:
- Choose glass with moderate solar heat gain coefficients (SHGC) rather than maximised values
- Combine low U-values with selective solar control rather than heavy tinting
- Prioritise external shading over reflective glass for aesthetic and performance reasons
Design Integration
Overheating risk assessment should inform retrofit design from the outset:
- Establish baseline overheating risk using modelling or risk screening
- Identify design options and test their combined effect through iterative modelling
- Document rationale for each design decision (shading type, ventilation strategy, window specification)
- Produce clear guidance for residents on window opening, blind use and summer management
- Plan post-occupancy review to validate modelling and adjust occupant behaviour if needed
Practical Considerations for Installation
- Commissioning: Verify ventilation components function as designed; test trickle vents and openable windows
- Occupant guidance: Provide clear, accessible information on summer ventilation and shading use
- Maintenance planning: Establish schedules for cleaning solar shading, clearing vents, and checking seals
- Monitoring: Consider installing temperature sensors in high-risk areas to capture real performance data
Effective retrofit balances energy efficiency with occupant comfort. Overheating risk is manageable through sound design practice, appropriate specification, and clear occupant engagement. Early assessment and integration of mitigation strategies into retrofit planning deliver buildings that perform well across all seasons.