Overheating Risk in Retrofit: Assessment and Mitigation

Improving building fabric performance is essential for decarbonisation, but retrofit interventions can inadvertently increase overheating risk if not carefully designed. This guide explains how to assess and manage this risk throughout the retrofit process.

Understanding Overheating in Retrofitted Properties

Overheating occurs when internal temperatures rise to uncomfortable or harmful levels during warm weather. In retrofit projects, the risk increases because:

• Enhanced insulation and airtightness reduce natural heat dissipation
• Reduced air infiltration limits passive cooling through ventilation
• Solar gains are trapped more effectively within the building fabric
• Window replacement may reduce opening areas for cross-ventilation

Vulnerable occupants—elderly residents, young children, and those with health conditions—face particular risk from sustained high indoor temperatures.

Assessment of Overheating Risk

Risk assessment should occur early in the design phase and be revisited as specifications develop. Use the following approach:

Step 1: Establish Baseline Conditions

Document existing thermal performance and occupant comfort:

• Current fabric U-values and thermal mass
• Window specifications and orientation
• Natural ventilation pathways and opening areas
• Historical temperature data if available
• Occupant feedback on summer comfort

Step 2: Dynamic Thermal Modelling

For properties at elevated risk, conduct dynamic thermal simulation using industry-standard software. This models hourly temperatures across a full year, accounting for:

• Climate data appropriate to the building location
• Internal heat gains from occupants and appliances
• Solar radiation on different facades
• Natural and mechanical ventilation strategies
• Thermal mass and time-lag effects

Compare results against assessment criteria such as CIBSE TM59, which defines overheating as more than 1% of occupied hours above 28°C (living areas) or 26°C (bedrooms).

Step 3: Identify High-Risk Properties

Properties at greatest risk typically include:

• Top-floor flats or properties with large roof areas
• Buildings with high solar gain (south or west-facing, limited shading)
• Poorly ventilated spaces with limited opening windows
• Properties occupied by vulnerable groups

Design Strategies to Mitigate Overheating

Effective mitigation combines multiple approaches rather than relying on single measures.

Solar Control

Manage solar gains through external measures wherever possible:

• External shading: Fixed or adjustable blinds, shutters, or louvres on south and west-facing windows are highly effective
• Vegetation: Deciduous trees and climbing plants reduce summer solar gain whilst allowing winter sun penetration
• Window glazing: Consider lower solar gain coefficients (g-value), though balance this against winter performance and daylight access
• Window orientation: Maintain or increase openable window area on north facades; reduce on south-west where feasible

Natural Ventilation

Design ventilation strategies that allow summer cooling:

• Maximise openable window area, particularly on opposite facades for cross-ventilation
• Ensure night-time cooling pathways; consider automatic window openers triggered by temperature
• Design ventilation routes to avoid noise and security issues
• Size and locate external vents to prevent weather-driven rain ingress

Thermal Mass

Exposed internal thermal mass (concrete, brick, stone) absorbs daytime heat and releases it at night, moderating daily temperature swings. Where feasible:

• Avoid carpeting concrete floors that could insulate thermal mass
• Position exposed mass on north-facing walls or internal partitions
• Ensure mass is accessible to air movement for effective heat transfer

Mechanical Ventilation

For properties where passive measures alone are insufficient, consider:

• Heat recovery ventilation (MVHR) with summer bypass modes to avoid heat recovery during cooling periods
• Whole-house mechanical cooling (expensive and energy-intensive; use only where passive measures fail)
• Spot cooling for high-risk individual rooms

Practical Implementation Guidance

Specification and Coordination

Ensure retrofit specifications explicitly address overheating:

1. State assessment criteria and modelling approach in design stage reports
2. Specify thermal properties of all envelope materials with justification
3. Detail ventilation and shading designs clearly in construction drawings
4. Identify commissioning requirements for ventilation controls and openings
5. Plan post-completion performance verification

Occupant Guidance

Overheating risk can only be managed in use with occupant understanding:

• Provide clear guidance on window operation, including when to open at night
• Explain the purpose of any new ventilation or shading systems
• Support vulnerable occupants with tailored advice and assistance
• Include overheating advice in maintenance and handover documentation

Summary

Overheating risk in retrofit demands systematic assessment and multi-layered mitigation. Early engagement with dynamic modelling, emphasis on external shading and natural ventilation, and clear specification will ensure retrofit improvements deliver lasting comfort and safety for all occupants, including the most vulnerable.