Overheating Risk Assessment

CIBSE TM59 is the technical memorandum published by the Chartered Institution of Building Services Engineers that sets out the methodology for assessing overheating risk in residential buildings. It defines pass/fail criteria based on the percentage of occupied hours during which internal temperatures exceed defined thresholds. For naturally ventilated bedrooms, the key criterion is that night-time temperatures must not exceed 26 degrees Celsius for more than 1 per cent of annual occupied hours.

TM59 applies to residential buildings (homes) while TM52 applies to non-domestic buildings such as offices, schools, and retail premises. They use different comfort criteria and assessment methodologies. TM59 uses fixed temperature thresholds appropriate for sleeping environments, while TM52 uses the adaptive comfort approach where acceptable temperatures vary with external conditions. The appropriate methodology depends on the building use.

Approved Document O is the section of the Building Regulations (introduced June 2022) that addresses overheating mitigation in new residential buildings in England. It provides a simplified method for demonstrating compliance through prescriptive limits on glazing areas and minimum ventilation provisions. Alternatively, compliance can be demonstrated through dynamic thermal modelling using the CIBSE TM59 methodology. Most planning authorities prefer the dynamic modelling approach as it provides a more accurate assessment of overheating risk.

The assessment should model current weather conditions and future climate scenarios. CIBSE publishes Design Summer Year (DSY) weather files that represent warm summers under different climate change projections. The London Plan requires modelling using the DSY1 weather file (a moderately warm summer representing the 2020s) as a minimum. Many authorities also request analysis under the more extreme DSY2 and DSY3 scenarios, which represent short intense heatwaves and prolonged warm spells respectively.

The cooling hierarchy is a sequential approach to managing overheating risk that prioritises passive measures before resorting to mechanical cooling. The hierarchy is: first, minimise internal heat gains from lighting, equipment, and hot water pipework; second, reduce solar gains through orientation, shading, and appropriate glazing specification; third, manage heat through exposed thermal mass and high ceilings; fourth, provide passive ventilation through openable windows and cross-ventilation; fifth, provide mechanical ventilation; and only as a last resort, provide active cooling systems such as comfort cooling or air conditioning.

If the dynamic thermal modelling shows that the development does not meet the CIBSE TM59 criteria, the design must be modified to address the overheating risk. Common mitigation measures include increasing external shading through brise-soleil, canopies, or deep reveals, reducing glazing areas, improving cross-ventilation, specifying solar control glass, and adding thermal mass. If passive measures alone cannot resolve the issue, mechanical ventilation with heat recovery or comfort cooling may be necessary.

Noise is a critical consideration because opening windows is the primary means of passive cooling in naturally ventilated dwellings. Where external noise levels exceed guideline values — particularly near busy roads, railways, or flight paths — windows may need to remain closed for acoustic comfort, significantly limiting ventilation. The overheating assessment must model these units with windows closed and demonstrate that acceptable temperatures can be maintained through mechanical ventilation or other means.

Yes. Single-aspect dwellings — those with windows on only one face of the building — are significantly more vulnerable to overheating because they cannot benefit from cross-ventilation, which is the most effective passive cooling strategy. South and west-facing single-aspect units are at greatest risk due to high solar gains. The overheating assessment should pay particular attention to these units, and design revisions to create dual-aspect layouts should be considered where feasible.

An overheating assessment is not typically required for householder extensions unless the extension involves a significant amount of glazing, such as a conservatory or orangery. However, Approved Document O applies to extensions that create new habitable rooms, and compliance must be demonstrated either through the simplified method or dynamic thermal modelling. For most standard extensions, the simplified method is sufficient.

A Design Summer Year (DSY) weather file is a dataset of hourly weather conditions representing a warm summer, published by CIBSE for use in dynamic thermal modelling. There are three DSY variants: DSY1 represents a moderately warm summer with a gradual build-up of heat, DSY2 represents a short, intense heatwave, and DSY3 represents a prolonged warm spell. The weather files are available for different UK locations and are updated periodically to reflect climate change projections.