For many years, building performance has been evaluated against weather data from 2016 taken from only 16 locations. However, amid rising average temperatures, CIBSE has updated its design methodology to mandate testing against predicted 2050 weather data from 28 areas. Evaluating building performance on outdated 2016 weather data means developers and architects are working with weather files that don’t accurately reflect today’s ever-changing climate. CIBSE’s updates to the weather data, as part of TM59:2026, are therefore long overdue.
Released in July 2026, the TM59:2026 updates provided an industry-standard methodology for evaluating overheating risk in newbuilds and refurbishments. The new UK standard references a weather file framework based on the latest climate projections for 2050. While modernising the assessment process, this framework shifts attention to how dwellings will perform in future climate conditions.
Currently, TM59:2017 is still required for compliance with Part O Building Regulations. However, CIBSE is in discussion with the Building Safety Regulator to identify how TM59:2026 can be incorporated into Approved Documents. To further aid energy performance and occupant comfort, the TM59 update defined a three-stage overheating assessment strategy. Consequently, modellers must illustrate how overheating risk is tackled at each stage, starting with passive design, before moving onto natural ventilation and only transitioning to mechanical intervention if required.
Mean night-time temperatures in bedrooms are also a major focus for TM59. For Category II dwellings, such as standard homes, general housing and apartments, the average temperature must stay below 27°C, with no more than four occasions of exceedance allowed between May and September. Reporting requirements are also more structured than ever before, with a new TM59 compliance checklist ensuring greater consistency and transparency across the board.
HEAT AND LIGHT MANAGEMENT
Rising summer temperatures, along with the UK’s increasing urbanisation and highly insulated newbuilds, are exacerbating the overheating challenges. UK buildings are designed and built to include thick insulation and double glazing while focusing on airtight construction principles. While useful in the colder months, this traps heat indoors and provides limited means for natural heat rejection in the summer.
In major cities especially, urban heat islands are an increasing problem. Created by the mass absorption and slow release of solar energy, this phenomenon often forces buildings to run air conditioning systems longer and harder to ensure cooler internal environments for occupants – driving energy consumption and expenditure.
INCORPORATING VENTILATION AND COOLING INTO BUILDING DESIGN
With the UK predicted to face colder winters and hotter summers, a balance of winter energy performance and summer heat dissipation is needed to align with the TM59:2026 assessment hierarchy.
Stage 1: Solar control
By specifying low G-value glazing and integrating external shading, such as brise soleils, overhangs, vertical fins or louvres, daylight autonomy can be optimised and excessive summer solar gains controlled. Any designed shading should account for the façade direction and angle of the sun.
Stage 2: Passive heat rejection
High-density interior materials should also be incorporated to absorb internal heat gains during peak daylight hours. Meanwhile, night purge ventilation can help mitigate overheating by removing accumulated heat overnight and cooling the exposed thermal mass.
Stage 3: Active cooling
Increasing airflow is rarely sufficient during heatwaves when outdoor temperatures are also elevated. Future UKCP18 projections suggest many UK buildings will require some form of active cooling during extreme summer conditions. Mechanical ventilation with heat recovery (MVHR) systems provide fresh air without losing heat during colder periods, using a summer bypass and night purge functionality to remove excess heat in warmer periods.
The changes to weather data and the overheating methodology will enable future-proofing to be incorporated into building design at the earliest stage. Consequently, housebuilders, architects and specifiers will be empowered to overcome the UK’s increasing overheating risk and ensure compliant design.
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