What Is TM59?
TM59 is a performance-based Overheating Assessment that has been created to evaluate overheating in new homes. It employs dynamic thermal modelling to model indoor temperatures under various conditions. Instead of making assumptions, it applies actual climate conditions and occupancy habits to check how houses perform during summer periods.
The test is especially necessary for new-build housing, urban developments, and high-density housing, where there is greater risk of overheating due to aspects such as:
- Large glazing areas on south or west orientations
- Inadequate ventilation strategies
- High levels of airtightness
- Urban heat island effect
By implementing TM59, designers, developers and building control officials can ensure that the houses are not only energy-skilled, but also comfortable and safe for the living..
TM59 Criteria Explained
The method provides two main criteria for compliance:
- Living Rooms, Kitchens, and Bedrooms (Daytime Use):
For these areas, the operative temperature should not exceed 26°C for more than 3% of occupied hours during the typical summer period. - Bedroom (Use of Night):
To ensure comfortable sleep, the operative temperature in the bedroom should not exceed 26 ° C for more than 1% hour between 10 am and 7 pm.
Why TM59 Matters
Overheating isn’t only about comfort—there are quite serious health implications. Long-term exposure to high internal temperatures has been associated with dehydration, disrupted sleep, and heat-related sickness. Climate change resulting in rising UK summers, the need for TM59 checks is increasing year on year.
From a developer’s viewpoint, a TM59 overheating report assists:
- Demonstrates compliance with planning policy
- Mitigates risk of resident complaints
- Prevent expensive retrofits post-construction
- Make homes more marketable by having them future-proofed against warmer temperatures
How a TM59 Assessment Works
An accredited assessor will:
- Create a dynamic thermal model of the building using specialist software.
- Enter design information like floor plans, glazing areas, insulation, and ventilation strategies.
- Use weather files based on local climate conditions (current and predicted)
- Try several occupancy situations to mimic real use.
- Compare results with TM59 compliance standards.
If the design is not successful, it can be recommended that the building be improved with solar shading, better cross-ventilation, smaller glazing areas, or low-g solar control glass.
TM59 overheating check has become a standard component of UK building design today. It assures that energy-efficient houses aren’t gained at the expense of comfort and safety for occupants. For landlords, architects, and developers, performing this check at the design stage saves them anxiety, compliance with regulations, and healthier living conditions.
