TM59 Overheating Assessment Designing Homes That Stay Cool

The UK has made huge progress in the construction of houses that are hot, greenery and more energy-efficient. The insulation is thick, windows are better seals, and buildings are designed to keep heat during winter months. But when it is great for cold weather, there is a negative quarrel in summer.

Hot heat, dense cities, and tightly sealed modern homes mean many people are now struggling with uncomfortable high indoor temperature. To deal with this, the Chartered Institution of Building Services Engineers (CIBSE) introduced the design method to assess overheating at risk in homes.

This guidance is now considered a benchmark to ensure that energy-skilled houses are also comfortable, safe and ready for the future.

What Is TM59?

CIBSE TM59 Assessment is a functioning to assess overheating risk in domestic buildings using dynamic thermal modeling (DTM). In simple terms, this means making a digital model of a house and testing how it behaves in various weather and use conditions.

Unlike basic checklists, TM59 provides evidence-based results. It accounts for:

• Local climate conditions (current and future scenarios)
• Building orientation and glazing size
• Occupant behaviour and internal heat gains
• Ventilation strategies and thermal mass

The outcome tells developers and designers whether a home is likely to overheat during summer—and what needs to be changed to fix it.

Why Overheating Is a Real Concern

Overheating is more than just an inconvenience. It’s a growing health and design challenge across the UK, driven by several factors:

• Climate Change

The heat is getting hot. The Met office predicts more frequent and prolonged heatwave, which directly affects indoor comfort.

• Urban Heat Islands

Cities such as Birmingham, Manchester and London experience artificially high temperatures in the form of solid, asphalt and dense buildings.

• Modern energy standard

Airtight, well -properly home reduces the cost of winter heating but limits natural cooling and ventilation in summer.

• Glazing design trends

Floor-to-decin window and waste-faceing glass walls add styling and natural lights, but also bring excessive solar benefits.

• High density housing

Apartments with limited airflow and shared walls often struggle to cool naturally, especially in mixed-use developments.

The effect is not only discomfort. Overheet homes may have health risk for sleep disruption, dehydration, decrease in productivity, and even weak groups such as elderly or young children.

TM59 Compliance Criteria

Cibse TM59 sets two major criteria that houses should complete:

• Living room, kitchen and bedroom (day time):

The indoor temperature for more than 3% of the annual business should not exceed 26 ° C.

1. Bedroom (night):

To allow healthy sleep, the indoor temperature should not exceed 26 ° C for more than 1% hour between 10pm and 7AM.

If either the situation is violated, the property is considered an overheating risk.

TM59 Evaluation How works

The process is detailed but straightforward when handled by a qualified assessor:

1. Data Collection
   Architectural drawings, floor layouts, insulation values, and material specifications are gathered.
2. Building Model Creation
   Using dynamic thermal modelling software (like IESVE or DesignBuilder), a 3D thermal model of the property is created.
3. Weather File Input
   Climate files from CIBSE (current, 2020s, and 2050s scenarios) are applied to test resilience under changing conditions.
4. Internal Gains and Occupancy
   Heat from occupants, appliances, and lighting is factored in, along with day/night occupancy schedules.
5. Simulation Run
   The model simulates building performance hour-by-hour through summer.
6. Results Analysis
   Outcomes are compared against TM59 criteria.
7. Recommendations
   If the design fails, practical solutions are suggested to bring it into compliance.

Design strategies to prevent overheating

When the assessment shows overheating risk, design changes can usually solve themCommon solutions include:

• Solar Control Measures
• • External shading (louvres, shutters, brise-soleil)
  • Solar control glazing or films
  • Overhangs or balconies to block high summer sun
• Ventilation Improvements
• • Cross ventilation
  • Safe night ventilation (trickle vent, lockable opening)
  • Mechanical ventilation with heat recovery (MVHR) system
• Thermal Mass Utilisation
• • Exposed brick, stone, or concrete absorbs heat by day and releases it at night
• Building Orientation Adjustments
• • Minimising west-facing glazing
  • Locating sensitive rooms (like bedrooms) away from high solar gain zones
• Green Solutions
• • Green roof or walls to absorb heat
  • Trees or landscaping planting to provide natural shade

 Assessment?

• Residential Developers : For new housing schemes, flats, and high-rise apartments.
• Architects: To validate the initial design options in the planning phase.
• Students increase housing housing and build-to-rant plans: Where density and occupancy patterns increase the risk.
• Local authorities and housing associations: To ensure compliance for construction rules and safety of residents.
• Renewal projects: Especially where insulation and glazing are upgraded.

Many local planning authorities now require TM59 compliance evidence as part of planning conditions.

TM59 vs Part O (Building Regulations)

Since June 2022, Part O of the Building Regulations introduced new requirements for overheating risk. While Part O provides prescriptive methods (like limiting glazing size or requiring openable windows), TM59 goes deeper by modelling performance dynamically.

In practice, TM59 offers a more accurate and future-ready assessment, while Part O sets the legal minimum. Many developers use both together.

Benefits of TM59 Assessments

• Regulatory compliance – Meets planning and Building Regulations expectations
• Future resilience – Designs tested against hotter climate projections
• Reduced risk of complaints – Happier tenants and homeowners
• Better marketability – Overheating-resistant homes are more attractive to buyers
• Avoidance of retrofits – Early adjustments save money compared to post-construction fixes

Final Thoughts

TM59 overheating assessment is no longer just a specialist exercise - it is rapidly becoming an essential step in modern building design. As the UK faces hot heat and strict regulations, the needs of homes that are both energy-efficient and comfortable are more than ever.

By including TM59 evaluation in the design phase, developers, architects and housing providers can:

• Provide compliance with local authorities
• Ensure healthy stay condition
• Avoid future legal and financial risks
• Distribute houses that are desirable for the coming decades

The message is clear: Energy efficiency alone is not enough. A home must also stay cool in summer. TM59 is the tool that helps achieve this balance.