TM59 Calculations for Overheating: What’s Changing?

& How Galaxy Technical Services Can Help

Following another long and hot summer in the UK, it's clear to see that temperatures continue to rise as the effects of global warming continue. Couple this with the fact that new homes grow increasingly airtight due to regulation, and the need for overheating calculations becomes apparent. CIBSE’s TM59 the Design Methodology for the Assessment of Overheating Risk in Homes remains critical for ensuring thermal comfort and compliance with Part O of the Building Regulations. CIBSE is now amending TM59 to reflect updated research, climate data, and practical feedback to better equip builders and design professionals for future challenges. Let’s unpack those changes and how Galaxy Technical Services can support you.

What Is TM59 (and Why It Matters)

TM59 provides a consistent, dynamic thermal simulation methodology tailored to assess overheating risk in homes, including flats, dwellings, care facilities, and student accommodation, using factors like internal gains, ventilation, orientation, shading, and weather data.

Buildings are evaluated on two main criteria:

1. Criterion A (naturally ventilated homes): Operative temperatures must not exceed 1 K above the adaptive comfort threshold for more than 3% of occupied hours (May–September).
2. Criterion B (bedrooms): Operative temperature must not exceed 26 °C from 10 pm to 7 am for more than 1% of annual hours.

For mechanically ventilated homes, a fixed temperature test applies (e.g., max 26 °C for no more than 3% of occupied hours).

TM59 also integrates future-focused weather datasets such as DSY1 (2020s) and further scenarios (2050s, 2080s) for resilience planning

To mitigate an overheating risk in residential properties, it is important for low G values to be specified at design stage as this would positively impact the TM59 Dynamic analysis.

TM59 – G Values Explained

A G Value, also known as Total Solar Energy Transmittance, is a coefficient used to measure the transmittance of solar gain through glazing or how much heat is transmitted through a window from the sun’s rays.

The G Value is a scale between 0-1 where:

• A high G Value of 1 represents the full transmittance of solar energy

• A low G Value of 0 represents all solar energy is blocked by the glass.

What’s Changing in the Latest TM59 Update

CIBSE is refining TM59 across several key areas to better align with current science, policy progression, and industry realities:

1. Revised Night-Time Criterion

Replacing the fixed 26 °C threshold, proposals are in testing that consider nights exceeding a mean temperature possibly up to 28 °C. This shift aims to better reflect actual discomfort and mitigation strategies

2. Mandatory Use of Future Weather Files

The 2050s DSY1 (Design Summer Year 1) will become the minimum required weather profile for compliance testing, ensuring assessments consider warming climate conditions

3. Expanded Internal Gain Profiles

New profiles will include emerging occupancy patterns particularly home-office usage, ceiling fan effects, and night-time window-opening behaviour’s (including open internal doors, except bedroom doors)

4. Enhanced Climate Data Accessibility

A forthcoming digital tool aims to make future weather datasets more accessible and user-friendly for long-term system planning

How Galaxy Technical Services Can Help

At Galaxy Technical Services, we’re ahead of the curve refining our TM59 assessment offerings to align with these updates and help you achieve robust, future-proof solutions:

1. Advanced TM59 Simulation & Analysis

Our dynamic thermal modelling services now include:

1. Use of 2050s DSY1 as standard (and longer-term DSYs on request)

2. Applications of refined night-time criteria and thresholds

3. Home-office gain profiles and ceiling fan modelling

4. Realistic window-opening and internal door strategies

2. Proactive Mitigation Strategy Development

We don’t just identify overheating risks, we design solutions:

1. Passive strategies: shading devices, optimized glazing, air-movement design

2. Active strategies: ceiling fans, MVHR systems, thermal mass utilisation

3. Case study inspired adjustments (e.g., side-hung windows, adjustable blinds, openable glazing)

3. Compliance Reports & Documentation

Our detailed TM59 reports include:

1. Clear pass/fail analyses against updated TM59 criteria

2. Climate-resilient weather scenarios and gain profiles

3. Visual heat maps and mitigation validation

4. Maintenance guidelines for shading and ventilation components crucial, as TM59 emphasizes that mitigation must be installed and maintained

4. Future-Proofing Advisory

With TM59 likely to influence the Future Homes Standard (due 2025), Galaxy can guide long-term planning:

1. Integration with upcoming Part O compliance requirements

2. Strategic use of future weather projections and risk resilience tools

3. Ensuring designs are both regulatory-compliant and climate-secure

Conclusion

The TM59 update is more than a regulatory tweak, it’s a shift toward a climate-aware, occupant-focused approach to thermal comfort. Galaxy Technical Services stands ready to equip designers, developers, and homeowners with robust, forward-looking TM59 assessments, mitigation strategies, and compliance documentation.

Ready to make sure your next project meets expectations using products that are cost effective and readily available and also tying the specification in with the SAP calculation which often conflicts with the overheating calculation, then reach out to Galaxy Technical Services.