What is TM59 and why is it important for my self-build home?

Since the introduction of Approved Document O in England and Wales and Scottish Building Standard 3.28, overheating mitigation is a Building Regulations requirement for all new residential buildings. TM59 is a Dynamic Thermal Modelling (DTM) method that assesses the risk of overheating in your home. It takes a holistic approach, considering all aspects of the building, including fabric, efficiency, shading, and glazing performance. TM59 aims to improve energy efficiency and reduce energy consumption and costs.

How does it work?
TM59 calculations consider solar gain, internal heat generation, and thermal mass. Software models simulate heat transfer within the building by taking account of insulation, ventilation, and shading. The results highlight potential overheating risks and can be used to make design adjustments to provide comfortable indoor temperatures without excessive energy use.

How is the risk of overheating calculated?
Two methods are currently in use: the simplified method and TM59.

• The simplified method is a basic prescriptive process considering floor, glazing, and openable areas. It is often used in the early stages of designing a home for quick assessments, but may only capture some aspects affecting living comfort.
• Dwellings with large amounts of south or west-facing glazing typically require the more detailed and comprehensive TM59 assessment. This method uses dynamic thermal modelling software to consider building orientation, materials, internal gains, occupancy patterns, and weather data.

Which method is best for my project?
Four criteria to consider are regulatory requirements, personal priorities, budget and design:

1. Check if your local authority mandates specific requirements for overheating assessments.
2. Reflect on your thermal comfort, energy efficiency, and sustainability goals and whether these align with TM59.
3. Budget is also a factor; simplified assessments start around £175 + VAT, while TM59 assessments range from £650 + VAT. However, investing in a TM59 analysis early in the design process can prevent delays and additional costs later.
4. Consulting with your architect or timber frame manufacturer can provide further insights.

Is a TM59 assessment mandatory?
It depends on your local authority. The Greater London Authority’s London Plan requires a robust approach to overheating analysis, for which TM59 is a suitable method of assessment. Other urban areas with progressive environmental policies may have similar requirements. Check your local authority’s planning guidelines to determine if TM59 or a similar detailed assessment is needed.

Practical design strategies based on TM59 results
To mitigate overheating, consider orienting your home to minimise solar gain, using shading devices, optimising cross-flow ventilation¹ and night purging², ensuring high insulation and air tightness, selecting efficient glazing, and incorporating green infrastructure. Material choices also play a crucial role.

Advice for large glazed areas
For large glazed areas, design with cross ventilation in mind. Optimise for cross-flow ventilation and consider high-performance glazing with low solar heat gain³. Use shading methods such as balcony or roof overhangs and brise soleils. Choose opening windows over fixed ones to enhance ventilation.

Does ‘Comfort Cooling’ mitigate against TM59 assessment?
Comfort cooling, through systems like air conditioning, directly addresses overheating by lowering indoor temperatures and maintaining comfortable conditions. The short answer is yes: by integrating cooling systems, you can effectively meet the requirements of TM59. However, this should be a last resort. While comfort cooling does remedy overheating, it also increases energy consumption and is a contributor to global warming. Which is why alternative mitigation measures should always be investigated first!

Is TM59 a service Fleming Homes offers?
Yes. Our planning and building regulations team will provide advice and guidance based on your design and assist you through the assessment process. For more information, call 01361 883785.

TM59 CASE STUDY

Once our clients were happy with the design and layout of their Fleming Homes’ house, our planning and building regs team carried out a TM59 assessment. As you can see from the results table below, although the bedroom areas met the overheating mitigation requirements the dining and living areas failed to do so. This was largely due to fact that these rooms had large, south/south west facing areas of glazing.

Following discussions with the clients, the following design changes were proposed:

• Fixed shading devices, comprising of the following
  • Overhangs above these openings:
  • Door – 3.0m x 0.5m external shading
  • Dining Room Window – 3.0m x 1.0m external shading
  • Living Area Window – 1.5m x 0.5m external shading

• Glazing design
  • South facing Door – G value of 0.37
  • Dining Room Window – G value of 0.37
  • Living Area Window – G value of 0.37

When a further TM59 assessment was carried out using the design recommendations, the criteria was met in all areas, as can be seen in the upgraded results below.

 

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¹ Cross-flow ventilation is where air can enter a property via a window, pass through the dwelling and exit it on the opposite elevation.

² Night purging uses the natural temperature difference between daytime and nighttime to reduce indoor temperatures and remove accumulated heat.

³ G-values measure of the transmittance of solar gain through glazing.