As the UK moves toward its net zero carbon goals, designers and developers are under growing pressure to build homes that are both energy efficient and resilient to climate change. One critical aspect of this is preventing overheating—especially in airtight, highly insulated buildings.
This is where TM59 comes in.
TM59 is a guidance document published by CIBSE (Chartered Institution of Building Services Engineers). It provides a method to predict and assess overheating risk in residential buildings using dynamic thermal simulation.
It’s widely used to demonstrate compliance with:
• Part O of the Building Regulations
• Net zero carbon housing frameworks
• Planning conditions for overheating mitigation
Why TM59 Matters in Net Zero Homes
Net zero homes typically feature:
• Airtight construction
• High levels of insulation
• Triple glazing
• Low solar gain materials
While excellent for reducing energy demand, these features can trap heat—leading to overheating during summer.
That’s where TM59 becomes essential. It models indoor temperatures based on:
• Local weather data
• Building design (layout, glazing, materials)
• Occupant behaviour and internal heat gains
• Ventilation strategy (natural or mechanical)
The goal is to prove that indoor temperatures won’t exceed comfort thresholds, even during heatwaves.
The Role of MVHR in TM59 Compliance
In net zero homes, natural ventilation alone often isn’t enough—especially in:
• Single-aspect flats
• Noise or pollution-constrained sites
• Highly airtight properties
This is where MVHR (Mechanical Ventilation with Heat Recovery) systems become vital.
MVHR Features That Support TM59:
• Summer bypass mode – avoids recovering heat when it’s not needed
• Boosted purge ventilation – delivers 4–6 air changes per hour when overheating is detected
• Night-time cooling – continuous low-noise airflow to reduce temperature spikes
• Low-resistance ducting – larger diameter (e.g. 125mm or 204x60mm) to support higher flow rates
• Smart controls – integrate with sensors to automatically activate purge ventilation
A well-designed MVHR system can significantly reduce overheating risk while preserving air quality in airtight homes.
How Is TM59 Used?
1. Thermal Modelling
Dynamic simulations are carried out using real weather data and internal loads.
2. Room-by-Room Analysis
Bedrooms and living spaces are assessed for frequency and severity of overheating.
3. Scenario Testing
Different ventilation rates, glazing types, and shading solutions are tested to achieve compliance.
4. Report Generation
A formal TM59 report is issued as part of a Part O submission or planning application.
Our Support for Net Zero Projects
We work with architects, developers, and M&E consultants to ensure:
• TM59 overheating compliance
• MVHR system design for airflow and duct sizing
• Part O reporting
• Integration with net zero or Passivhaus frameworks
Need Help Designing Overheating-Resilient Net Zero Homes?
We offer:
• TM59 thermal modelling
• MVHR specification & layout
• Support for ceiling void coordination (low-profile ducting)
• Expert guidance on integrating Part O, MVHR, and net zero goals
Get in touch for a free consultation
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