When Simplified Method Doesn't Work
TM59 Dynamic Thermal Modelling
Hour-by-hour thermal simulation for Part O compliance. More accurate than simplified, more design flexibility.
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TM59 Dynamic Simulation
What is TM59?
CIBSE methodology for dynamic thermal modelling. Simulates building performance hour-by-hour.
When do I need it?
When simplified fails, for single-aspect flats, noise-constrained sites, or complex designs.
Why is it better?
Accounts for orientation, thermal mass, shading, actual weather patterns. More accurate.
What are the criteria?
Bedrooms: max 3% hours over 26°C. Living areas: assessed differently.
How much does it cost?
From £250 per dwelling. Simplified fee credited if tried that first.
How long does it take?
2-4 working days for most projects.
Dynamic modelling often passes designs that fail the simplified method. More work, but more design freedom.
What Is TM59 Dynamic Simulation?
TM59 is the CIBSE methodology for assessing overheating risk in dwellings using dynamic thermal modelling.
Unlike the simplified method (which uses lookup tables), TM59 creates a thermal model of your dwelling and simulates its performance hour-by-hour through a Design Summer Year - a weather file representing typical hot summer conditions.
This detailed approach accounts for factors the simplified method ignores: thermal mass, actual solar gains, shading from adjacent buildings, and the way heat moves through the building over time. The result is a more accurate assessment that often passes designs the simplified method rejects.
When Do You Need TM59 Dynamic Modelling?
Dynamic modelling is required or recommended when:
- The simplified method fails due to glazing or ventilation limits
- Single-aspect flats (one external wall) - simplified doesn't apply
- Sites with noise constraints preventing adequate natural ventilation
- High-rise residential where wind-driven ventilation is unreliable
- Complex designs where simplified tables are too conservative
- Client/architect wants to maximise glazing while proving compliance
- Planning conditions require TM59 assessment specifically
Why Dynamic Modelling Passes More Designs
TM59 accounts for factors the simplified method ignores:
Thermal Mass
- ✓ Heavy construction absorbs daytime heat
- ✓ Releases heat at night when windows open
- ✓ Smooths temperature swings
- ✓ Simplified method ignores this benefit
Shading Effects
- ✓ Adjacent buildings reduce solar gains
- ✓ Balconies and overhangs provide shade
- ✓ Trees and landscaping help
- ✓ Actual shading modelled, not assumed
Real Orientation
- ✓ Simplified uses ±30° bands
- ✓ Dynamic uses exact orientation
- ✓ South-east performs differently to south-west
- ✓ More accurate solar gain calculations
Ventilation Strategy
- ✓ Models actual window opening patterns
- ✓ Accounts for wind-driven ventilation
- ✓ Night purge cooling modelled
- ✓ More realistic occupant behaviour
TM59 Pass Criteria
TM59 sets specific criteria for different room types:
Bedroom Criterion
Hours where operative temperature exceeds 26°C must be less than 3% of annual occupied sleeping hours (typically 2,928 hours, so max ~88 hours over 26°C).
Living Area Criterion
Uses the CIBSE Guide A criterion: operative temperature should not exceed the comfort threshold for more than 3% of occupied hours. The threshold varies with outdoor temperature.
The bedroom criterion is typically the most challenging. Upper-floor bedrooms with south or west glazing need careful design.
TM59 Modelling Pricing
Priced per dwelling. Simplified fee credited if you tried that route first.
TM59 Single Dwelling
From £250
Full dynamic simulation for one dwelling. Includes mitigation recommendations if needed.
TM59 Per Flat (Blocks)
From £150
Reduced rate when modelling multiple flats in same block. Worst-case units assessed.
With SAP
From £350
SAP + TM59 together for efficient Part L + Part O compliance.
How TM59 Modelling Works
We build a thermal model and simulate performance through design summer conditions.
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1
Send us detailed drawings
Floor plans, elevations, construction details, glazing schedule, and ventilation strategy. More detail = more accurate model.
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2
We build the thermal model
We create a 3D thermal model of the dwelling in approved dynamic simulation software.
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3
Run annual simulation
The model is simulated hour-by-hour through a Design Summer Year to predict internal temperatures.
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4
Receive results and recommendations
Pass/fail for each room with mitigation recommendations if needed. All reasonable revisions included.
Common TM59 Mitigation Strategies
If initial modelling shows overheating risk, these changes often achieve compliance:
- External shading - Brise soleil, external shutters, or balcony overhangs. Most effective mitigation.
- Solar control glazing - g-value of 0.4 or lower significantly reduces solar gains.
- Increased thermal mass - Exposed concrete soffits, solid floors, masonry walls.
- Improved ventilation - Larger openable areas, add windows for cross-ventilation.
- Reduce glazing - Sometimes the most cost-effective solution.
- Night cooling strategy - Secure ventilation options for overnight heat purge.
We rank recommendations by cost-effectiveness so you can make informed decisions.
TM59 FAQ
What software do you use?
Is TM59 the same as Part O?
Do you model every room?
What if my flat block has many unit types?
Can TM59 model mechanical cooling?
Need TM59 dynamic modelling?
Professional TM59 assessments accepted by Building Control nationwide.