The UK Building Regulations (Part O, 2021) and the CIBSE (Chartered Institution of Building Services Engineers) TM59 methodology provide two complementary frameworks for assessing overheating risk in residential dwellings. Originally developed in the UK context, these approaches are increasingly relevant for design engineers and specifiers working on residential projects with high solar exposure — including in continental Europe.
The TM59 Methodology (CIBSE)
TM59 is based on dynamic thermal simulation (DTS) of the building. It assesses overheating risk against two primary criteria:
-
Adaptive criterion (living spaces): the number of hours during which operative temperature exceeds the adaptive comfort threshold defined by EN 15251 (or EN 16798-1) must not exceed 3% of annual occupied hours.
-
Absolute criterion (bedrooms): operative temperature must not exceed 26°C for more than 1% of sleeping hours (between 22:00 and 07:00).
These thresholds are assessed using weather data representative of a Test Reference Year (TRY) or a Design Summer Year (DSY).
Part O (Building Regulations England, Part O)
Part O came into force in England in June 2022. It requires a regulatory demonstration of overheating risk for all new dwellings or significant extensions. Two approaches are available:
- Simplified method: compliance check using pre-calculated tables defining maximum glazing area ratios by orientation, combined with solar protection requirements.
- Dynamic method: full dynamic thermal simulation per TM59, with numerical demonstration that hourly criteria are met.
Orientation, the glazing-to-floor area ratio, and solar protection devices (internal or external) are the three primary levers for compliance.
The Role of Solar Films in TM59 / Part O Simulations
Solar control films feed into dynamic thermal simulations through two parameters:
- The total solar factor (g-value) of the glazing-plus-film assembly, which determines the fraction of solar energy entering the space. A film that reduces the g-value of the glazing from 0.62 (standard clear double glazing) to 0.30 cuts solar heat gains by more than 50%.
- Visible light transmission (VLT), which governs available natural daylight and must be factored into lighting assessments.
Both values appear on film technical data sheets and can be integrated directly into dynamic thermal simulation software (IES-VE, TRNSYS, EnergyPlus, DesignBuilder) via material libraries. The design engineer inputs the glazing-plus-film g-value into the construction parameters, which modifies the hourly solar gain calculation per zone.
TM59 Threshold Values and Interpreting Results
In a TM59 simulation, a dwelling is considered to present an overheating risk if:
- More than 3% of annual occupied hours exceed the adaptive threshold in living spaces;
- More than 1% of night-time hours (22:00–07:00) exceed 26°C in bedrooms.
Applying a high-TSER solar control film to exposed glazing (particularly east, south, and west orientations) may be sufficient to bring results below these thresholds in borderline cases. For limit cases, the combination of film and night-time ventilation (a passive approach) is frequently recommended by thermal engineers. This combined strategy is detailed in a dedicated article on passive summer comfort using solar film and night ventilation.
Compatibility with French Regulations (RE2020)
Since 1 January 2022, RE2020 incorporates a summer comfort indicator (DH — Degree-Hours of overheating). Solar control films reduce the simulated DH value by lowering solar gains (Qs). While RE2020 does not explicitly reference Part O or TM59, the calculation methods are compatible, and design engineers frequently use TM59 as a complementary diagnostic tool on projects with high solar exposure. For projects subject to strict energy certification (LEED, BREEAM) or RE2020 compliance, adherence to the standard specified in the project brief remains mandatory.
Practical Recommendations for Specifiers
- Identify critical glazing: south, east, and west orientations with high glazing area ratios.
- Enter film technical data into the DTS software using the correct g-value and VLT values.
- Distinguish between the glazing g-value alone (without film) and the assembly g-value (with film).
- Cross-check TM59 results against a glare assessment: films that significantly reduce the g-value may affect VLT and the visual quality of the space.
To select a high-performance solar control film meeting regulatory overheating requirements, the Solar Screen range offers products with EN 410-compliant technical data, ready for direct use in thermal simulations.