What is Climate Based Daylight Modelling?

Climate Based Daylight Modelling (CBDM) is a sophisticated method of assessing daylight performance within buildings by simulating how natural light varies over time in a specific location.

Unlike traditional static metrics like the Daylight Factor, which assumes a fixed overcast sky, CBDM uses real-world weather data to provide dynamic, location-specific insights into daylight access. This allows designers, developers, and planners to make informed decisions that support health, sustainability, and compliance with modern daylight standards.

Traditional vs Climate-Based Approaches

Historically, daylight assessments relied heavily on the Daylight Factor (DF), a simple ratio comparing indoor illuminance to a theoretical, constant outdoor light level under a uniform overcast sky. While useful for basic analysis, this method does not account for seasonal variations, building orientation, or climate differences.

In contrast, CBDM incorporates actual climate data, usually in the form of a Typical Meteorological Year (TMY) file, to model natural light in a given location over the course of an entire year. This results in a much more accurate understanding of how spaces will be lit in practice.

Key Climate Based Daylight Modelling Metrics

CBDM is grounded in several core metrics that help quantify daylight performance:

Daylight Autonomy (DA): The percentage of occupied hours during which a space receives enough daylight (typically 300 lux or more) without artificial lighting.

Spatial Daylight Autonomy (sDA): The percentage of floor area that achieves the required daylight levels (e.g. 300 lux for 50% of the occupied time).

Useful Daylight Illuminance (UDI): The percentage of time daylight levels fall within a desirable range (e.g. 100 to 2000 lux), helping identify areas that may be too dark or too bright.

Annual Sunlight Exposure (ASE): The percentage of floor area receiving excessive sunlight (e.g. more than 1000 lux for over 250 hours annually), highlighting risk of glare or overheating.

Why CBDM Matters

1. Better Design Decisions

By accurately modelling daylight over time, CBDM allows design teams to optimise:

Window placements and sizes

Glazing specifications

Room depths and layouts

Solar shading strategies

2. Health and Wellbeing

Numerous studies confirm the physiological and psychological benefits of natural light. CBDM helps ensure that indoor environments support circadian rhythms, reduce reliance on electric lighting, and improve occupant wellbeing.

3. Energy Performance

CBDM informs energy modelling by quantifying when and where natural light can reduce the need for artificial lighting. This leads to more energy-efficient buildings and supports compliance with sustainable certification schemes like BREEAM, LEED, and WELL.

4. Planning and Compliance

In the UK, Climate Based Daylight Modelling aligns with BS EN 17037: Daylight in Buildings, which replaces the older BS 8206-2. The new standard encourages a performance-based approach, using CBDM to evaluate illuminance levels across key living and working spaces.

CBDM also complements the BRE Report BR209 - Site Layout Planning for Daylight and Sunlight (2022 Edition), which recognises climate-based assessments as a preferred method for evaluating daylight provision.

CBDM in Practice

1. Inputs Required

To run a CBDM analysis, practitioners need:

Accurate building geometry

Reflectance values for interior surfaces

Window specifications (glazing type, transmittance, framing)

Obstructions (external buildings, trees, balconies)

Location-specific weather data (e.g. TMY files)

Occupancy schedules and daylight targets

2. Software Tools

Several software packages offer CBDM functionality, including:

Radiance

Daysim

Honeybee/Ladybug (Grasshopper plugins)

IES VE

DIALux Evo

Autodesk Insight

Each tool varies in complexity and capability, but all use validated algorithms to simulate dynamic daylight performance.

3. Interpreting Results

CBDM results are usually presented as coloured floor plans or heatmaps, illustrating spatial variations in daylight levels. Key outputs include:

Areas meeting or failing target lux thresholds

Locations of potential glare or overexposure

Seasonal daylight availability

These insights guide design revisions and help demonstrate compliance with planning policies or sustainability benchmarks.

UK Standards and Guidance

BS EN 17037 (UK National Annex)

This standard outlines how to assess daylight provision using CBDM. Key elements include:

Minimum target illuminance: e.g., 100 lux (bedroom), 150 lux (living room), 200 lux (kitchen) for 50% of daylight hours

Assessment plane: typically 0.85 m above floor level

Calculation grid: spacing of 0.3 m for robust analysis

BRE BR209 (2022)

This guide recommends using climate-based metrics to:

Assess new developments

Safeguard existing buildings

Optimise layout for sunlight in open spaces

Appendices in BR209 also highlight how CBDM complements Vertical Sky Component (VSC) and No Sky Line (NSL) tests, especially in complex urban environments.

Limitations of CBDM

While powerful, CBDM does require careful interpretation:

Results are sensitive to input accuracy (e.g. reflectance, glazing, geometry)

Simulations can be computationally intensive

Expertise is needed to select appropriate metrics and interpret results meaningfully

Nonetheless, for projects seeking robust, future-proof daylight analysis, CBDM is the gold standard.

FAQs

What is the difference between Climate Based Daylight Modelling and the Daylight Factor method?

CBDM uses real weather data to assess daylight dynamically over the year, while the Daylight Factor method uses a single overcast sky condition and does not account for orientation, seasons, or climate.

Is Climate Based Daylight Modelling required for planning in the UK?

While not mandated, CBDM is increasingly expected by local planning authorities and is encouraged in BS EN 17037 and BRE BR209 (2022). It demonstrates a higher standard of analysis and aligns with sustainable design goals.

Which buildings benefit most from CBDM?

All buildings can benefit, but it's particularly valuable in:

Dense urban sites

Schools and hospitals

Offices

Residential developments requiring daylight and sunlight impact assessments

Can Climate Based Daylight Modelling predict glare or overheating?

Yes. Metrics like Annual Sunlight Exposure (ASE) and Useful Daylight Illuminance (UDI) can highlight areas prone to glare or excess solar gain, helping inform mitigation strategies.

How long does a CBDM analysis take?

Depending on building complexity and software used, it can range from a few hours to several days. Accurate inputs and simplified geometry can speed up the process.

Conclusion

Climate Based Daylight Modelling represents a major advancement in how we evaluate natural light in buildings.

By considering real weather conditions and time-based variations, it enables better design decisions, enhances occupant wellbeing, and supports compliance with the latest UK guidance.

Need Help with Climate Based Daylight Modelling Assessments?

At Anstey Horne, we specialise in delivering clear, accurate, and robust daylight & sunlight assessments for planning applications and appeals in London. If you’re dealing with a complex site or want to understand how a Climate Based Daylight Modelling could help with the assessment of affect daylight & sunlight within a development, get in touch with us today.

For more advice on how we can help support a planning application with a daylight & sunlight assessments please give us a call. If you would rather we contacted you please fill in our Contact Form and we will be in touch.

For more information on all aspects of Internal Daylight & Sunlight Assessments for planning see the collection of articles on our blog page.

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