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Climate-based Daylight Modelling

Climate-based daylight modelling is a relatively new technique that was developed at the turn of the millennium and involves using climate-based data to predict the quantity and quality of daylight within a space. Most building designers would enthusiastically agree that natural light is vital to a successful building and to the happiness of the buildings occupants. However few would confidently know how to achieve excellent levels of daylight, or even how to measure it.

The drive towards sustainable, low-energy buildings places increasing emphasis on detailed performance evaluation at the early design stage.  Over the past decade, a new family of dynamic daylighting metrics have been developed to describe and evaluate daylight in spaces. These metrics summarise the daylight availability over the whole year and throughout a space.

Climate-Based Daylight Modelling (CBDM) is one approach to design that answers this question, with a level of precision that improves designs and building outcomes. The advantage of using a climate-based lighting simulation over more traditional techniques, such as the daylight factor approach, is that it takes into account sunlight and skylight in a way that is consistent with our experience of the naturally lit environment. CBDM is a tool that allows buildings to be shaped to the climate and site in which they sit, utilising natural resources both beautifully, and efficiently.

Given the self-evident nature of the seasonal pattern in daylight availability, an evaluation period of a full year is needed to fully capture all the naturally occurring variation in conditions that is represented in the climate dataset for a specific location. CBDM, takes into consideration the whole year and the seasonal variations of sunlight throughout. The effects of surrounding buildings, local topography etc. are all considered to complete the picture. In the UK, The Department for Education facilities output specification for daylighting sets two criteria to be met: Spatial Daylighting Autonomy (SDA) and Useful Daylight Index (UDI).

  • Daylight Autonomy (sDA) is a measure of how often a minimum work plane illuminance threshold can be maintained by daylight alone. This criterion is aimed at delivering an energy-efficient space. The output specification sets a minimum target for each learning space of 300 lux or more on 50% of the working plane for 50% of the time.
  • Useful Daylight Illuminance (UDI) is the annual occurrence of illuminance distribution across the working plane that are within a range considered “useful” by occupants. This is subdivided as follows:
    • UDI-a (x to y lux) where daylight is acceptable and electric light would not be needed for the majority of the day; achieving a high UDI-a percentage signifies the space is predominantly daylit throughout and glare is controlled
    • UDI-e (above y lux) where the amount of daylight would be considered excessive and a source of glare and blinds would need to be operated.
    • UDI-s (below x lux) where the light would be considered insufficient without electric lighting.

Climate Based Daylight Modelling improves the design, function and efficiency of your building. QuinnRoss Energy will help you to meet any CBDM related planning permission requirements.

What is that?

Climate based daylight modelling (CBDM) is the prediction of various radiant or luminous quantities using daylight conditions derived from standard meteorological datasets. Climate-based modelling delivers predictions of absolute quantities (e.g. illuminance) that are dependent on the location and the building orientation, in addition to the building's composition and configuration. I.e. its daylight and sunlight modelling that uses specific geographical climate.

Why is it different from a standard daylight and sunlight assessment?

Although it uses the same methodology of standard daylight and sunlight assessment, the calculation process is very different. It uses a Spatial Daylight Autonomy (SDA) and Annual Sunlight Exposure (ASE) calculation to seek a balance of daylight and occupant comfort. Standard daylight modelling, for example, analyses a single static period under an overcast sky. Whereas climate-based modelling takes into consideration the whole year and the seasonal variations of sunlight throughout.

Is it significant?

Yes. Most noteworthy is that it’s a mandatory requirement for all UK schools. It’s also the preferred method for the WELL building standard and LEED assessments. Its also viewed in the industry as the next step in daylight analyses, as the current guidance is now over 10 years old.

Is QuinnRoss Energy ready for these as well?

Again, yes. We already have industry recognised compliant software capable of said calculations and have already performed a number of assessments on UK schools, so have the experience as well.

Need Help With Your Project?

Contact one of our energy assessors today on 01795 845 780, or email us at hello@quinnross.energy


RIBA stages

When do I need Climate-based Daylight Modelling?

Services for RIBA 0 - 7

Whether you need a SAP for a bungalow, or overheating analysis for a penthouse, our technical specialists will work with you to find the best solutions for compliance within your deadlines and budget, explaining them in plain English.