Climate-Based Daylight Modelling

Climate-based daylight modelling (CBDM) is the prediction of daylighting quality on a specific indoor space using sun and sky conditions that are derived from standard meteorological datasets, taking into consideration the illumination effect of the sun and non-overcast sky conditions, orientation and building configuration.

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.

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).

SDA is the amount of time a space can expect to reach a target illuminance level on the working plane. 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.

UDI is defined as the annual occurrence of illuminances across the work-plane that is 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 wouldn’t 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.

The Output Specification sets a minimum target of an average of 80% UDI-a for each learning space and currently sets x at 100 lux and y at 3000 lux. If average lux levels fall below 100 or above 3000 on each working plane for more than 20% of the time, this is considered a fail with a mitigative solution required.

At L2 Energy Consulting we have the experience and knowledge to help demonstrate these requirements within new or existing learning spaces. Please call us on the number above or ask for a quote electronically.