What is Climate Based Daylight Modelling

Climate Based Daylight Modelling is a relatively new way of assessing the quality and quantity of natural daylight and sunlight the rooms within a building will receive.

Thanks in part to the Priority Schools Building Programme (PSBP) and the ESFA Output Specification Generic Design Brief, the method is gaining popularity as the preferred way to carry out daylight analysis on new schools and other types of developments.

Using Climate Based Daylight Modelling instead of daylight factors provides much greater detail about light distribution and intensity within a space, allowing the building’s design to be adjusted to maximise the use of sunlight and daylight.

Actual location dependant annual weather data is used to calculate lux levels and targets can be set which are relative to the end users needs.

Useful Daylight Illuminance (UDI) and Spatial Daylight Autonomy (DA) are the two main criteria in Climate Based Daylight Modelling to assess whether a room will be sufficiently day lit or not.

Useful Daylight Illuminance (UDI)

Useful Daylight Illuminance (UDI) is defined as the annual occurrence of illuminances across the work-plane that is within a range considered “useful” by occupants. This is generally within the range 100 to 3000 lux, which is then subdivided further:

  • UDI-s (below 100 lux) where the light would be considered insufficient without electric lighting.
  • UDI-a (100 to 3000 lux) where electric lighting is acceptable and electric lighting 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 3000 lux) where the amount of light would be considered excessive and a source of glare and the blinds would be closed.

Daylight Autonomy (DA)

This criterion is aimed at delivering an energy efficient space.

Daylight Autonomy (DA) is the amount of time a space can expect to reach a target illuminance level on the working plane.

Priority Schools Building Programme (PSBP)

The full Priority Schools Building Programme (PSBP) document can be found here.

To summarise, it states that a minimum target of 80% UDI-a for each teaching space is considered acceptable.

Also that a minimum DA of 50% for each teaching space should be targeted.

ESFA Output Specification Generic Design Brief

Technical Annex 2E Daylight Electric Lighting of the ESFA Output Specification Generic Design Brief provides the guidance that must be followed and the criteria that must be met to satisfy the climate based daylight modelling requirements.

The following table shows the UDI and DA targets that must be met by different types of school spaces.

For Primary Schools the working plane height should be set at 550mm from the floor and for Secondary Schools it is set at 700mm. The calculation is also run on a daily occupation time of 08:30 – 16:00.

Key Features of a Good Climate Based Daylight Design

Incorporating some of the below features into your building design will help to ensure a good climate based daylighting result.

  • Single side lit classrooms should be ideally no more than 7.2m deep.
  • Rooms greater than 7.2m deep should attempt to have natural light entering from two sides of the room, or through one side and rooflights.
  • Windows should be placed at high level to allow light to penetrate deep within the space.
  • Ceilings should be as high as possible.
  • South facing rooms should incorporate solar shading to ensure excessive light levels are not received.
  • Classroom spaces that are adjacent to atriums should include internal glazing to allow secondary light into the back of the room.

How we can Help

If you require Climate Based Daylight Modelling carried out on your building, Elmstead Energy Assessments and Building Services are experts in how Climate Based Daylight Modelling can improve the design, function and efficiency of your building. Helping you to meet any planning permission requirements as well as satisfy the PSBP and ESFA requirements.

We can also carry out detailed daylight and sunlight assessments and Average Daylight Factor Calculations if required.

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