Daylighting design is an important part of designing a green building. Well, daylit spaces do not only conserve energy as lighting can be turned off more frequently but also increase occupant wellbeing and productivity. The following sections provide an overview of different daylight matrics and calculators for estimating the daylighting levels within a space.
The following calculator provides a rule of thumb for how deep a floor plan can be daylit based on the window head height.
The Daylight Factor (DF) is a simple representation of the daylight distribution within a space. The value is expressed as the percentage of illuminance on a horizontal surface measured indoor versus the instantaneous external illuminance.
The Malaysian Standard 1525, Table 1. Daylight factors and impact recommends a daylight factor between 1.0 and 3.5 in Malaysia.
The following calculator can be used to convert targeted Daylight Factor to lux levels and vice versa.
The Daylight Factor is for design cases simulated with CIE Overcast Sky condition often a 10,000lux sky to find the percentage. In reality, the daylight factor would vary depending on the outdoor illuminance levels. The following graphs show the annual average conversion of exterior illuminance levels to corresponding daylight factors. The representation uses the annual average diffuse radiation for Kuala Lumpur in W/m2 which is then converted to lux with a factor of 120Lux / W/m2.
ANNUAL DAYLIGHT MATRICS
Annual daylight matrics are characterised by using weather files to more accurately predict the daylighting for a space. A wide range of matrices have been developed with the most commonly used in green building certifications being:
Daylight Autonomy (DA)
Annual Sunlight Exposure (ASE)
Spatial Daylight Autonomy (sDA)
Useful Daylight Illuminance (UDI)
These annual matrics are often used in combination with each other as seen in the Singaporean green building scheme, Green Mark.
Green Mark requires the daylight to be measured as Daylight Autonomy (DA) where overlit areas are reported and deducted by using the Useful Daylight Illuminance (UDI) if they exceed a certain level. Daylight Autonomy is defined as the percentage of time the interior illuminance levels are meeting the required levels for a percentage of the floor area. For example DA300lux,50% means that the Daylight Autonomy has to be 300lux for 50% of the occupied time (Green Mark default hours are 08:00 - 17:00, whereas LM82-12 used by LEED and WELL uses 10 hours a day 08:00 - 18:00) the output is then the percentage of floor area where this requirement is forefilled. Green Mark similarly requires overlit areas to be deducted from the useable daylit, this is defined as UDI3000Lux,10%. This means that the Useful Daylight Illuminance (UDI) cannot exceed more than 3000lux for more than 10% of the time. If this value is exceeded for more than 10% of the floor area then it will have to be excluded from the daylight calculation as it is considered overlit. Green Mark has provided Pre-Simulated Daylight Autonomy tables where the Daylight Autonomy has been derived from more than 4,000 simulations of a reference shoebox model. We have translated the results in to a simple daylight calculator below showing how far into the space a Daylight Autonomy of 300lux for 50% of the time (DA300lux,50%) will get.
A more complex calculator is shown below where Daylight Autonomy requirements and obstructions can be adjusted. the results are the average of all the data when multiple values are selected.
GLARE PROTECTION AND BLINDS
The Daylight Autonomy and matric for Green Mark do not consider the engagement of blinds. However, other certification schemes such as LEED and WELL uses a Spatial Daylight Autonomy (sDA). The sDA matric is characterised by including blinds, which has to be activated when a threshold above 1,000 lux of direct sunlight is reached. The simulation method is thereby getting closer to how the building or the space will be operated. However, as Gregers explains in our recent article (Daylight Autonomy: lightshelf versus Manual/Automated Blinds) there are still differences.
Interested in learning more or do you have a project which could use our daylighting expertise and/or simulation capabilities? Please contact us at firstname.lastname@example.org for a custom quotation.