DAYLIGHT
 

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 matrixes and calculators for estimating the daylighting levels within a space. Click here for a brief introduction to this page.

The following calculator provides a rule of thumb for how deep a floor plan can be daylit based on the window head height.


DAYLIGHT FACTOR 
 

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 outdoor horizontal external illuminance, measured at a place where the view to the sky is unobstructed (for example at the roof). The daylight factor only applies to the distribution of diffuse light only, i.e. during overcast conditions without direct sun light.

The Malaysian Standard 1525, Table 1. Daylight factors and impact recommends a daylight factor between 1.0 - 3.5% in Malaysia.

MS Reference.jpg
 

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. In Malaysia, for example, the annual average illuminance from diffuse daylight during the daytime is about 30,000 lux. 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.

The following figure exemplifies the Daylight Factor for an office with Daylight Trough (DT). The figure shows how the daylighting levels drop the further you get from the facade or daylight applications such as the Daylight Trough. The full paper can be downloaded here.


ANNUAL DAYLIGHT MATRIX
 

Annual daylight matrixes are characterised by using weather files to more accurately predict the daylighting for a space. A wide range of matrixes 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 matrixes 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 at least 300lux for 50% of the occupied time between 08:00 - 17:00 (Not that LEED and WELL uses 08:00 - 18:00, refer to LM82-12). The DA result is then the percentage of floor area where this requirement is fulfilled. However, Green Mark also requires overlit areas to be deducted from the useable daylit area, this is defined as UDI3000Lux,10%. This means that the Useful Daylight Illuminance (UDI) cannot exceed more than 3,000lux 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 into a simple daylight calculator below showing how far the daylit zone stretches from the facade into the space for the Green Mark Daylight Autonomy of 300lux for 50% of the time (DA300lux,50%) with deduction of overlit areas.

 

A more complex calculator is shown below, where Daylight Autonomy requirements and obstructions can be adjusted. The shown results are the average of all data when multiple values are selected. The following figures illustrate how the Window Shading Angle (referred to as Overhang Obstruction Angle (OOA) in Green Mark) and Urban Obstructions (referred to as Average Urban Obstruction Angle (AUOA) in Green Mark) can be found.

Illustration by IEN Consultants after GM NRB:2015, Technical Guide and Requirements; Appendix B: Effective Daylighting Simulation and Pre-Simulated Daylight Availability Tables Methodology and Requirements

Illustration by IEN Consultants after GM NRB:2015, Technical Guide and Requirements; Appendix B: Effective Daylighting Simulation and Pre-Simulated Daylight Availability Tables Methodology and Requirements


GLARE PROTECTION AND BLINDS
 

The Daylight Autonomy and matrix 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 matrix is characterised by including blinds, which has to be activated when more than 2% of the analysis points receive direct sun, defined as an interior horizontal measured direct beam of more than 1,000lux.  The simulation method assumes automated blinds that adjust instantaneously. Once the window blinds are adjusted manually, a significant drop in daylight autonomy is observed, as explained in our article, Daylight Autonomy: lightshelf versus Manual/Automated Blinds. Our article also points out that occupant glare from the diffuse sky is a significant factor in the tropics, something the sDA method also does not take into account, hence, resulting in an overestimation of the daylight autonomy.

Interested in learning more or do you have a project which could use our daylighting expertise and/or simulation capabilities? Please contact us at order@ien-consultants.com for a custom quotation.

 
Customised quotation: Email us at order@ien-consultants.com
 


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