Daylight and Energy Use.
Daylight is the most sustainable way of supplying light in the workplace. The amount of daylight available to occupants will be an indicator of the quality of the workspace. Four key objectives for daylight design should be satisfied:
Adequate task illuminance from daylight for as much time as possible
A view to outside
A glare free working environment
Artificial lighting systems that complement the daylight design
The task illuminance required for both daylight and electric lighting are the same- to provide sufficient illumination to carry out the task. Both BREEAM and LEED include points for demonstrating the space is well daylit. Following the BREEAM and LEED, a well daylit office space achieves an average daylight factor of between 2% and 5%.
This would translate into daylight illuminance >300 lux for between 55% (2%daylight factor) and 80% (5% daylight factor) of the annual working hours for a typical office with working hours of 9am to 5pm.
While daylight has a huge positive effect on human wellbeing, it is also the greatest source of glare in the working environment. This could be controlled through the selection of appropriate shading. The SLL Lighting Guide 10: Daylighting and Window Design provides a summary of the shading strategies.
Floor plate studies
As a general rule, well daylight space is within 6m of façade, receiving the benefits from good access to daylight, as well as a low reliance on artificial lighting.
The space that is 6-12m deep has benefits from the view of daylight.
Space that is deeper than 12m will be considered to have no benefits from daylight.
Interior finishes and room brightness
Good daylight will provide brightness to the surfaces within the space. This will vary throughout the day and weather conditions. Dark interior finishes will absorb light and reduce the sense of daylight in the space. High reflectance finishes are recommended to reduce the contrast between the views out of the window and the surfaces. The designer must ensure the reflectances of the surfaces are coordinated with the architect.
Daylight and energy saving
Daylight can contribute to significant energy savings. Up to 25% of lighting energy can be saved when daylight and artificial lighting are combined with occupancy and lighting controls. The daylight design should also be included in the overall environmental strategy.
Part L2 of the Building Regulations is one of the primary tools for reducing CO2 emissions. There are 3 criteria within Part L relating to lighting.
Criterion 1- Setting carbon-limiting emission levels for the building. This criterion is mandatory, the other criteria are for guidance only. The calculated CO2 emissions of an actual building must be less than the calculated CO2 emissions of a “notional’ building. This is based on the notional building using lighting with an efficacy of 60lm/W.
It is recommended that the power and heat load allowance for the office lighting is designed to 10W/sqm.
Criterion 2 - Performance of fixed lighting systems. This criterion sets limits on the performance of the building fabric. The minimum lighting efficacy for offices in Part L 2014 is set to be 60lm/W.
Criterion 3 - Limiting the effects of solar gains in summer.
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