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Offices
Daylight quality can be judged by its ability to satisfy the requirements of occupants at their work space; in terms of:
• a view toward the outside,
• the supply of sufficient light on the work plane: (200 to 600 lux as often as possible) requiring daylight factors in the range of 2 to 4% if possible for most European locations.
• Glare control using adaptable shading devices, with typical transmittances of awnings below 20%, to provide enough attenuation of sunlight for instance
• reduction of disturbing reflections (‘veiling reflections’) on computer screens, avoiding daylight coming from behind the terminal user.
• Balanced luminances in the field of view near the workplane, with surounding luminances not exceeding 10 times the one of the task luminance. The energy-saving potential can be achieved with automatic controls of lighting in response to daylight or occupation. High energy savings can be achieved if lighting is separated into ambient lighting (easily controlled in response to daylight) and task lighting.
Circulation zones can easily be daylit using secondary daylighting windows since they may be considered as well-lit even with daylight factors as low as 0.5 or 0.25%.
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Glazing adapted to each facade orientation provides different levels of daylight attenuation
Tractebel building, Brussels, BELGIUM
Glazing types and blinds largely modify the daylight distribution in rooms as well as visual comfort conditions.
Aerial view of the Tractebel building.
62,000m2 of office floor for 2,600 people
The Tractebel building is a contemporary office building built in 1990 and designed by M. Jaspers & Partners. The building, of 62,000m2 floor area, is occupied by 2,600 people mainly employed by Tractebel, a large Belgian engineering company. Functionality and flexibility were principal issues in the design. The internal layout consists of fixed and moveable offices, modular furniture, modular moveable walls, ceilings and lighting. The general shape of the building is a double cross with facades facing NW, NE, SW and SE. The junction of the two crosses forms a courtyard. Facades have large glazed areas: window area is equal to 44% of the facade area. Glazing types differ according to the orientation of the facade. SE and SW facades use double glazing with selective heat-reflecting glass on the exterior pane. NE and NW facades use double clear glass.
In addition, the acoustic properties of the glazing are adjusted depending on the adjacent noise conditions. Acoustic insulation is greatest on facades facing the local highway.
A typical standard office for two persons in the Tractebel building.
The effect of the difference in glazing properties was monitored on the 1st and the 3rd floor of wing E of the building, which has NW and SE facades.
Daylighting from facades with heat reflective glass
The three windows of the monitored office are 1.60m high and 2.60m in width. The typical glazing ratio of office space is rather high at 21% (ratio of glazing area to floor area). Although the luminous transmittance of the glazing is only 43% (compared to 76% for the double clear glazing with a low-E coating), daylight penetration is rather good. The selective properties of the glass make the radiative transmittance significantly lower at 23%, which is close to the theoretical minimum value by comparison with the luminous transmittance. The rather high illuminance values which are reached are due to bright wall finishes (wall reflectances vary from 60 to 90%).
Viewed from the outside, the heat-reflecting glazing gives a slight silvery reflection. However colour rendering toward the inside is very good (CRI=96) and the glass appears to be clear to occupants sitting in their office. For occupants in the middle of wing E and seeing NW and SE facades simultaneously, the tint of the heat reflective glass is hardly noticeable; one notices only the difference in transmittance.
Daylighting from facades with clear glass
Clear glass was used to the NE and NW facades.
In wing E, the north-west facade faces a courtyard. From the first floor, the obstruction is significant: 38 degrees above the horizon at the level of the work plane.
In room 4, the measurements demonstrate that the good luminous transmittance of the clear glass roughly compensates for the reduction of daylight due to the building wing across the courtyard.
Site plan, showing location of wing E where measurements were taken.
Daylight factor distribution on the work plane in rooms 1, 2 (Third floor) 3 and 4 (First floor). NW facade is equipped with clear glass, SE with heat-reflectin...
