By Eric Loki
Over the years, research on the treatment of openings in tropical buildings has focused only on the extreme cases, with most offered solutions so far being based on the need for strict control of direct solar radiation. Reflected daylight through the often heavily sun-shaded side openings has been recommended with top-lighting, almost considered a vice.
There is no denial however, that the challenge is very real. Tropical Climates present an interesting paradox: ambient luminance levels can be very high yet very little of this finds its way into interior spaces as useful daylight without leading to overheating. Much of it is in the form of direct sunlight as the predominantly blue skies give lower illumination. Since 50 percent of solar radiation is infrared (heat), cutting direct sunlight to reduce overheating results in a dramatic decrease in daylight levels leading to even ‘darker’ interiors due to the very bright exteriors.
The emphasis however, on an almost exclusive use of side windows for day-lighting has led to the negligence of other innovative strategies, especially top-lighting, which is known to provide higher and better distributed daylight levels. This is despite the many studies that have been carried out in recent years showing that the poor daylight penetration from side openings is made far much made worse by the use of solar control features and the dependence of externally reflected component of light is not always reliable.
Contrary to an overcast sky’s luminance (quantities between 5000 lux and 20000 lux and typically only three times greater at the Zenith than at the horizon), a clear sky’s brightest part, which is in the direction of the sun, is about ten times brighter than the darkest part of the sky, which is found at about 90 degrees to the sun. Overall illumination is quite high, with values up to between 60,000 lux and 100,000 lux (see figs. 01 & 02).
Recent comprehensive work on top-lighting (or ‘Zenithal light’ as he calls it) by Spanish architect Elias Torres however, clearly shows that admitting daylight through high openings is a practice that cuts through all climatic regions. Even though there are clear variations to suit the thermal conditions of each region, the principles are the same. The key is to understand the daylight and thermal performance of any top-lighting strategy within a particular climate, and provide a system that befits the thermal and daylight requirements of a space.
Kenya’s capital Nairobi, in particular, presents a very interesting case. The city, located at latitude 1.20 South and at an altitude of between 1600 and 1800m above sea level, experiences a tropical upland type of climate. This is characterized by relatively lower temperatures and more cloud cover than other more extreme tropical regions. Despite the high solar radiation levels characteristic of such altitudes, the city exhibits some cases, albeit very few, of top-lighting. This raises the question as to whether the city can benefit from an extensive use of top-lighting systems by taking advantage of the rather mild tropical climatic conditions.
During this period in time when there is a dire need for the development of low energy and ‘cleaner’ solutions for running buildings in a bid to reduce global warming and deal with the depletion of world fossil fuel reserves, every effort is needed to exploit any opportunity that presents such.
Particularly in Nairobi, where the use of electrical light during the day is considered a norm, the benefits of additional daylight from top-light sources to supplement side openings would be immense. This would not just be for the city alone but actually for the whole country as Nairobi consumes over 50% of the country’s electrical energy.
From this, it is very clear that top-lighting can help provide daylight in Nairobi’s buildings, and in particular allow for deeper plans without over dependency on artificial lighting. When solar gains are not desirable, the strong radiation can be intercepted away from the opening, loosing much of the heat to the outside. Reflected diffuse light can then help illuminate interior spaces. When solar radiation is desirable, or when the intensity is low, direct sunlight can be allowed indoors for good day-lighting. Control is however important to ensure maximum comfort.
The most important thing to remember though is that the comfort levels, both visual and thermal, of any interior space lit by a top light in Nairobi is a function of the type, size and location of the top light; the size of the space (area and volume); and the air movement (and temperature).
