Even though only the visible part of the solar radiation spectrum provides daylight useful for visual tasks, the whole spectrum, visible and invisible, contributes to heat gains when absorbed into the room. Numerous ways of improving daylight performance have been developed over the years. These are based on the use of special glass, orientation of openings in relation to the sun, as well as on the use of solar control/light diffusing elements.
Glass reflects, admits and absorbs solar radiation in different proportions depending on the type. For normal clear glazing, the reflected component is about 15% of the incident value but this value increases strongly when the angle of incidence increases beyond about 600. The transmitted component is about 80% of the incident radiation, absorbing about 5%.
Several types of special glass that improve daylight and thermal conditions are available for use today. Tinted/absorptive glass contains pigments which increase its absorption. This reduces transmittance typically from 40% to as low as 10%. The absorbed energy heats up the glass, partly ending up in the room and partly being lost to the outside.
Reflective glass has a thin metallic coating which increases the reflected component. Energy is therefore lost to the outside without getting absorbed into the room. However, the glare that results can be a nuisance to neighboring users.
Low-emissivity glass (low-e) absorbs all long wave radiation, reducing emitted radiation. It is however important to note that the original purpose of this type of glass was to reduce heat loss by radiation during the heating season in cold climates. In tropical regions where heat loss is highly desirable, low-e glazing can result in very uncomfortable environments.
Other types of glazing include those of varied properties. Photo-chromic glass reduces its transmittance with increase in light levels, while thermo-chromic glass responds to an increase in temperature. However, both of these types are reactive, and cannot anticipate conditions. Their spectral transmittance is poor as well.
The special glazing materials described above are aimed at reducing heat gains associated with useful daylight. In the predominantly warm conditions of the tropical regions, this is a priority. However, these types of glass lead to lower daylight levels and are not 100 percent effective in controlling heat gains. Their extensive use in tropical regions only results in unnecessary cooling loads.
Solar control and diffusing elements assist in controlling solar gains and useful daylight. These include overhangs, fins, blinds, louvers etc, and can either be fixed or movable. Fixed devices can be sized to cut solar radiation when solar gains are not desirable and admit when desirable. Movable devices on the other hand, can be adjusted to modulate their transmittance.
In addition, all these devices can improve the distribution of light due to their ability to reflect light from specific directions and/or obscure light from specific directions.
Other ways of improving daylight performance include the use of reflective finishes to allow for the propagation of light deeper into spaces. These can be applied on the interior wall surfaces, and/or on the surfaces of the daylight and solar control devices above.
To achieve high performance in day-lighting requires an integrated approach that takes into consideration the thermal and daylight requirements for the function in question, the properties of materials and devices to be employed, as well as the climatic conditions. The task and problems of day-lighting in tropical climates can be summarized as follows:
- To provide adequate useful daylight with minimum solar gains.
- To reduce interior ‘gloominess’ and glare which is very likely due to the extremely bright exteriors
Solutions to these challenges should however be sought within the context of all day-lighting strategies for maximum benefit.
The author is an architect and Environmental Design Specialist working with Planning Systems Services Ltd
