No. 28, Spring/Summer 1989
By Warren R. Hampton
|"Daylighting in hot dry climates requires an integrated design approach toward energy performance of the building envelope and illumination of the interiors. Architectural morphology must respond to passive heating, cooling, and ventilation requirements, and be properly configured to capture and distribute natural illumination effectively and efficiently to the interior spaces."||
This article might have been entitled "An Alternative to the Well-considered Window." While this author strongly advocates skylighting as a unique and utilitarian form of natural lighting for spaces in hot dry climates, he does not recommend elimination of windows designed for view and ventilation. Windows alone, however, do not provide satisfactory daylighting of deep spaces due to poor penetration and distribution of the illumination within the space; this can result in glare and local overheating. Skylighting in combination with properly designed windows can effectively illuminate deep interior spaces and offers opportunities for natural ventilation.
To allow the penetration of daylight into a building interior requires the architect or lighting designer to evaluate two components of sunlight--the thermal and the luminous. This evaluation should include the analysis of thermal gains and illumination from direct solar radiation, and from diffuse sources such as the skydome and adjacent reflective surfaces including the ground itself. It is important to consider aperture size as a function of light transmission and its resultant thermal impacts upon the building interior. Improperly designed apertures can result in overheating of the interior spaces, visual discomfort from glare and unwanted deterioration and fading of furnishings and other materials.
Daylighting in hot dry climates requires an integrated design approach toward energy performance of the building envelope and illumination of the interiors. Architectural morphology must respond to passive heating, cooling, and ventilation requirements, and be properly configured to capture and distribute natural illumination effectively and efficiently to the interior spaces. The architect or lighting designer must sensitively apply the design principles for thermal performance and actively pursue the investigation of options using computer simulation and photometric model testing.
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Toplighting is a very flexible strategy for daylighting; skylights may be located for specific illumination of sculpture or other architectural features, or spaced evenly across the roof for uniform illumination of the space below. Lam points out that when utilizing toplighting there is no need to overlight one area in order to get sufficient light for adjacent areas, commonly a problem with sidelighting (1). Interior areas adjacent to the window aperture receive very high levels of illumination that drop off abruptly a short distance into the space which can result in problems with veiling reflections and glare. Reflections from adjacent surfaces can also negatively affect the quality of light available through a window.
Toplighting (as measured by illumination levels in relationship to heating and cooling loads) can be very efficient in low-rise buildings with daylit atria, on the upper floor(s) of high-rise buildings, and in deep residential spaces. A uniform distribution of illumination of the space can be achieved with a minimum glared aperture.
The sun is the ultimate source of daylight. Daylight received on the Earth's surface is a combination of direct beam radiation and sunlight diffused in the atmosphere. Under the predominantly clear skies of a hot dry climate the total amount of natural illumination received at any location is dependent upon solar positions latitude, and local atmospheric conditions. On clear days, the bright and intense sunlight results in illumination levels on the ground surface exceeding 10,000 footcandles (107,600 1x). For a clear skydome, typical of hot dry climates the horizon is approximately 3 times brighter than the zenith luminance. The clear skydome, however, is considered brightest in the region nearest the sun, and darkest 90° from the sun's position. According to Hopkinson the deep blue sky may have a very low luminance from the horizon up about 30° elevation during the hours around midday, and may consequently be insufficiently bright to act as the principal source of interior illumination (2). As a result, sidelighting which attempts to capture this illumination is not the best solution to daylighting in a hot dry climate.
Intense sunlight, clean atmosphere, and reflective ground surfaces bring about the problem of glare. Glare results from excessive brightness in the visual field originating from sky luminance, light reflected from the natural landscape and/or from man-made features of the environment such as buildings. In hot dry climates, Hopkinson estimates the ground can be 4 times as bright as the sky and as such can be the primary source of glare (2). Windows transmit this glare into building interiors.
In citing historical precedents for skylighting, Lam states that the use of small horizontal openings to provide light and ventilation in thermally massive, vaulted roof structures is uniquely appropriate to the predominantly sunny, dry, desert climate with its hot days and cool nights (1). This combination ensures that the ceiling vault and upper walls will be the primary reflecting surfaces, minimizing glare at eye level.
During both winter and summer conditions, the level of interior illuminance from an overcast sky is remarkably higher than from a clear sky. In a hot dry climate with primarily clear skies, skylighting must utilize either direct beam radiation or reflected sunlight to maintain a minimum roof aperture consistent with the desired thermal performance of the structure. Lam points out that optimizing the performance of toplighting today implies using light indirectly (1). Incoming sunlight must be baffled and redirected to avoid glare and local overheating and to provide the light distribution desired. Lam also states that skylights may be the best option in equatorial locations where their horizontal orientation will maximize collection of the incident sunlight from a high solar altitude. Under similar conditions a very small area of glazing in a deep well can illuminate a large area effectively. The skylight should be no larger than necessary to provide the desired illumination under sunny conditions (i.e., the aperture should be +-1% of the floor area) (1).
In summary, skylighting offers the architect and lighting designer opportunities for supplementing windows designed for ventilation and view. Appropriately designed skylights direct the light into deep interior spaces and provide better distribution of the light indoors.
Proper design for daylighting interior spaces requires consideration of both energy and lighting issues. In hot dry climates these lighting issues and energy issues must be carefully integrated and the design responses must be balanced in the final building design. Some of these issues follow.
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To preserve the thermal integrity of massive walls and insulated roof structures, penetrations are usually minimized. Fewer, smaller openings result in dimly lit, sometimes gloomy spaces and interior wall surfaces of low luminance. Visual discomfort in the form of glare results when the intense light entering through these openings is viewed in contrast with surrounding wall/ceiling surfaces. The rule must be "minimize the opening while maximizing the amount of daylighting potential."
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Proper detailing of skylight openings can reduce glare in interiors. Contrast grading, through the use of color shading, and spreading the intense natural illumination across a ceiling or wall surface softens and diffuses the light. Likewise, locating skylights next to vertical wall surfaces or in sloped or vaulted ceilings lessens glare. Coffers and lightwells effectively control the entering light.
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A twentieth-century solution to the problem of glare is the use of tinted glass. This glass limits the amount of transmitted illumination and therefore controls the differences in apparent brightness between the interior and the exterior. Glare-reducing and low-transmittance glazings also impart a color tint to the daylight; this color tinting can result in an unnatural appearance of the exterior environment. Such glazings are often chosen for their thermally reflective properties with disregard for their light-related qualities. Specialized glazings can offer effective solutions to glare and overheating, but they can also significantly limit the use of daylight in building interiors.
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Skylighting is a unique opportunity for combining natural ventilation options with lighting. Operating skylights located in the upper reaches of the space can utilize natural convection to remove excess heat. Properly designed and oriented skylights can also serve to catch natural breezes and direct cool air to the interior.
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In summary, the design of openings for a hot dry climate requires that window/skylight areas be minimized to control thermal loads on the structure and yet allow for effective lighting of interiors. For conditions in hot dry lands, the Building Research Station in the United Kingdom suggests that a minimum glass area as low as one-sixteenth of the floor area to be lit should be adequate for normal residential buildings (4). To maintain physiological and psychological comfort within a space a high quality of daylight is desirable but the quantity of daylight must be minimized to avoid thermal complications. The primary concerns for natural lighting must be the following: 1) to maximize the amount of daylight penetrating through an opening, and 2) to minimize the effects of heat gain, glare, and deterioration and fading of materials.
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Warren R. Hampton is a lecturer in the UA College of Architecture and consults professionally through the Architecture Library/Center for Desert Architecture. His research interests include the impact of solar energy, daylighting, and wind/natural ventilation upon building design.
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