A look at daylighting strategies and why few designers are actually doing it right.
Statistics on the much-heralded benefits of daylighting are not hard to come by. Studies reveal that natural light has improved test scores in schools, increased retail sales, enhanced productivity in offices, and accelerated recovery rates for healthcare patients. Click here for more information on daylighting benefits.
What we’re not as familiar with are the complexities that accompany a daylighting design and the problems of its often erroneous implementation. To reap the benefits, daylighting strategies must be designed and installed correctly--a task that isn’t as simple as it seems.
Daylighting affects the building’s architectural form; electrical, mechanical, and interior design; and facility and occupant operations, making its design a truly multi-disciplinary process.
When building team members do not collaborate to create a successful daylighting scheme, the result can be: uncontrolled direct solar penetration, untamed glare, mechanical systems over- or under-load, and architectural aesthetic divergence.
Uncontrolled direct solar penetration
Blinding occupants with direct sunlight is one of the most common daylighting failures. This often occurs when light enters from a large clerestory in a top lighting scheme or above an improperly designed lightshelf in a side lighting scheme (see sidebar on daylight strategies ). In both scenarios, the occupants cannot control the sunlight and often leave the workspace.
Take the following extreme example: A building initially designed with light shelves to control direct sun entering from a high window has to remove the shelves for budgetary reasons, without specifying a replacement strategy. The result? Occupants were blinded for five hours a day by direct sunlight. Consequently, manual mini-blinds were added that remain permanently closed, preventing daylight from entering the building altogether.
Designers of another facility applied a top lighting strategy knowing that direct sun would penetrate the space from 6 to 8 p.m. during the summer months, when no occupants would be present. After implementation, the owner added a second work shift. A change in building programming was the cause, but the result was a daylighting element that blinded its occupants.
The solution to these scenarios lies within each facility’s initial design, as well as its occupants’ expectations and understanding of the daylighting system. Owners, architects, and engineers must collaborate to create realistic daylighting solutions that will carry a building through its lifecycle.
Knowing your audience and working together will also reveal appropriate spaces where direct solar penetration may be welcome. For example, in an airport terminal, having direct sunlight is good for passengers. In an academic setting, direct sun may be welcomed in corridors, as opposed to the classroom. In a library, visitors often enjoy curling up with a book under the sun.
The human eye is an incredible organ--it can read in moonlight and in direct sun. That’s roughly a 1:1 million ratio in amount of light that the eye can adjust to. However, the eye will always constrict to the brightest thing in view, thus an evenly illuminated room will appear darker if the light source is visible than if it is blocked. Glare is simply the ratio of the brightest thing in view to the darkest.
Take a typical perimeter-enclosed office space that’s 10 ft wide by 15 ft deep. With just the light entering from the window, there are 250 foot candles (fc) at a desk adjacent to the window and 25 fc at a desk on the interior wall. With a 10:1 ratio, the interior wall will feel very dark. Most people in this situation would be uncomfortable and subsequently turn on the overhead lighting, providing an evenly distributed 75 fc. The room now has 325 fc by the window and 100 fc at the interior. With a glare ratio of 3.25:1, the overhead lighting has helped create a more balanced and comfortable light throughout the space.
When creating daylit spaces, it’s important to not only transfer the daylight from the perimeter to the interior, but also make sure that the window itself isn’t a source of bright illumination. Look at each area. See what the typical person will be seeing. Success here can be achieved though balance and the distribution of light.
As this example illustrates, integrating daylight with the building’s electrical lighting system is difficult, but crucial. In addition to combining the light distribution, the high color temperature of daylighting (5,000 to 6,000 K), makes the electric lighting (3,100 to 4,100 K) appear orange. Architects, designers, and owners need to appreciate this difference and create the appropriate balance.
Daylighting has a direct effect on both the sizing and operation of mechanical loads, as it has a lower heat ratio than electric light, providing less heat gain to its spaces. If the systems designer is assured that the building’s daylighting system is going to be used regularly, the entire system can be designed for the lesser load. If it’s subsequently not used, however, the building runs the risk of not meeting its heating and cooling needs.
The opposite holds true as well. If the systems designer creates the mechanical load for a facility that won’t use their daylighting strategies, and they subsequently do, the mechanical system will be oversized. (With an oversized system, the building runs the risk of poor system performance and a deterioration of latent capacity, i.e. dehumidification).
Again, addressing this issue is only possible by realizing the intended building operations from day one, and with the coordination of all building design disciplines.
Architectural and interior aesthetic
Integrating daylighting strategies into the architecture is another big design challenge. Proper daylighting generally requires a complimentary architectural program, a strategies have a considerable aesthetic impact. Furthermore, certain building types naturally lend themselves to the practice while others do not. A long, skinny, flat building is generally easy to daylight with a medley of different techniques. Alternately, designing a 50-story core and shell building can severely limit the number of applicable strategies.
As referenced earlier, daylighting delivers a higher color temperature of light: lots of blue and grey coloring into a building. So if the facility’s interior design scheme incorporates these colors (as do many office buildings), the light will actually make the space feel clinical. If a significant amount of daylighing is set to illuminate a specific area, try changing its color scheme to a warmer palette to achieve the desired results.
Additional daylighting resources:
Top 5 daylighting strategies
Today’s daylighting strategies are far from new. A glance at the architecture of ancient Rome reveals the foundation for many of today’s techniques. Of course, we’ve perfected them since.
1. Top lighting %%MDASSML%% Light enters the building from above through vertical (clerestories) or horizontal (skylight) apertures. Top lighting can be augmented with a luminous ceiling, where a transparent or translucent material covers the ceiling and light entering the plenum is diffused through it, gently illuminating the space. For example, a perforated metal ceiling will allow light to bounce through its holes and reach the space below.
2. Side lighting %%MDASSML%% Light enters the building from the side through both diffuse (flat white surface) and specular (mirror-like surface) light shelves. Diffuse light shelves are the most common application, as they provide adequate daylight, while reducing the amount of total illumination at the window and bouncing it deeper into the space. Specular light shelves are made of a curved, mirror-like surface that redirects the light entering the space. The specular shelf controls sunlight far better than the diffuse light shelf, as it can redirect light almost 40 ft into the space. Without proper design and execution, however, it can create additional solar penetration and glare issues.
3. Louvers %%MDASSML%% These age-old “mini-blinds” have been around for at least a millennium, used all over Europe and the Middle East. With automated controls that track the sun and tilt them according to the solar position, louvers are generally installed on the interior of the building, but can be externally mounted as well.
4. Ambient daylighting %%MDASSML%% Typically used to light large atria or other high-ceiling spaces, ambient lighting is diffuse daylight that enters a building fully shaded, generally under a large overhang that prevents direct sunlight from coming into the space. This can often provide the most consistent and gentle daylight, but has a huge aesthetic impact.
5. Transmitted daylighting %%MDASSML%% Light is collected, transmitted, and diffused. The most common applications of transmitted daylight are tubular devices where a specular, 21-in. tubular duct with a diffuser at the bottom can carry light as far as 80 to 100 ft in from the roof. Decoupling where the light is collected and delivered can provide the design with the architectural flexibility that other strategies do not.
For more information on these strategies, click here .