The use of daylighting is a high-performance lighting initiative that building professionals can implement to reduce electric lighting use and energy demands, while providing the utmost in occupant comfort, health and productivity.
Daylighting involves the design of windows, light shelves, blinds, ceilings and wall surfaces that take natural light, diffuse it via the appropriate building elements and enhance the workplace while reducing overall energy costs.
The Benefits of Daylighting
On average, lighting costs account for one-third to one-half of a building’s total electricity costs. Effective daylight distribution through reflection and diffusion can reduce electric lighting power costs and improve the usefulness of natural light.
Daylighting brings more natural and diffused light into buildings with less heat than artificial lighting, which means cooling systems use less energy to offset added heat from light fixtures. As a result, daylighting often results in lower energy bills. Estimates of overall energy savings from installing high-performance technologies and daylight controls are as high as 75%. Having control over dimming of electric lighting can also reduce peak energy use and reduce overall electrical costs.
Additionally, daylighting creates a visually pleasing, healthier and more productive environment for building occupants. Studies on the benefits of daylighting reveal higher test scores for students, increased sales for retailers, higher work output for employees, and better regulation of hormones and the circadian rhythms that control our sleep.
Measuring Daylight Autonomy
Daylighting effectiveness is measured in terms of daylight autonomy, the percentage of floor area that receives the acceptable illumination level for a critical percentage of annual occupied hours.
Spatial daylight autonomy (sDA) is a dynamic measure of the level of lighting achieved from daylighting averaged over a year. The LEED standard uses the sDA metric, requiring 50% of occupiable hours to receive between 300 and 3,000 lux.
In addition, LEED stipulates that a second measurement, annual sunlight exposure (ASE), does not exceed 1,000 lux for longer than 250 hours during the year, and in no more than 10% of occupied space.
Measuring and quantifying daylighting statistics requires an objective analysis involving all spaces. Typically, measurements are taken at around desktop height, or 2.5 feet off the floor. Collecting this data hourly across the course of a year is achievable through computer simulations.
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Daylighting Modeling Analysis Tools
Daylighting modeling tools are a good way to forecast daylighting design outcomes for an existing or planned structure. These tools can evaluate potential daylight levels at different times of the year based on weather and climate conditions, and they work in tandem with programs to project building energy implications.
A lighting designer or consultant can provide this kind of analysis. In existing buildings, this analysis would show whether there is sufficient daylighting coming in through existing openings and if blinds or other interior retrofits would improve performance at a reasonable cost. It is also important to evaluate the shading of other structures on a building.
4 Ways to Manage Daylighting
The greatest benefits of daylighting result from maximizing a building’s northern and southern windows while minimizing its eastern and western windows. Northern and southern lighting is easily controlled. Northern light is relatively diffuse, with little glare, and often does not require the use of external shading. Southern daylight is abundant, with more opportunity to direct lighting deeper into a space with light shelves or screens, but glare must be controlled to manage this opportunity. Windows to the east and west, as well as unshaded southern windows, can cause excessive glare due to low sun angles and excessive cooling loads due to difficulty in shading.
Tools are available to control each of these exposures and come in several options, including layering external light shelves, overhangs and fins, internal light shelves, blinds and shades.
1. Interior Shading and Light Shelves
Interior blinds, sunshades and light shelves provide additional options for high-performance daylighting in both new construction and existing buildings. Although internal blinds can help to control glare and light, the energy has already entered the building and must be cooled. However, if selected correctly, the blinds and shades can reflect some solar energy back outside through the glass.
Light redirection, diffusion and solar shading must be designed for each orientation and room configuration to optimize natural light distribution into the space while controlling heat and glare.
2. Interior Sunshades
Interior sunshades block glare and regulate daylight distribution inside a building but do not significantly reduce solar heat gain and often compromise views. In existing buildings, installing appropriate blinds and educating the tenant management and occupants in the use of the blinds can be beneficial. Newer blinds that can be operated from the top and the bottom (bidirectional blinds) have many advantages over conventional blinds, maintaining daylight entry above and views below.
3. Light Shelves
A light shelf is an architectural element that conveys daylight deep into a building by directing it to the ceiling. An interior light shelf can be used with bidirectional blinds to improve daylight penetration into a space. An interior shelf is the most feasible in an existing building since it is not exposed to the weather.
The combination of light shelves and shades provide far more effective daylighting and glare control, but at a higher cost. All interior systems should support manual or automatic control by the occupants.
4. Horizontal Blinds
Traditional horizontal blinds are very useful for daylighting, especially if the blades are inverted to reflect light up to the ceiling. When combined with independent upper and lower control, they can optimize daylight and view without glare. Vertical blade blinds are not effective for augmenting daylighting. They do not direct the light to the ceiling plane where it can bounce deeper into the space, and they make glare control difficult.
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Ceiling Design for Light Reflectance
The design and reflectiveness of ceilings affect the performance of lighting systems, the amount of daylighting provided to a building and ultimately its lighting energy costs.
Ceiling systems are becoming a pivotal aspect of energy conservation. Well-designed ceilings with high light reflectance:
- Improve space illumination
- Allow for fewer light fixtures
- Reduce the need for electrical light
- Reduce maintenance costs and overall cooling load
Ceilings are also key to an indirect lighting system because they reflect light into the space. The light resistance (LR) value is the percentage of light striking the panel that is reflected (from 0 to 1, with 1 being the highest reflectance), meaning the LR is the ability of a ceiling to reflect light.
Increasing the reflectance of a ceiling has a direct, positive effect on the lighting and energy used in a building. Commonly used acoustical ceilings have an LR of 0.70 to 0.81, but there are products that can reach 0.82 or higher.
The best result stems from an increase in light reflection in conjunction with indirect lighting. Studies have shown that a ceiling with a LR of 0.89 versus one of 0.75 can increase light levels by 25% with indirect lighting, 18% with direct/indirect lighting, and up to 4% with direct lighting.
Daylighting and electrical lighting are the two primary lighting resources available in a building. The light reflectance of the ceiling, floor and wall surfaces plays the second most important role in overall illumination of a room.
This article is adapted from BOMI International’s High Performance Sustainable Building Practices, part of the BOMI-HP designation program. www.bomi.org.
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