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Daylighting refers to the practice of using natural daylight entering the interior of buildings through windows, skylights, and other openings to satisfy interior illumination needs. Prior to the advent of electrical lighting in the late 19th century, when the only other light sources were candles, fires, and gaslights, buildings were generally designed with greater care given to daylighting needs. Throughout large stretches of the 20th century, the availability of cheap electric lighting from incandescent and fluorescent sources led to the construction of a huge class of buildings still in existence today that are no conducive to effective daylighting practices. 

Now, in the age of more expensive energy and an urgent need to reduce greenhouse gas emissions from the use of electricity, it is time to reintroduce the art of  good daylighting into the designs of our commercial buildings and to retrofit existing buildings to take advantage of daylighting opportunities. 

Some of the content and graphics in this section is based on content from the Daylight Pattern Guide website. 

Systems for the delivery of daylight are generally classified as either sidelighting or toplighting. There are also innovative systems that use fiber-optics or reflective tubes to bring daylight in interior spaces, and some very advanced systems involving mirrors that track the sun and guide daylight to interior spaces deeper in the building.

Sidelighting generally refers to windows of all sizes and shapes as well as clerestory openings as illustrated below. Windows may additionally be outfitted with exterior and/or interior lightshelves to direct daylight deeper into the space. In addition to daylight, sidelighting solutions generally also provide views to the exterior. 

Toplighting refers to skylights, roof monitors, and atria/courtyards that bring in daylight from above, generally without affording views. Toplighting openings can provide access to the brightest part of the sky and typically are exposed to solar daylight for longer periods than any other glazing orientation. 

Let's start with a simple question: Why should we use electric lighting to illuminate office spaces when free daylight is available to do the same? Rational economic behavior would surely dictate that the free resource is the preferred choice, wouldn't it? The fact is, however, that huge swaths of New York City's and the nation's buildings are artificially lighted even during those times when sufficient daylight is available. By taking advantage of the tremendous daylighting potential in our existing buildings, we can reduce energy use, save operational expenses, and reduce greenhouse gas emissions on a very significant scale.

Lighting in New York City's office buildings consumes a tremendous amount of energy (more than any other electrical end use) and accounts for nearly one third of Con Edison's commercial sector electricity delivery, according to a 2010 Con Edison study. Furthermore, New York City has a unique concentration of office buildings - the biggest office market in the United States by a significant margin. 

Not only is the New York City office market enormous, decision-making for these buildings is concentrated among a relatively small number of owners, managers, and tenants. According to a 2010 report, a handful of self-managing building owners control approximately a third of all commercial real estate in Manhattan, and a relatively small group of large third-party managers are responsible for almost all of the borough's remaining office space. 

To increase the efficiency of the commercial lighting systems in the city's commercial building stock, Local Law 88 was passed in 2009 requiring that lighting systems be brought into compliance with the energy code by 2025. The law is estimated to affect 1.25 billion square feet of space and represents a singular opportunity to realize substantial energy savings including peak demand reductions. 

In summary, the scale of New York City's office market, coupled with regulations requiring broad retrofits of lighting systems and the relatively concentrated control over the city's commercial real estate, present an unprecedented opportunity to drive market transformation of lighting control systems and to meaningfully reduce New York City's carbon footprint.

Taking advantage of daylighting opportunities has strong potential to reduce overall energy use and to substantially reduce demand during peak periods. Lighting energy savings in office buildings, enabled by daylighting, typically result from: 

  • Reducing the connected lighting load;
  • Reducing the hours that lights are in use; and
  • Providing only the lighting output actually needed for a given task (typically through dimming).
Electricity for larger buildings is billed for two quantities: the amount of electricity consumed (energy used, measured in kilowatt-hours, or kWh), and the peak demand  (expressed as kilowatts, or kW) set in any given billing period. For most customers, "peak demand" is the highest amount of kW used during any 30 minute period in a given months, and for a large number of the City's office customers the highest demand set in any month is carried as the per kW charge for the full year. Peak periods are when all available utility generating equipment needs to run, including the oldest, least efficient, and dirtiest power plants feeding into the grid, so reducing buildings' peak load has significant environmental benefits. 

The following charts, developed by the Building Energy Exchange for the Let There Be Daylight report, illustrate that the daily distribution curve for energy use for lighting follows a building's overall energy demand curve, and that daylight is typically available when demand for lighting in commercial buildings is highest during the mid-day working hours. This temporal coincidence creates a tremendous potential to drive down energy consumption in commercial buildings by using daylight instead of electric light when available. 

Furthermore, reducing lighting energy can result in lower internal loads and a reduced demand for air conditioning leading to further energy savings and peak demand reduction. However, this benefit has to be carefully balanced against the need to limit solar heat gain through windows. 

In addition to the economic benefits from reduced energy use, studies indicate that interior spaces with good daylighting and access to views enhance the comfort, well-being, and productivity of the occupants. While demonstrating the direct causal link between daylight and occupant health and productivity is difficult given the wide variety of factors that impact such metrics, strong correlations have consistently been shown.

Because the cost of employees heavily outweighs all other business expenses, even mild increases in productivity (through impacts like reduced absenteeism) can have positive financial impact that dwarf the typical ROI considerations of energy conservation measures. 

New York City's existing buildings are well situated to capitalize on the benefits of daylight. Many of the City's older office buildings were designed to utilize daylight, as they were built in electric lighting's infancy or prior to the advent of cheap energy that made general reliance of electric lighting possible. As for new construction projects, obviously there are no hard barriers that would prevent project teams from designing and implementing advanced (day)lighting systems to take full advantage of inherent daylighting opportunities. 

Prior to the 1950s, buildings in New York City and throughout the US were generally designed for daylighting with high ceilings and floor plate layouts where all workstations benefited from relative proximity to perimeter punched windows, to provide occupants with access to natural ventilation and daylight. The introduction of cheap energy, fluorescent lighting, and mechanical ventilation subsequently reshaped commercial buildings. As a result, windows were separated from their once essential function of providing fresh air and daylight.

At the sane time, executive offices moved to the perimeter, while most workers were pushed deeper into the interior of the floors. The nature of office work has also changed: It used to be primarily a "headsdown" activity, but the widespread use of desktop use computers now means that most people work facing forward and are more sensitive to glare from windows and artificial lights.  Glare control is therefore a higher priority today.

Furthermore, over the past decade we have witnessed an unprecedented trend toward the "all-glass" building, be it commercial or residential. Floor-to-ceiling glazing, however, is seldom the best choice for effective daylighting and thermal control, since all-glass curtain walls, even those utilizing high-performance systems, experience greater thermal gains and losses and require more hours of shading. Glass closest to the ceiling if the most effective location to maximize daylight penetration, while glass closest to the floor typically serves no useful purpose. Therefore we find that, ironically, buildings constructed before the 1950s with punched window facades are often more conducive to daylighting than newer all-glass structures. 

To maximize energy reductions from the installation of smart modern lighting systems, it is imperative to move away from a paradigm that for too long guided lighting designs: Basing the determination of required electric lighting levels in commercial office workspaces on the assumption that no daylight is ever entering the space - in effect, designing to satisfy the worst case conditions which occur at night when most office buildings are unoccupied. Rather, good daylight design should consider the amount of available daylight to realize designs where electric light and daylight "team up" to deliver efficient, high-quality lighting environments. 

Critical considerations for effective daylighting design includes overall building orientation and more specifically, the orientation of the windows in the building facades relative to the sun, as well as floor plate geometry parameters, such as perimeter to core depth, aspect ratio, and programmatic organization. 

In general, useful daylight levels decrease with the distance from the perimeter openings. A good rule of thumb is that useful daylight levels are not available past twenty feet from a window. Deep floor plates with occupied areas far away from the windows are therefore not well suited for daylighting design. "Section depth" refers to the distance between perimeter windows and the innermost wall of an occupied space, often a core wall. In most office building the section depth is the most crucial determinant of whether all occupants will be able to benefit from daylight and views. 

Many office buildings have deeper floor plates with larger section depths, thereby creating interior areas away from the perimeter where daylight is not available. From an analysis of representative New York City office buildings we develop the concept of "daylight zone area" (DZA): the area within fifteen feet of the building perimeter walls with windows where electric lighting could be significantly reduced or turned off. below are some illustration of the DZA in commercial floor plan layouts that are common in New York City. 

A common misperception about building facades is that more glass is always better. In fact, too much glazing will likely introduce unwanted glare and solar heat gain. Therefore, the area of glazing relative to the total wall area, known as the window to wall ratio (WWR), the choice of glazing, and the availability and use of exterior shading devices must be carefully balanced.

The window to wall ratio is a key parameter when designing for daylighting. Beyond the ratio itself, the distribution of the wall area across the facade is also very important. Window head height and spacing are key factors. Higher window heads allow daylight to penetrate deeper into the space, and a uniform window spacing or continuos band will result in more uniform daylight distribution patterns. However, the tops of windows in many older buildings are often obstructed by dropped ceilings necessitated by the installation of upgraded HVAC equipment. 

Some designers working with deep floor plates may be tempted to over-glaze the perimeter in an effort to drive light deeper into the building. However this commonly increases glare and contrast to a degree that blinds are deployed continuously to maintain visual comfort, thus defeating the purpose of glazing and substantially compromising both daylight illuminance and views. A far better option is to use less glazing to provide even distribution of daylight across a realistic daylight depth and to ensure that occupied areas are situated within reach of daylight. Rather than trying to "get more light into the building", a better strategy is to "get more of the building into the light". 

In most climates, control of direct sun penetration is a key design objective to avoid unwanted heat gain and excessive glare. Interior blinds can be very effective at preventing glare but are only marginally effective at minimizing heat gain. Installing exterior automated motorized blinds can be a very good strategy to minimize both heat gain and glare. However, these devices can be cost prohibitive, and the motorized mechanisms may fall into disrepair. Fixed architectural shading strategies, such as exterior overhands, vertical fins and lightshelves can minimize both heat gain and glare and greatly reduce the number of hours per heat that manually operated blinds as required. 

Interior partitions and open office furniture have to be designed carefully to ensure the preservation of daylight and views. In side-lit spaces, office partitions should ideally be kept low (42 inches or less) and unobtrusively parallel to the direction of daylight entering the space. If higher partitions are required for privacy or to create a sense of enclosure, they should still be oriented perpendicular to the perimeter glazing.

Furthermore, interior finishes of partitions, furniture, ceiling and flooring should be selected considering their reflective properties. Light, diffuse reflective surfaces are considered most conducive to successful daylighting outcomes.

Workstations should be designed so that the primary visual field (the direction that most occupants face while performing visual tasks) is parallel to daylight openings wherever possible. This helps avoid visual discomfort from building users looking into their own shadow, or worse, from the excessive contrast that can occur when a visual task area (commonly a computer screen) is surrounded by the brightness of a view to the exterior.

In all daylighting applications the desire to bring in natural light has to be balanced against the need to avoid glare. Therefore, controlling direct beam sunlight is critical to providing a visually (and thermally) comfortable space. Interior blinds afford a range of methods to control direct sunlight and redirect diffuse daylight.

In office spaces the most common type of blinds are standard louver blinds or fabric roller blinds. These are most commonly manually operated, and one of the primary barriers to sustaining daylighting performance in office spaces over the long term is the fact that office workers cannot be relied upon to operate the blinds in accordance with daylighting objectives. In extreme cases this can render an otherwise well daylit space into one that is virtually non-daylit. Advanced motorized roller shades or louver blinds offer a solution to this problem, as they can be programmed to be repositioned open after a set time (or at least each morning), or to use irradiance sensors to control blind position in response to sky condition and sun position.

To take full advantage of daylighting opportunities in a commercial building or space the systems controlling the infrastructure of electric lighting have to be designed to accommodate that goal. Older systems with inflexible control mechanisms may not allow for practical ways to turn off or dim electric lighting when daylight is available to provide sufficient illumination at no cost.

The following is a brief list of lighting control strategies conducive to daylighting:
  • Proper Zoning: Zoning refers to the creation of several lighting control zones that can be independently controlled to turn on/off and perhaps dim all the luminaires within it. Zoning is an essential lighting control prerequisite that does not result in energy savings by itself, but that enables other control strategies to be effective. It is particularly important for larger spaces where tasks, occupancy, access to daylight, and the need for artificial light at different times may vary across the floor plate.
  • Vacancy Controls require that occupants manually turn on the lights in their space empowering them to keep them off during those times that they feel the available daylight levels are sufficient for the tasks to be performed.
  • Daylight Harvesting Controls enable the reduction of artificial lighting levels during times of available daylight, employing photosensors to monitor interior lighting levels and reduce electric lighting levels in proportion to the available daylight.
  • Personal Dimming and Tuning: Dimmable lighting fixtures that can be controlled by the occupants enable them to tune the lighting levels in their workspace to their needs.
For a more in-depth description of lighting control strategies, refer to the Lighting Controls section of this website. 

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Welcome to the Building Energy Exchange! Located in downtown Manhattan, we’re a resource center dedicated to advancing efficient energy and lighting design in buildings. Explore this site for helpful case studies and reports, check out our calendar for a wide range of activities and events, and follow our lively blog, Insight. Our site is growing all the time—drop us an email to contribute or comment!