Successful daylighting is achieved by integrating climate, site and solar access into the design of the building envelope. In many, new buildings, the design team attempts to:

• maintain high visible light transmittance (VT) to connect occupants to the outside,
• provide views, and
• exploit natural daylighting.

• windows should face north or south
• windows facing east and west should be minimized, and
• interior surfaces should help distribute the daylight into the interior space.

The size of the window and shape of the window sill, jamb and head also enhance or detract from its daylighting ability. A conventional window can light an interior space to a depth approximately 1.5 times the window head height. Daylighting devices such as light shelves are necessary to increase penetration to 2.0 times head height and are used to improve light distribution. Interior light shelves are most effective for relatively clear climates at mid-latitudes for south orientations.

For south and north elevations, tall windows maximize light penetration. Clerestory windows also can increase the effective height of transom lites without increasing window-to-wall ratio (WWR). Even relatively low WWR provides more than ample natural daylighting, if properly oriented and directed.

Windows with deep frames, canted to the interior, will increase daylight penetration and decrease contrast between the glazed area and the adjacent wall. Glazing that abuts interior walls allows more daylight into the space and softens transitions.

Ultra-clear glass is not required to achieve the U.S. Green Building Council’s LEED® Rating System™ Indoor Environmental Quality Credit 8.1 and 8.2. In fact, an Effective Aperture target (the product of WWR and VT) of 0.30 is recommended. This leads to a darker glass in larger window openings and clearer glass in smaller window openings. In addition, glass selection can be optimized by elevation – using clear substrates and coatings on north-facing façades and “darker” glass on south, east and especially west façades.

Achieving the highest level of LEED certification, the University of Illinois’ Business Instructional Facility maximizes natural light in its four-story atrium with a structural-glazed curtainwall, while the classrooms and offices feature triple-glazed, operable windows. Other energy-saving features include solar panels to help power the building, roof plantings to insulate the building and reduce water runoff, and filtered, humidity-controlled, low-velocity air delivered at ambient temperatures to improve indoor air quality year-round.

“Basic passive integrations of smart building, shaping, siting and carefully tuning conventional wall systems -- especially the windows -- by the design team contributed more toward the sustainability and LEED Platinum certification than any high-tech element,” says Craig Copeland, senior associate at Pelli Clarke Pelli Architects, and the project’s design team leader. “The variety of glazing systems is a big contributor to the building’s overall quality and environmental intelligence. There was a great deal of consideration given not only to thermal efficiency, but also to daylighting and ventilation for enhancing the building’s learning environment.”

Whether in a school, courthouse, hospital or other commercial building, a daylight-optimized system uses high-transmission glass above a light shelf, moderate transmission below the light shelf, and sloped openings to admit more daylight. Interior, operable shades provide occupants with control of direct sun and glare. The use of high-reflectance, diffuse surfaces on ceilings, walls and window framing also helps reduce glare and contrast. For greater comfort, orient occupants at right angles to the windows used for natural daylighting.

Christopher Cuttle’s 1983 research study of office building occupants found that excessive artificial lighting was associated with short-term stress and long-term psychological distress. Of the occupants participating in the study, 80% stated a daylit environment is more comfortable. Furthermore, the research found that the study’s respondents perceived larger windows to be more desirable in an office environment.

Lower absenteeism, higher morale, increased accuracy and greater productivity also have been reported in studies of building occupants with access to daylighting. Building owners have noted higher tenant retention and lease rates for properties with green design features that include daylighting.

For owners, energy savings is another key factor in maximizing daylighting. Artificial lighting accounts for 40% of the energy used in a typical commercial building and generates at least 3 Watts of heat for each Watt of visible light. Sunlight through high-performance glazing produces 175 lumens of visible light for each Watt (175 lm/W) of heat gain that must be cooled by mechanical means. An incandescent lamp provides 14 lm/W and a high-output fluorescent lamp produces 95 lm/W. Sunlight is the most energy-efficient lighting source when used in conjunction with spectrally-selective insulating glass.

In net-zero energy buildings (facilities designed to produce as much power as they consume), a combination of effective daylighting and automatic electric lighting controls are required to achieve the objectives. Photo-sensitive controllers should be used to dim or extinguish indoor lights when unnecessary. Daylight-corrected fluorescent lamps integrate with natural daylighting strategies. Luminaires should be zoned and placed in a direction parallel to the windows.

These representative lighting strategies were employed for the National Renewable Energy Laboratory’s (NREL’s) net-zero energy, LEED Platinum certified, Research Support Facility. Every workstation has 100% daylighting and no workstation is more than 30-feet from an operable window. As a result of the whole-building design’s complex energy modeling, at least eight different types of glass were specified, including triple-pane insulated glass. Interior light “louvers” and exterior sun “bonnet” shades also were custom-manufactured and installed.

In addition to the automatic electric lighting controls, NREL building’s climate sensors automatically open the clerestory windows to purge unwanted heat from the building at night, naturally cooling it for the next business day. In the colder months, a “labyrinth” thermal storage system stores and returns heat into the building to maintain a comfortable, indoor temperature.

Energy efficiency, made possible through an integrated façade design, has a more significant impact than daylight for buildings pursuing certification through the LEED Rating System. Aluminum remains the framing material of choice for commercial and institutional buildings’ façades. Choosing aluminum with a high percentage of recycled content, durable finishes and a thermal barrier system also may assist in meeting LEED criteria. Aluminum framing using a high percentage of recycled content, and extra-wide polyamide thermal barrier, can achieve overall U-Factors below 0.20 (R-5) meeting best-in-class building envelope performance in any U.S. Climate Zone.

Optimizing for the specific Climate Zone and application, window system manufacturers offer selection tools based on the U.S. Department of Energy’s modeling software. Effective daylighting design must include thermal analysis. Solar heat always should be considered along with conductive heat loss in façade design. Exterior sun shades reject solar heat gain before it can impact a building’s HVAC loads and soften the light entering a window.

A complete daylighting system provides shading from direct sunlight, glare protection and daylight redirection. Approached from a whole-building design model, this not only integrates the exterior and interior components of the building envelope, the natural and artificial lighting strategies, and the many building team specialists, but also addresses the facility’s full functionality and intended lifecycle.

When is a new construction project a candidate for natural daylighting?
An innovative, daylighting system may be an appropriate design objective under these conditions:

• Early enough for integrated design
• Unobstructed site in mid-latitudes with a clear climate
• Primarily daytime occupancy, for example, an office or school
• Whole-building energy modeling is planned
• Expansive north- and south-facing façades
• Open plan program or glazed interior wall partitions
• Atriums, courtyards or light wells
• High ceilings
• Dimmable artificial lighting controls are available for critical spaces
• Task illumination required
• Blinds, drapes or shades to be used for glare control