D.6.4  Designing for Daylighting

When properly designed and effectively integrated with the electric lighting system, daylighting can offer significant energy savings by offsetting a portion of the electric lighting load. A related benefit is the reduction in cooling system capacity because the electric lighting operates less, lowering a significant component of internal gains. In addition to energy savings, daylighting generally improves occupant satisfaction and comfort. Recent studies show improvements in productivity and health in daylit schools and offices. Windows also provide visual relief, contact with nature, time orientation, the possibility of ventilation, and emergency egress.

Dayligh entering the space through clerestory windows is reflected off the bright white ceiling to provide diffuse daylight throughout the ACE Hardware Store at the BigHorn Center in Silverthorne, COlorado

Consider daylighting possibilities for every space unless a strong programmatic function does not allow daylight.

• Within the spaces that can use daylight, place the most critical visual tasks in positions near the window.

• Try to group tasks by similar lighting requirements and occupancy patterns.

• Carefully place the window in relation to the occupant to avoid extreme contrast and glare.

• When possible, locate computer monitors so that they are facing a window.

• Consider interior glazing that allows light from one space to be shared with another. This can be achieved with transom lights, vision glass, or translucent panels if privacy is required. Hallways can often be lit entirely by shared light.

D.6.4.1  Window Design Considerations

A standard window typically provides daylight illumination to a depth of about 1.5 times the distance between the floor and the top of the window. Light shelves, described later, or other reflector systems can increase this distance two or more times. As a general rule of thumb, the higher the window is placed on the wall, the deeper the daylight penetration. In most cases, daylighting designs are most effective within the first 25 feet from the window.
Daylight within a space comes from three sources:

• The exterior reflected component includes ground surfaces, pavement, adjacent buildings, wide window sills, and objects. Remember that excessive exterior reflectance can result in glare.

• The direct sun/sky component is typically blocked from occupied spaces because of heat gain, glare, and ultraviolet (UV) degradation issues. Direct sun/sky is acceptable only when using patterned glass to diffuse the light.

• The internal reflected component is the daylight reflected off the surrounding wall, ceiling, and floor surfaces. Surfaces that are reflective but not specular reflectors will bounce the daylight around the room without creating uncomfortable bright spots.

Window frame materials should be light-colored to reduce contrast with the view, and should have a nonspecular finish to eliminate glare. The window jambs and sills can be beneficial light reflectors. Deep jambs should be splayed (angled to open toward the interior) to reduce the contrast around the perimeter of the window.

Sources of Daylight Contributions

The most important interior light-reflecting surface is the ceiling. High reflectance paints and ceiling tiles are now available with 0.90 or higher reflectance values. Tilting the ceiling plane toward the daylight source creates a “bright” ceiling and improves the feeling of “brightness” in the space.
In small rooms, the rear wall is the next most important. The rear wall should have a high-reflectance matte finish. The side walls, followed by the floor, have less impact on the reflected daylight in the space.

Major room furnishings such as office cubicles or partitions can have a significant impact on reflected light. Light-colored materials are important on those components as well. Here are some suggested room surface reflectances:

• Ceilings: > 90%

• Walls: 50-70%

• Floors: 20-40%

• Furnishings: 25-45%

The proportions of the room are more important than the dimensions. A room that has a high ceiling relative to its depth will have deeper penetration of daylight whether from sidelighting (windows) or toplighting on the reflected daylight in the space. (skylights and clerestories). Raising the window head height will also result in deeper penetration and more even illumination in the room.

D.6.4.2  Effective Aperture

Sources of Daylight Contributions

Light shelves on this buildings bounce light onto the ceiling for deeper daylight penetration.

Separating the view aperture from the daylight aperture with a light shelf can improve the daylighting effectiveness by bouncing light deep into the room while at the same time maintaining comfortable luminance ratios through the view aperture.

Skylight Performance

Lightpipe Cross-Section

Clerestory section shown with the structure passing through and supplemental night time lighting.

Clerestory section shown with the structure passing through and supplemental night time lighting.

Sawtooth section shown with vertical north-facing glazing, supplemental ngith time lighting, and roof-mounted solar panels.

One method of assessing the relationship between visible light and the size of the window is the effective aperture method. The effective aperture (EA) is the product of the visible transmittance and the window-to-wall ratio. The window-to-wall ratio (WWR) is the proportion of window area compared with the total area of the wall in which the window is located. For a given EA number, a higher WWR (larger window) requires less visible transmittance.

D.6.4.3  Light Shelves

A light shelf is a horizontal light-reflecting overhang placed above eye-level with a transom window placed above it. This design, which is most effective on southern orientations, improves daylight penetration, creates shading near the window, and helps reduce window glare.

Exterior shelves are more effective shading devices, and actually increase the amount of light through the daylight aperture as compared to interior shelves. A combination of exterior and interior shelves will work best in providing an even illumination gradient.

D.6.4.4  Toplighting Strategies

Large single-level floor areas and the top floors of multi-story buildings can benefit from toplighting. The general types of toplighting include skylights, clerestories, monitors, and sawtooth roofs.

Horizontal skylights are an energy problem because they receive maximum solar gain in summer at the peak of the day. Their daylight contribution also peaks at midday and falls off severely in the morning and afternoon.

High-performance skylight designs address these problems by incorporating translucent insulating material, reflectors, or prismatic lenses to reduce the peak daylight and heat gain while increasing early and late afternoon daylight contributions.

Another option is lightpipes, in which a high-reflectance duct channels the light from a skylight down to a diffusing lens in the room. These may be advantageous in deep roof constructions.

A clerestory window is vertical glazing located high solution to control this glare is to install interior vertical overhead. A properly designed horizontal overhang can effectively shade south-facing clerestories from direct sunlight.  It is best to slope the interior north clerestory wall to reflect the light down into the room. Use light-colored overhangs and adjacent roof surfaces to improve the reflected component.
If calculations show that direct winter sun on work surfaces is a problem, use a lightly patterned glass in the clerestory windows to diffuse the light. Another solution to control this glare is to install interior vertical baffles at the clerestory windows.

North-facing clerestory windows do not need over- hangs.  Direct gain through these windows is rarely a problem. East- and west-facing clerestory windows are problematic because of glare issues and excessive solar gain. Use simulations to determine if east- and west-facing clerestories are beneficial.

A roof monitor consists of a flat roof section raised above the adjacent roof with vertical glazing on one or more sides. This design often results in excessive glazing area when glazed on all sides, which leads to higher heat losses and gains than a clerestory design. The multiple orientations of the glazing can also create shading problems.

A sawtooth roof is an old design often seen in industrial buildings. Typically one sloped surface is opaque and the other is glazed. A contemporary sawtooth roof may have solar-thermal or solar-electric (photovoltaic) panels on the south-facing slope and north-facing daylight glazing. Unprotected glazing on the south-, east-, or west-facing sawtooth surface results in high heat gains.

In general, designs accommodating vertical glazing are preferred. Vertical glazing minimizes unwanted solar gains and reduces the potential for maintenance problems.

D.6.4.5  Daylighting Integration Issues

A daylit building without an integrated electric lighting system will be a net energy loser resulting from heat losses and gains through the windows and the electric lighting system operating more than needed. An integrated lighting system has energy-efficient lighting fixtures that operate only to supplement the available daylight. The savings from reducing the electric lighting load will offset - and exceed - the added thermal loads. See Appendix D, section 7 for lighting control strategies.

Coordinating the electrical lighting system design with the daylighting design is critical for the success of the system. The layout and circuiting of the lighting should correspond to the daylight aperture. In a typical sidelighting design with windows along one wall, it is best to place the luminaires in rows parallel to the window wall, circuited so that the row nearest the windows will be the first to dim or switch off, followed by successive rows.

To maintain the designed performance of the daylighting system, the person responsible for interior finishes and furnishings must be aware of the desired reflectance values. Dark interior finishes can compromise an otherwise good daylighting design.

D.6.4.6  Daylighting Resources

Physical models are an effective way to analyze day-lighting performance. Even simple models can begin to inform the designer of how daylight will behave in the building. For more detailed studies, the model should be at least 1” = 1’ scale. This size model allows easy viewing to assess the daylight contribution and potential glare sources. The daylight apertures must be accurately modeled and the model must include the reflectance values of the surface materials. The model can then be tested on the actual site or under artificial sky conditions in a daylighting laboratory. A sundial for 38.8° North latitude (Fort Carson) attached to the model base allows the designer to simulate various dates and times of the year.
Computer analysis is another method of testing day-lighting solutions. Several lighting programs such as Lumen-Micro,Radiance, and Lightscapehave daylighting calculations. Typically, a three-dimensional digital model is constructed using computer-aided design software that is then imported into the lighting soft-ware. The programs then require the operator to define all surface characteristics, sky conditions, location, date and time. Lumen-Micro and Radiance can produce photo-realistic renderings of the proposed design, while Lightscape is useful for less detailed analyses early in the design process.

Appendix d