Effective daylighting is difficult to achieve as a retrofit to an existing house. Although skylights are an obvious approach, they often cause overheating in the summer and heat loss in the winter. Triangular "roof monitors," with vertical glazing, are a more energy efficient approach. If you are considering installing skylights, see the EREC reference brief, Skylights for Residences.
You can also enhance your use of existing daylight through careful interior design. Bright interior colors help reflect daylight into the interior of the house. Desks, reading chairs, and dining room tables can be strategically located to best use the available light, but beware of glare problems if locating a computer in a day lit area.
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 capacity and use by lowering a significant component of internal gains. In addition to energy savings, daylighting generally improves occupant satisfaction and comfort. Recent studies are implying improvements in productivity and health in day lighted schools and offices. Windows also provide visual relief, a contact with nature, time orientation, the possibility of ventilation, and emergency egress.
- This section includes the following:
The Daylight Zone
Window Design Considerations
Top lighting Strategies
The Daylight Zone: High daylight potential is found particularly in those spaces that are predominately daytime occupied. Site solar analysis should assess the access to daylight by considering what is "seen" from the various potential window orientations. What proportion of the sky is seen from typical task locations in the room? What are the exterior obstructions and glare sources? Is your building design going to shade a neighboring building or landscape feature that is dependent on daylight or solar access?
It is important to establish which spaces will most benefit from daylight and which spaces have little or no need for 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. Avoid placing the window in the direct line of sight of the occupant as this can cause extreme contrast and glare. It is best to orient the occupant at 90 degrees from the window. Where privacy is not a major concern, consider interior glazing (known as relights or borrow lights) that allow 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.
The floor plan configuration should maximize the perimeter daylight zone. This may result in a building with a higher skin-to-volume ratio than a typical compact building design. A standard window can produce useful illumination to a depth of about 1.5 times the height of the window. With light shelves or other reflector systems this can be increased to 2.0 times or more. As a general rule-of-thumb, the higher the window is placed on the wall, the deeper the daylight penetration.
- Window Design Considerations: The daylight that arrives at a work surface comes from three sources:
The exterior reflected component. This includes ground surfaces, pavement, adjacent buildings, wide windowsills, and objects. Remember that excessive ground reflectance will result in glare.
The direct sun/sky component. Typically the direct sun component is blocked from occupied spaces because of heat gain, glare, and UV degradation issues. The sky dome then becomes an important contribution to daylighting the space.
The internal reflected component. Once the daylight enters the room, the surrounding wall, ceiling, and floor surfaces are important light reflectors. Using high reflectance surfaces will better bounce the daylight around the room and it will reduce extreme brightness contrast. Window frame materials should be light-colored to reduce contrast with the view and have a non-specular finish to eliminate glare spots. The window jambs and sills can be beneficial light reflectors. Deep jambs should be splayed (angled toward the interior) to reduce the contrast around the perimeter of the window.
Remember that the most important interior light-reflecting surface is the ceiling. High reflectance paints and ceiling tiles are now available with .90 or higher reflectance values. Tilting the ceiling plane toward the daylight source increases the daylight that is reflected from this surface. In small rooms the rear wall is the next important surface because it is directly facing the window. This surface should also be a high reflectance matte finish. The sidewalls 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 so select light-colored materials.
- Suggested Room Surface Reflectance:
Ceilings: > 80%
Since light essentially has no scale for architectural purposes, the proportions of the room are more important than the dimensions. A room that has a higher ceiling compared to the room depth will have deeper penetration of daylight whether from side lighting (windows) or top lighting (skylights and clerestories). Raising the window head height will also result in deeper penetration and more even illumination in the room. Punched window openings, such as small, square windows separated by wall area, result in uneven illumination and harsh contrast between the window and adjacent wall surfaces. A more even distribution is achieved with horizontal strip windows.
Effective Aperture: 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 defined as 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 to the total wall area where the window is located. For example, if a window covers 25 square feet in a 100 square-foot wall then the WWR is 25/100 or 0.25. A good starting target for EA is in the range of 0.20 to 0.30. For a given EA number, a higher WWR (larger window) results in a lower visible transmittance.
Example: WWR = .5 (half the wall in glazing),
VT = .6, EA = 0.3
Or WWR = .75, VT = .4 for same EA of 0.3
Typically lowering the visible transmittance will also lower the shading coefficient but you must verify this with glazing manufacturer data since this is not always the case.
Figure 104: Lighting Shelves: Light bounces off the top of the light shelf into the ceiling of the first floor offices. The overhang shades the window below it.
Light Shelves: Since luminance ratios or brightness is a major consideration in view windows, it is often wise to separate the view aperture from the daylight aperture. This allows a higher visible transmittance glazing in the daylight aperture if it is out of normal sight lines. Since the ceiling is the most important light-reflecting surface, using this surface to bounce daylight deep into the room can be highly effective. Both of these strategies are utilized in light shelf designs. 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 than interior shelves. A combination of exterior and interior will work best in providing an even illumination gradient.