Letting light into a house is an important function of windows. Natural light is important to the health and well-being of the occupant. Even though people have become more reliant on electric light in their houses, good home design can provide most, if not all, the needed daytime light. The introduction of natural light is also a powerful architectural tool in shaping and defining the interior spaces. However, providing daylight requires thoughtful window placement and interior design that addresses a number of concerns such as visual comfort, balanced light levels, color, and fading of furnishings.
Providing Balanced Lighting
A balance of light is important both for visual comfort and to perform visual tasks. Too much contrast between dark and light, from very bright light to very dark shadows, can be uncomfortable for the eyes. Although the eyes can adjust to changes in light levels very quickly and can simultaneously see a wide range of light intensities, the human eye is more comfortable with a ratio of the brightest to the darkest level of no more than twenty to one. Light levels for reading in the home or office range from 100–2000 lux, and an overcast sky can provide outdoor levels of 5000–20,000 lux. A beam of bright sunlight will provide up to 100,000 lux on a surface. In order for direct sunlight to be useful visually, it should be diffused and reflected around the room. When the sunlight is spread out over a larger area, it provides more comfortable light levels.
It is important to recognize that different uses of interior spaces have different ranges of acceptable lighting level. In a corridor, the amount of illumination can range above or below desired levels with little adverse impact. However, in a home office with a computer screen, visual comfort depends on careful control of brightness ratios. Artwork and artifacts, particularly those with paints, dyes, paper, or fabric that are light-sensitive or susceptible to fading, must also be protected from excessive light levels.
The balance of light in a space depends on the overall number and size of windows, their location, and the average reflectance of the interior surfaces of a room. A room with only one window will inevitably have bright areas close to the window and dark areas farther from the window. This gradation in light will be further exaggerated if the room has dark surfaces and furnishings. An improved balance of light can be created by providing light from at least two directions, such as windows located on different walls or a skylight balancing the light from a window. Shadows created from the first window source are balanced by light from the second direction, and the overall contrast is reduced.
As the sun moves through the sky during the day and in different seasons of the year, its angle of penetration into a room changes significantly. In the middle of the day during the summer in the northern hemisphere, for example, the sun comes in high overhead and strikes near the sill of a south-facing window. However, the low sun to the east in early morning and to the west during the afternoon will penetrate deep into a room, strike back walls, and may shine directly into people's eyes. In the winter, the sun is lower in the sky throughout the day for all orientations, and tends to penetrate more deeply into rooms. This may provide useful heat but must be managed to control glare. In order to be visually useful, direct sunlight must first be reflected off of a floor or wall and then diffused around the room. For instance, east- or west-facing windows can be placed near a corner with a north wall of a room, which will catch and reflect the sunlight before it penetrates too deeply. In addition to designing the room itself properly for glare control, venetian blinds, translucent shades, and drapes can be used to diffuse entering light. In hot climates, it is usually best to avoid direct sun completely during the peak cooling season.
Using Reflective Outdoor Surfaces to Increase Daylight
Direct light from the sun and sky is an important source of daylight, although in hot climates, it is usually best to avoid direct sunlight completely during the cooling season. In all climates there are situations in which the light reflecting off of outside surfaces can be very useful. A view of a light-colored sunlit wall can actually provide more light than a view of the north sky. And large reflective horizontal surfaces outside a window, such as snow, a lake, or a sandy beach, greatly increase the amount of light entering the window.
People often think of south windows as the sunny windows; however, the view from a south window might look out at the dark, shady "back" of a building, which is in stark contrast to the bright, sunlit foreground. On the other hand, a north window can look out onto a brightly sunlit wall or garden, providing both a great deal of reflected light and a cheerful view of sunlit flowers or surfaces.
Avoiding Glare from the Sun
While sun penetration at south windows in winter can create some glare problems, east- and especially west-facing windows are usually the greatest offenders year round. People inevitably orient windows toward the most interesting view. But when that view includes a reflective surface such as snow, water, or sand, especially if the window faces the east or west, the penetrating low sun problem is intensified. These windows create the most difficult situations for solar control. While the view may be highly desired, the glare and excess heat of the sun and its reflections are not. Typically, these windows require active operation of shades or blinds by the homeowner, and, until recently, the only permanent solar control technology was tinted or reflective glazing, plastic films applied to the glass, or screens.
Direct sun at low angles can be blocked by trees, shrubs, or garden walls. Such landscape elements can be strategically placed to reduce glare as well as heat gain through east- and west-facing windows. The most important time to block low-angled sun is in the summer when the sun rises and sets farther to the north of direct east and west, so plantings should be located to account for this pattern.
Diffusing Direct Sunlight
Overhangs and other solid external architectural elements can block direct sunlight completely for some time periods, while more open elements such as lattice structures diffuse the daylight before it enters the windows. This can assist in reducing glare from the direct sun or sky, and can illuminate spaces with a larger and more diffuse light source. Dark-colored woven fiberglass or perforated metal screens mounted on the exterior of windows can also reduce glare while still maintaining a high degree of visibility to the outside on sunny days. Light-colored screens diffuse the transmitted light but do not allow as much view to the outside.
Interior shades, drapes, or blinds can block or diffuse direct sunlight. The ideal drapery to reduce glare from bright windows, while still allowing a clear view out, is a loosely woven dark-colored drape. A loosely woven light-colored drape will diffuse the daylight from the window about the room and provide maximum privacy. However, light-colored drapes will appear very bright in direct sunlight and it may be difficult to see through them. Roll-up shades can be translucent, allowing some diffuse light to enter, or opaque, which block all light. Shades can also be made from woven screen materials in varying densities and colors, producing a range of light control and visibility. Reflective and tinted plastic roll-up shades reduce the sun's intensity and provide a clear view out, but do not diffuse or scatter the incoming light. When adjusted to the correct angle, horizontal venetian blinds or miniblinds can block direct sunlight but permit diffuse light to enter the room and often provide views out. Vertical blinds serve a similar function and are particularly useful for controlling low-angle sunlight at east- and west-facing windows.
Translucent glazing materials, such as frosted or patterned glass, can diffuse sunlight very evenly. Glass block and translucent fiberglass panels also provide light diffusion and visual privacy. The use of translucent glazings for skylights and clerestory windows, which are not in the visual field, is an excellent way to diffuse sunlight evenly throughout a space. When used on windows at eye level, however, the materials may become too bright for visual comfort. A frosted window in the low afternoon sunlight will seem to glow with the intensity of a searchlight. Because of this effect, clear glass with additional interior shading devices generally allows for better control of the light from sunlit windows.
Skylights and roof windows can provide high levels of daylight, but view is not a concern as it is with conventional windows. The direct light from above can be diffused by using frosted glazing, by using a light well where light-colored vertical surfaces just beneath the skylight reflect and diffuse sunlight, or by using interior shades or blinds operated from below. A single skylight is far more effective at lighting a larger space on a sunny day if the sunlight is diffused either by the glazing or the light well surfaces. The size, shape, and color of light well surfaces influences their ability to diffuse and distribute light.
When a skylight and light well are difficult to incorporate into the room design, light tubes are becoming a popular approach to admit sunlight into a room. A clear plastic dome (normally 10–14 inches in diameter) is mounted over a hole in the roof and is connected to a small light diffuser in the ceiling by a highly reflective tube. Sunlight enters through the domed opening and bounces numerous times until it emerges at the ceiling diffuser. Although some of the light is lost in the multiple reflections in the tube, these systems can provide as much light as a bright lighting fixture on a sunny day. The overall efficiency of the system depends on the time of day and location of the domed opening as well as the length of the tube and its internal reflectivity.
The American Architectural Manufacturers Association's (AAMA) Skylight Council has a fact sheet, "Daylighting Basics: Daylighting and Energy Savings" that provides useful information on top lighting and its benefits. The document is available to download from AAMA's web site.
Using Light from the Sky
Light from the sky, as distinguished from light from the sun, is cooler, gentler, and diffuse. While sunlight is normally considered to appear white, the early morning and late afternoon sun takes on a yellow or red hue, an effect that can be heightened by dust and pollutants in the atmosphere. Depending upon the position of the sun and type of clouds present, light from the sky will provide illumination levels of 5000–20,000 lux, or 5–20% of that provided by bright direct sunlight. Light from a clear blue sky has the additional advantage that there is more visible light, with a significantly smaller infrared component.
In situations where daylight is desired with minimal solar heat gain, north windows can provide the best quality of daylight of any orientation. While the use of north-facing glass may be desirable in terms of daylighting and avoiding solar heat gain, it is not an effective strategy for providing useful solar gain in winter.
You can increase the levels of natural daylight in your home by the use of certain interior design strategies. You can use interior windows to bring natural light deeper into spaces. These windows can be clear or translucent, large or small, operable or fixed, placed in various places on a wall surface — all part of the design of the space.
You can also increase the light levels in your home by the choice of interior surface materials. You can use materials such as light-colored or reflective paints or wallpaper, mirrors, metallic, or shiny surfaces that reflect or redirect light.