Good daylighting design saves energy in many ways. The obvious one is lighting energy, which can represent a major portion of the total energy consumed by many buildings.

With daylight comes free heat, which can save energy during the cooler seasons. Some products used mainly for providing natural light can also significantly reduce the amount of heat lost when substituted for more traditional products.

This daylighting-focused web page addresses key differences between fenestration products installed primarily vertically (windows, doors, curtain walls and storefronts) and those installed primarily overhead (skylights, roof windows and tubular daylighting devices [TDDs]) and their relation to daylighting and energy savings.

The common set of terms used by daylighting professionals for these two fenestration categories is side-lighting and top-lighting respectively. Generally, all fenestration products can be sources of quality daylight and passive solar energy; however, additional factors need to be considered when choosing side-lighting and top-lighting products.

The basis for the difference is quite simple; side-lighting products face the horizon and top-lighting products face the sky.

Side-lighting from windows and doors provides daylight and solar energy along the perimeter of a building. Good daylighting design should consider these side-lighting characteristics:

• Most daylight is provided through ambient lighting from the sky. The amount of daylight available will vary throughout the day depending on the direction the fenestration is facing. External obstructions may reduce access to the available daylight.
• Orientation (north, east, south, west) with respect to the sun’s path is a critical factor.
• Shading to avoid excessive glare may be necessary when the sun is low in the sky.

Top-lighting can provide daylight and solar energy throughout the interior of a low rise building, on the top floor of a building or in an atrium. It should complement side-lighting in any good daylighting design, where conditions permit:

• Daylight is consistently available throughout the day from both ambient lighting from the sky and direct exposure to the sun.
• Shading to avoid direct sun may be necessary when the sun is high in the sky. Modern transparent and/or translucent glazing can be utilized to avoid glare, aid in capturing sunlight at low angles and diffuse light to wider areas of floor space.
• Shading accessories can be used on many product options to manage light levels when desired or necessary.
• Even on a cloudy day, top-lighting can provide excellent daylight.

Effective daylighting design results in a system that includes side-lighting, top-lighting, electric lighting controls (automated if possible) and a building explicitly designed to optimize the usefulness of daylight.

In applications where daylighting is the primary goal, factors which impact the efficient application of side-lighting and/or top-lighting include:

Building Purpose

Non-residential
In a retail building, for example, daylighting is typically focused on the public retail areas and not on the lesser-used areas for storage and offices. Top-lighting can provide daylighting into any interior floor space area, a critical consideration for large floor plans.

Residential
Daylighting is most beneficial in common areas. Top-lighting is particularly useful further from the perimeter of the building.

Building Features

Ceiling height, surface colors and textures, light shelves, room dividers and partitions all impact the usefulness of the daylight available to the space and should be considered in daylighting design. All of the newest U.S. energy conservation codes specify that many suitable buildings must now use skylights (and automated lighting controls) over specified portions of the occupied floor area because of the energy savings that are achievable in such buildings.

Building Siting/Orientation

Optimizing exposure to the sun’s path is critical to any daylighting system. Top-lighting obtains consistent exposure for long periods of the day.

Multi-story vs. Single-Story

Side-lighting is easily provided on all floors of a multi-story building. However, through proper building design and/or use of integral light wells or TDDs, top-lighting potential still exists for lower floors in multi-story buildings.

Typical Climactic and Daylight Conditions

Even in moderate climates with typically cloudy weather trends, top-lighting provides excellent daylighting potential all day long.

Interior Climate Control System

Appropriately designed HVAC systems are critical in any energy conservation effort.

Non-residential
Automated lighting controls are critical in multi-user environments such as offices and retail spaces to ensure lighting energy is not used when daylighting is sufficient. Annual energy modeling approved for code compliance on these buildings takes lighting energy into account, so it is feasible to justify good daylight on this basis alone.

Residential
Although automated systems are becoming more readily available, generally no additional automated controls are required as homeowners are increasingly conscious about energy conservation and savings. In contrast to non-residential applications, energy modeling for residential code compliance only accounts for heating and cooling energy. This makes official recognition of the value of daylighting harder to obtain. However, one manufacturer has published a study using those modeling tools that was designed to evaluate the changes to heating and cooling energy use when fenestration is optimized for good daylighting and efficient natural ventilation. One of the outcomes of that study shows that using toplighting where appropriate allows the total fenestration area in the modeled buildings to be reduced significantly, thereby saving annual energy in every climate zone of the U.S.

Fenestration Design

Advances in both glass and plastic glazing, as well as other system components, have improved thermal performance characteristics, such as insulation and solar heat gain control. Modern glazing can reduce the amount of glare resulting from direct sun exposure and/or diffuse the light into a larger area of interior spaces.

For top-lighting, light well design and/or TDD products can be useful in directing/reflecting light into larger areas of floor space.

Remember, the overriding goal of any daylighting design is how well it uses the available light. However, uncontrolled daylight may result in excessive heat gain and potential discomfort. It is important to ensure that the fenestration is appropriately sized and located and that the correct glazing and accessories are selected. Use the fenestration area wisely to help insure the energy benefits balance the costs.

Further considerations for proper daylighting design are health, comfort, solar tuning and other design aspects. For more information about daylighting, visit the Skylight/Sloped Glazing Council Resources web page. Another resource is the FGIA Window and Door Selection Guide, which is available in the online store.