The dynamic nature of daylight, throughout the day and throughout the year, poses numerous challenges when designing buildings that seek to utilize this abundant natural resource to meet the illuminance requirements of architectural spaces. This part of the Daylighting Primer addresses the intricacies of what constitutes good daylighting design, and outlines a general design methodology that considers the many factors involved in good daylighting design.
Why design buildings to use the abundant daylight available at the site? What are the potential benefits of an effective daylighting design?
What is good daylighting design and what is bad daylighting design? What strategies and approaches can lead to good daylighting designs, and what issues should a designer be concerned about?
When during the architectural design process should daylighting be considered? How can a designer best integrate daylighting design into the architectural design process?
How can an architect design for good daylighting: that is, what design and analysis tools and “ daylight “ products are available and how are they best used / integrated to create an effective daylighting system?
The two primary reasons for using daylight to meet the illumination requirements of an architectural space are the psychological benefits and the energy savings benefits. Good daylighting has been shown to improve the overall attitude, satisfaction and well being of building occupants. A number of research studies, a few of which are listed below, have shown a variety of benefits of daylighting in different building types and functions, among them improved retail sales in big box stores, increased worker productivity and reduced absenteeism in office buildings, improved student educational performance in K–12 schools, and improved patient recovery times in hospitals. Exposure to daylight has also been shown to improve general health and circadian rhythm. These psychological benefits can easily justify any extra design effort or added expense associated with introducing controlled daylight into buildings.
Daylighting, with proper electric lighting controls, can result in significant energy savings by reducing electric lighting loads and associated cooling loads. In addition, with proper solar control, solar gains during cooling load periods can be mitigated and solar gains during heating load periods can be utilized, reducing the energy requirements of both cooling and heating a space.
Office / Retail / Service:
The benefits of introducing daylight into a space can be negated by improper handling of the daylight being introduced. Uncomfortable glare and thermal conditions, as well as veiling reflections and high contrast ratios, are examples of unfavorable conditions that can be created by poor daylighting. Therefore, it is important to implement a thorough and efficient approach to daylighting design. The recommended approach to daylighting design begins by defining daylighting performance goals and design criteria, and then developing and evaluating daylighting design alternatives that achieve these goals and criteria. Listed below are general daylighting performance goals for daylit spaces that represent successful daylighting design characteristics.
Provide ambient lighting requirements during daytime hours for the majority of the year.
Create uniform distribution of daylight to reduce uncomfortably high brightness ratios.
Control direct sunlight when necessary and utilize beneficial passive solar strategies when appropriate.
Allow for user adjustment and override.
Ensure adequate daylight to all occupants of the daylit space.
Provide view and connection to the outdoors.
Fully integrate with the architectural expression of the building inside and out.
Fully integrate with other building systems -- HVAC, Electrical, Lighting, Structural, Interiors.
Implement within overall construction budget of the project.
Achieve significant energy savings by reducing lighting energy costs and associated cooling energy costs.
Daylight designs are most effective when properly integrated into the overall architecture of a building. Daylighting and solar control strategies that are addressed as an afterthought and added to an already designed building typically do not achieve a successful integration with the building design and space layout. Additionally, these types of strategies tend to be more costly to implement and more problematic in general. Therefore, it is best to address daylighting and solar control issues early in the design, when programming the various spaces.
When considering daylighting strategies early in the design process, the following issues should be considered:
- Daylighting Appropriateness – For each space in a building, consider whether daylighting is appropriate. Spaces with very specific lighting requirements, where excessively high light levels could be problematic ( i.e. an electronic laboratory using lasers ), should be a low daylighting priority. Additionally, spaces that are seldom used and/or used for short durations, such as storage rooms, restrooms, and copier rooms, should have a low daylighting priority. These types of spaces would ideally be located in the core of the building where there is limited access to daylight. Spaces that are continually occupied and where daylighting would more beneficial to the occupants and to the energy efficiency of the space should have a high daylighting priority. These spaces should be located towards the perimeter of a building where there is a plentiful daylight resource.
- Direct Sunlight Tolerance – For each space with a high daylighting priority, consider its tolerance to direct sunlight relative to glare and solar heat gains. In more public, transitory spaces, some direct sunlight, especially during the colder winter months or colder mornings, can be very pleasant and tolerated, adding sparkle and warmth to the space. These spaces are ideally located towards the south, east, and west sides of the building. To use solar gains to help heat a public space on cold mornings, orient the space to the east.
- Solar heat gain control is always important; designers must control direct sunlight penetration, shading the higher summer sun angles but possibly allowing the lower winter sun angles to penetrate the spaces. Stairwells or other less critical building components could be located in such a way as to shade more critical spaces from afternoon sunlight. Critical spaces could also be located to the north, where incident sunlight is minimal. Space layout is important, since the location of interior walls can help in providing adequate direct sunlight shading and dictate the cut-off angles required. It is important to consider direct sunlight control strategies early in the design process to avoid having to resort to more expensive and less streamlined solar control strategies down the road.
- Views and Connection to the Outdoors – Even when the daylight resource is not adequate to provide the necessary lighting requirements, a view to the outdoors provides much of the psychological benefit of daylighting. For spaces that can be located close to a daylight resource, view lines should be considered when determining the space layout. Consider the use of transom glass and other clear glass applications to maintain views to the outside for all building occupants.
- Integrated Architecture and Daylight Harvesting - When considering introducing daylight into a building, always consider synergistic strategies; ways to use other building elements (i.e. architectural, mechanical and structural) to serve multiple functions by including a daylighting function as well.
In order for a daylighting design to effectively improve the energy efficiency of a building, it is essential that both the electric lighting and the solar heat gain of the daylit spaces are effectively controlled. Inadequate control can result in glare issues and a reduction in energy efficiency.
Dynamic (movable) solar control is often the best choice for east and west facades or cloudier climates, where incident direct sunlight is variable and occurs less than two-thirds of the time. These strategies optimize the daylight resource both when direct sunlight is present as well as under overcast skies when the dominant daylight contribution is from a relatively glare-free sky dome. Static solar control is often the most effective strategy in sunnier climates and for southern facades (within 20o of due south) that receive significant incident direct sunlight. Static systems typically have reduced maintenance requirements and costs compared to an automated or dynamic strategy.
Efficient Electric Lighting Integration
Along with adequate solar control, integration with the electric lighting system design of a space is essential for a daylighting design to effectively provide increased building-wide energy savings.
The following electric lighting system design strategies should be considered for non-daylit spaces as well as daylit spaces. These will help to lower the Lighting Power Densities (LPDs) of the various spaces and reduce energy use while maintaining the required light levels.
1. Task/Ambient Electric Lighting Design Approach
A task/ambient electric lighting system utilizes two levels of lighting to provide the illuminance requirements of the space. One level provides sufficient ambient light for circulation and general tasks, and one level provides greater localized illumination suited to the specific tasks that require it. Typically, the ambient lighting is provided in a more diffuse and uniform manner. Ambient lighting is provided by indirect means whenever possible to provide a comfortable and shadow-free environment which often integrates more effectively with daylighting than direct lighting. However, high luminaire efficiencies and ceiling reflectances are important when using indirect systems as they can have reduced efficiency compared to a direct system. The task level of lighting should be provided in a more localized manner only where it is needed, i.e. the desktop, workbench, etc.
Since daylighting is an effective method for providing the ambient lighting needs of a space, but is not as effective in maintaining high localized illuminance requirements, a task/ambient lighting approach can integrate effectively with daylighting. The intent is for daylighting to provide the ambient illuminance and to control the level of ambient electric lighting in response. In many cases, a daylighting strategy can be designed to provide adequate ambient illuminance whenever daylight is available, and the electric lighting can be controlled with on/off timer or photosensor-based controls. In addition to better integration with daylighting, a task/ambient system often results in lower Lighting Power Densities (LPD’s), resulting in a greater base level of energy savings, because illuminance is only provided when and where it is needed.
2. Differentiate Nighttime and Daytime Needs
Humans have evolved under high daytime illuminance levels and relatively low nighttime illuminance levels. Therefore, people are psychologically accustomed to these conditions and will often feel more comfortable under lower light levels at night. In fact, recent studies show that light levels are one of the main triggers that keep our human circadian rhythms (the daily rhythms that impact numerous biological functions) in sync. Once disrupted, our out-of-sync rhythms can contribute to various maladies such as SAD (seasonal affective disorder), sleep deprivation, and general lack of energy.
Electric lighting system design can take advantage of these rhythmic luminous needs, especially when daylighting is integrated into the lighting system design. In spaces with adequate daylight saturation, the electric lighting system can be designed to the lower nighttime requirements, since during the daytime it is truly just supplementing the more desirable daylight resource. This results in lower Lighting Power Densities as well as a better psychological rhythm to the luminous environment.
3. Electric Lighting Control
Electric lighting control is essential in providing energy savings and can be addressed in several ways from a very simplistic approach to more complex photosensor control systems. In the most simplistic approach, adequate daylight saturation can allow for a general reduction in the electric lighting requirements. If daylight is present throughout the day, the electric lighting is purely supplemental and should be designed for nighttime functions only, where reduced lighting levels are often adequate and even preferred. Complex control can allow for maximum energy savings and involves using photosensors, timers, or other central electronic control strategies to turn of zones of electric lighting when the daylight resource is adequate.