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Principles and Impacts of Daylighting
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Daylighting is a moving target, one that shifts from moment to moment, morning to evening, and season to season. Its allure lies in its complexity, which makes it challenging to implement for professionals charged with the delivery of a homogeneous environment, principally defined by ASHRAE Standard 55. But if your organization is serious about the triple bottom line, daylighting provides powerful incentives to strive for more.
Daylighting is the simple concept that seeks to control natural light in a space and reduce or eliminate electric lighting. Technical guidance for the application of daylight in buildings is provided by the USGBC’s Leadership in Energy and Environmental Design (LEED) suite of rating systems.
Daylight and views provide a strong connection to place and time. They promote healthy circadian rhythms, reduce stress, and improve productivity, attentiveness, and mood. Although many studies have attempted to quantify the human benefits (employee retention, reduced absenteeism, and improved student testing scores), definitive research is still lacking.
Economic benefits come principally from energy savings (kW and kWh) due to reduced electrical lighting and cooling loads. Because sunlight has less heat per unit of light than electrical lighting, cooling loads will be smaller if windows are appropriately sized and oriented, and electric lights are automatically switched off. Smaller cooling loads mean smaller and less costly HVAC systems. Of course, energy usage is also affected by other variables, including geographic location, climatic zones, glazing, and wall properties.
In terms of the environment, energy efficiency is the most cost-effective means of reducing greenhouse gas emissions though reduced generation of electricity. Daylighting substitutes a clean, plentiful resource (the sun) for an expensive, limited resource (electricity).
Building a Daylighting Plan
Due to the many issues surrounding daylighting, it’s essential at a project’s start to have a plan in place to track metrics, set benchmarks, and coordinate the team throughout the design, construction, and commissioning process.
The responsibilities of individual team members include:
- Architect: Site analysis, orientation, massing, window-to-wall ratios, and glazing selection.
- Mechanical engineer: Climate analysis, energy modeling, and HVAC system sizing.
- Electrical engineer, lighting designer, and/or daylighting consultant: Fixture and controls selection, daylight modeling, and design integration.
- Contractor: Cost estimating, installation, and coordination.
- Interior designer: Material selection (color, reflectance, etc.).
- Commissioning agent: Commissioning and monitoring the system.
- Facilities management: Employee training, ongoing maintenance, and troubleshooting.
These responsibilities can be reassigned or curtailed as long as they are addressed in the plan. In fact, successful daylighting is primarily a matter of coordination; all team members must be involved early in the design process in order to meet each of the following core principles:
- Usability: This principle tracks how much daylight is delivered and for how long. The most common expression of this would be the daylight factor. (See USGBC LEED for New Construction Credit 8.1 Daylight & Views: Daylight 75% of Spaces, Path 2: Prescriptive for more information.) Additional considerations are user controls that allow adjustments to the amount of light admitted to the space.
- Thermal comfort: Commercial window assemblies have an average whole unit insulation value of R2. Balancing heat gain and heat loss throughout the envelope is a primary factor for the design concept.
- Glare control: An ideal daylighting plan provides uniform distribution that minimizes glare and hot spots.
- Energy savings (kWh): Savings can be achieved by turning off or dimming the lights when not in use.
- Peak electric load reduction (kW): Peak loads are typically coincident with the optimal daylight conditions. They result in “ratchet charges” that can make up half of a customer’s utility bill. Additionally, the size of the cooling equipment is directly proportional to the peak load of a facility. Reducing the peak load has a significant impact on both operational costs and first costs by accommodating smaller equipment.
- Views: Perhaps the most subjective metric, but arguably the most important. Excessively dark glass can undercut an otherwise exemplary daylit design. Similarly, clear glass can admit too much glare, resulting in drawn shades and unwanted use of artificial light. A balance must be established.
Good daylighting design means using strategies that address the climatic conditions of the building location. In climate zones with more cooling requirements, designers should employ strategies that reduce solar and conductive heat gain while maximizing natural light. In contrast, strategies for zones with more heating requirements should balance the heat loss from reduced artificial lighting and conduction with potential heat gains from daylighting.
Some of these qualities are objective and measurable, while others are subjective. Rough approximations of daylighting availability, such as daylighting factor or single point in time metrics, provide a cost-effective means of analyzing the quantity and quality of daylighting in a space. More accurate models, such as daylight autonomy or daylight saturation percentage, offer more reliable outcomes in exchange for a more costly and time-consuming design process. Space use, energy savings goals, and the probability of value engineering should be key factors in the planning process. Putting windows in a building does not constitute daylighting; similarly, performance-based metrics do not tell the whole story about daylight’s effect on the finished building.
Linking to Other Sustainable Strategies
For new construction projects, LEED Indoor Environmental Quality (IEQ) Credit 8.1 (IEQc8.1) and Credit 8.2 deal explicitly with daylighting and views respectively, but do not guarantee that all of the principles cited here will be used. LEED Energy & Atmosphere (EA) Prerequisite 2 and Credit 1 are synergistic with the daylighting credits, and address the central component of energy efficiency as a result of daylighting.
As codes have become increasingly stringent, advanced strategies for saving energy are necessary to meet the minimum two-point requirement for EA Credit 1. IEQ Credit 6 Controllability of Systems addresses the challenge of coordinating the project’s automated controls for daylighting while allowing some autonomy for the well being of the building occupants. IEQ Credit 6.2 Controllability of Systems and IEQ Credit 7.1 Thermal Comfort reference ASHRAE Standards 62.1 and 55, both central to the amount of light and heat that daylighting brings. Most importantly, integrated daylighting provides the opportunity for a project to propel the business model of the organization by requiring transparency in a project’s design, connecting natural light with the ancient metaphor for knowledge, and putting people first.
There are four ways to achieve LEED for New Construction daylighting credits:
- Simulation: Demonstrate daylighting performance through computer modeling of light levels. This is typically the most expensive approach to document credit compliance, but it is also the lowest risk, especially for projects with complex floorplans.
- Prescriptive: Meet credit requirements through a simple calculation involving geometry, glass size, and transmittance. This is a very cost-effective approach to achieving the credits, but it can be difficult to achieve due to its reductive nature. There is also a risk that the prescriptive approach will provide too much daylight if not used with caution.
- Measurement: Record indoor light measurements in the space after construction is complete. This is the most accurate calculation of a space’s daylighting performance, but it has inherent risk as a post-occupancy strategy for verification.
- Use a combination of the strategies above.
In contrast to LEED for New Construction’s two points for Daylighting and Views, LEED for Existing Buildings: Operations & Maintenance IEQ Credit 2.4 gives a single point for either Daylighting or Views.
Even if the scope of the project is limited to retrofitting an existing space for daylighting, it’s important to consider all of the elements that affect daylight penetration. Consider them in order of importance: first, a building’s context (climate, site, orientation, and neighboring obstructions); second, the active measures (lighting, lighting controls, and HVAC sizing); and finally, the passive measures (window-to-wall ratios, glass types, materials, and surfaces).
Projects can execute all of the daylighting details to perfection yet fail to consider the orientation of the building. As a result, blinds remain closed to prevent glare and heat gain, the lights remain on, and the views inaccessible to the occupants. On other projects, orientation and active controls can be perfect, but the interiors contain dark, light-absorbing materials that result in low light levels, negating the benefits of the lighting controls.
Assessing Existing Spaces for Daylighting Potential
It is more difficult to adapt an existing space to daylight access than to design one from scratch, but there are still a number of opportunities for upgrading to a more daylit space. One unique advantage of retrofitting an existing space is having a solid baseline from which to work.
Begin by assessing the nearby obstructions (or opportunities): other buildings, trees, views, or parking lots. Western exposures are typically bad due to glare and heat gains from the low afternoon sun. Even, indirect northern light is usually the best quality, but remember to balance the potential for heat loss in colder climates.
Assess the interior space layout to determine the daylit zone. For example, in a typical office floorplan, executive suites are located along the window wall and have access to the best views and natural light. Placing the open office at the perimeter of the building and moving individual offices to the core increase the daylit zone by removing full-height obstructions.
Lastly, address lighting controls. Typically, controls are the most challenging element of a daylighting plan. They are costly and difficult to calibrate and maintain; however, they provide the foundation for energy savings. Automated shading should also be considered, especially for buildings where orientation is not optimal.
Not surprisingly, most historic buildings built before the turn of the century have highly tuned daylighting designs. They have smaller floor plates so that daylight can penetrate deeper into the core of the building. The floor-to-ceiling heights are typically higher (12 feet or more) than newer designs, as are the corresponding window head heights, which also facilitate light penetration.
In historic buildings, natural light fulfilled a fundamental need to operate. Today’s buildings rely on technology to replace that need, but at what cost?
Tate Walker is a licensed architect, LEED Accredited Professional, and vice president of the Wisconsin Green Building Alliance. At the Energy Center of Wisconsin, he provides building design assistance and education for commercial high-performance building initiatives.
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