Designing for Daylight
Daylight modeling provides a clear picture of how daylight will impact a space
There are many advantages to daylighting, which brings natural light into spaces. The more natural light that can be brought into interior spaces, the less electrical energy is required to light those same spaces, saving money for both building owners and tenants.
One way for architects to figure out their best options for daylighting a building is through daylight modeling. According to Steve Schohan, marketing and communications manager at Austell, Ga.-based YKK AP America Inc., the goal of daylight modeling is to maximize the use of natural light and energy efficiency in buildings as an integral part of the design process. “The benefits are vast, including a healthier, more productive and energy-efficient environment, improved occupant comfort, and the ability for architects and building owners to better meet certifications like LEED.”
Daylight modeling allows one to assess all potential variables that could influence design decisions. “The variables include window size and location, incorporations of views to the interior or exterior of the building, overall light pollution, lighting design and controls, and positioning of interior and exterior shading,” says Anastasia Flanegin, LEED AP BD+C, O+M, WELL AP, Fitwel Ambassador, National Fenestration Rating Council (NFRC) Certified Simulator, and energy engineer at Apogee Renovation, an initiative of Apogee Enterprises Inc., Minneapolis. “The design team can assess each variable with ease if the daylighting model is available, allowing them to compare options to meet their desired goal.”
An Integrated Design Concept
Daylight modeling ensures a building is effectively utilizing natural daylight to reduce glare, while improving occupant comfort, minimize electric lighting and heating and cooling demands, says Jim Leslie, general manager at EXTECH/Exterior Technologies Inc., Pittsburgh. It is not a one-size-fits-all process and there are a variety of aspects that need to be taken into consideration while developing an effective, integrated daylighting solution. These include the building siting to the architectural massing and programming, occupant characteristics and visual/physical needs, as well as the local climate.
“The application of an effective daylighting solution requires the design team to thoughtfully integrate many key architectural and building system elements to achieve the remarkable energy, human performance, and wellness benefits that effective daylighting can provide,” explains Neall Digert, Ph.D., MIES, vice president of Vista, Calif.-based Solatube International Inc., and second vice president of the Fenestration and Glazing Industry Alliance (FGIA) Skylight and Sloped Glazing Council. “As a result, daylight modeling plays a key role in informing the architectural and building system design, building system design, and space development process. Daylighting modeling helps to identify energy savings potential and, if done properly, identify potential energy- and human-performance issues that must be addressed through architectural detailing and/or synergistic building system integration. Accurate and thorough daylighting modeling is a key part of achieving a successful, energy-efficient, sustainable and human-centric building design.”
By using a 3-D model of the building, along with the time, date and sky condition calculations, simulation results typically generate colored images of the building’s floor plan at a specified elevation. “The resulting color-coded diagram shows how many foot candles of light will be cast on the plane, during the duration of the simulation,” explains Leslie. “A simulation could show just the amount of light on a particular day/time of the year, or the total amount of light over a chosen period.” This can help architects determine the placement and orientation of the building, the amount of glazing and locations, as well as the type of glazing to use, such as glass or translucent polycarbonate, window treatments, skylights, electrical lighting, etc.
In addition to giving the design team an assessment of how well daylit a building design is, Helen Sanders, Ph.D., general manager of Technoform Glass Insulation NA at Technoform, Twinsburg, Ohio, notes that daylight modeling allows various design scenarios—and shading control strategies—to be assessed in order to inform design choices and improve the design’s daylighting performance before irreversible design decisions are made. “Annual daylight simulation is the most robust simulation methodology as it assesses daylight intensity hitting the floorplate of a design throughout the course of a typical year. Typical weather files are used to simulate the incident light levels and sunlight intensity on each elevation of the building every hour each day through the year. Other methods choose to assess single hours of a single day around each equinox and/or in June and December to assess performance.”
And, as Rafael Rivero, director of business development at Kingspan Light + Air, North America, Lake Forest, Ill., notes, modeling can also provide a glare analysis, which can show where glare from certain materials may appear in a design.
Types of Daylight Analysis
Brian Grohe, commercial sales manager at VELUX America LLC, Fort Mill, S.C., notes there are two primary types of analysis for daylight modeling purposes that focus on energy efficiency and lighting distribution. A program like SkyCalc can show the expected energy savings delivered by a turnkey daylighting system, including the impact to the building’s HVAC usage. “This not only allows for the proper size and quantity of skylights to be used, but also for the evaluation of the electric lighting system components,” he explains. “In doing so, the building design can leverage the contributions of daylight while maximizing the efficiency of the lighting system.”
On the other hand, a lighting distribution model can provide the design team a clear picture of how daylight will impact the space. “This is important to understand so the right balance of light can be achieved,” Grohe says. “Programs like AGi32 can show the spread and intensity of light in the space, thereby allowing designers the ability to incorporate other design elements, such as shading systems, lighting controls, etc. Understanding the type of light and amount desired will make for a much smoother modeling and design process.”
While the final output of an analysis depends on the purpose of the model, Flanegin says in general, each daylight model would evaluate an overall interior illuminance. “The illuminance data demonstrates which spaces, and at what capacity, receive daylight. Based on that data, architects could establish design shortcomings and pursue corrective options.”
Starting the daylight modeling early in the design phase allows architects enough time to make the necessary adjustments. “This will provide flexibility and enough time to model and evaluate potential alternatives without compromising other disciplines (mechanical, electrical, plumbing, etc.), which could lead to a lot of expensive changes if done later in the life of the project,” explains Flanegin.
For optimal results, Digert says the design team should consider the use of daylight as a primary source for interior illumination early in the initial building design process. “The first step is always to develop a clear set of daylighting performance objectives—for both energy and human wellness—that must be achieved for the building spaces being designed,” he says. “Once those objectives are understood, the design team must then start analyzing the ways in which building siting and climate affect the daylight resource that is available for interior illumination, which will then help guide the process of defining the building’s form and selection of the skylight/fenestration solution that yields beneficial daylight harvesting and performance.”
“It is important to base the daylighting system on the outcome desired, whether that be lowering energy costs, providing a connection to the outdoors, or some other reason,” adds Grohe. “This will play a critical role in system considerations and ultimately the design.”
Once the intent is determined, the architect can select a system that will maximize results while not disrupting the overall building design. This way, by the time a design makes it to a manufacturing company, Schohan says they are able to take the design and performance spec and deliver the product and engineering solutions that meets the need.
Location and Orientation
Where a building is located, as well as the direction it faces, plays a substantial role in determining its optimal daylighting design. For example, a building in an urban setting surrounded by several nearby buildings will have very different daylighting options than a building situated in a more residential or rural area. “The building’s physical location, climate, siting and roof plane and façade orientation all play important roles in determining the final, optimal daylighting solution,” notes Digert.
Since important factors such as a building’s massing and orientation are decided in the first parts of the design process, Sanders says daylighting should also be taken into consideration at the concept development stage. “It is much easier to daylight a long skinny building than a square one since the perimeter to area ratio is higher (less core versus perimeter),” she says. “A north-south orientation for the long elevations of the building is most desirable since it reduces glare issues, or at least makes direct sunlight glare more manageable, and allows a higher quality of daylight while minimizing solar gains. Window aperture size, position and overall quantity are also important. Putting glazing higher up increases daylight penetration (higher window head height, use clerestory and skylights) and focused on north and south elevations help comfortable daylighting. Counterintuitively, punched opening windows can create more glare than ribbon windows because of the contrast from wall to window.”
A daylighting model will account for a building’s geographic location as well as its orientation. “This is used to compute the incidence angle of the sun at each point in time during the simulation, and in turn the amount of light absorbed,” explains Leslie. “If possible, orient main glazing areas to the north to take advantage of the sun for warmth and natural light.”
An accurate daylight modeling assessment can be done as long as the annual weather files with solar intensities by the hour is available. A building in the southern United States will have different daylighting needs than one in the Pacific Northwest or Midwest, which will impact the strategies architects use to control daylight and solar gains.
And, Rivero notes, considerations should be taken into account for thermal performance as well as light. “Certain areas of the country will require different skylight considerations based on thermal transmittance,” adds Grohe. “Incorporating these factors into the design will ensure the effectiveness of the entire building.”
Daylight modeling can provide the required documentation to show compliance with credits within LEED and WELL certification programs. “Daylight modeling provides an additional level of intelligence in the design process to maximize natural light, which improves a building’s energy efficiency and makes it possible to meet LEED certification requirements,” says Schohan.
Daylight modeling can aid building certification in a variety of ways. “First, the modeling can show where daylight can be introduced to a building and where the reach is possible,” explains Grohe. “This is important to understand how much daylighting can be incorporated, the areas where electrical lighting needs should be addressed, where and what size windows could be used, and other design considerations. Second, the efficiency of the system can be modeled to show the impact on the building. In LEED, for example, there are credits available for improving the energy efficiency of the building against a baseline. Daylight modeling can show the skylight system’s impact on the energy systems and how much energy can be saved, thus resulting in credit considerations.”
“The benefit of daylight simulations is that the design team will know if advance if a space qualifies, rather than waiting until construction is completed,” adds Sanders.
As Digert explains, in today’s world of sophisticated skylight glazing and fenestration technologies and complex building systems, quality daylight modeling and analysis has moved well beyond the single point-in-time or representative hour/day analyses that used to guide critical architectural design and fenestration choices. “Today, in order to properly select, apply and detail a comprehensive daylighting solution, it is important to analyze the hourly performance and interaction of the daylighting systems based upon a building’s location, climate, energy-impacts and occupant behavior. Today’s sophisticated design tools allow a space to be analyzed relative to a number of useful design metrics, including hourly heating and cooling loads based upon skylight fenestration orientation and application, as well as hourly daylighting system performance metrics including visual task illuminance, melanopic illuminance, spatial Daylight Autonomy (sDA), Annual Sunlight Exposure (ASE), Useful Daylight Illumination (UDI), and spatial glare analysis. When used as a suite of performance metrics, the daylighting design process can be informed, thereby allowing an effective solution to be implemented given the design and performance criteria that the project requires.”
When properly done, Digert says, “the effective application of daylight (using skylights) as the primary source for interior illumination is one of the most important elements in creating a sustainable building solution that maximizes both energy efficiency and human wellness! The key is being able to effectively interpret and apply the numerous performance metrics that can now be calculated.”
And small design changes can make big differences. “Small changes, such as window heights, can create drastic changes in the use of natural light in the building,” Rivero says. “Being open to small changes can greatly impact the quality of light in the space as well as help lead to daylighting autonomy.”
Since daylight simulations are complex, Sanders recommends running daylight simulations in tandem with energy modeling and also thermal comfort modeling, since all these aspects of building design need to be optimized together. “Optimizing one element without managing the others could lead to poor performance in another,” she says. “For example, appropriate levels of glazed areas may be needed for daylighting, but in order to meet energy targets, the appropriate glazing selection needs to be made to manage thermal conduction losses/gains and solar heat gains. We recommend having experts in daylighting design and fenestration system/glazing at the table at the earliest stages in design.”