by Tim Metcalfe
Structures of all sizes can benefit from bringing natural light into a space and connecting the building’s occupants to the outside environment. The use of natural light on its own, or integrated with architectural lighting, provides energy savings and creates an aesthetic environment.
Controlled use of light in and around a building is the basic definition of daylighting. By employing daylight modeling, design professionals can determine the right amount of light for a certain space, as well as achieve Leadership in Energy and Environmental Design (LEED) or Collaborative for High Performance Schools (CHPS) daylighting criteria.
Daylight modeling
Daylight modeling is a computer simulation allowing project teams to use real-life conditions to properly design a building or space before purchasing materials that may not have provided the natural lighting levels required. By treating the building as a luminaire, opportunities to adjust the design can be pinpointed. To do this, a 3D model is created using a software program including site conditions such as surrounding buildings and ground conditions, as well as other considerations that can affect the daylight reaching the analysis area.
Then, materials are assigned to different surfaces within the model (e.g. floors, walls, and windows) that express the design. The model, however, is only as good as the technician and the material files used. In the hands of the untrained or inexperienced, the model can provide inaccurate or flawed results.
The next step is to import a weather file from a source such as the U.S. Department of Energy (DOE). The weather file contains information about conditions including the number of rainy, sunny, and cloudy days, plus average temperatures over a 15-year span. The weather file is also project-specific for areas, cities, or towns. Having an accurate weather file means the program will be using site conditions for a specific project and not a computer-generated condition.
The next step is to export the model to a validated ray-tracer software system. A ray-tracer takes the model information into account and then, simply put, turns ‘on’ the sun for the model’s global position and sky condition. The ray-tracer sends millions of rays out, recording the levels as they bounce around hitting the analysis grid. The final step is to import the results back into the model. Once the results are imported, designers can determine if the design meets the criteria and the model can be adjusted accordingly to optimize the daylight within the space.
Putting up shades or blinds to control the daylight entering the space can negatively impact a building’s solar heat gain coefficient (SHGC). Letting in heat without taking advantage of turning off the electric lights can create an environment that uses much more energy and cause the mechanical systems to work harder then required to keep up with the demand.
Daylight modeling demonstrates not only how much available light there is, but also where it goes within the space. This knowledge allows designers to form an electric lighting scheme that includes sensors and ballast controls that turn off or dim electric lights when they are not needed. This saves on electricity costs that can be applied to other expenses, making the building more sustainable in the long run.
Conclusion
The use of daylight is all about the occupants. If daylighting of a space is done right, occupants will feel energized, making them more productive. Additionally, daylighting saves on energy costs and makes buildings more sustainable.
A daylight model is a valuable tool to help create a thoughtful design that takes both form and function into account. Making conscious decisions that impact the end-user in a positive way will lead to an energy-efficient, comfortable space that reflects the intent of the spatial and overall design while considering the environment around it.
Tim Metcalfe is the director of sustainable technologies at Duo-Gard Industries Inc. He heads the company’s integrated building modeling strategies and his expertise includes a special focus on daylight modeling, which demonstrates how effective designs are and how they can be improved. Metcalfe can be reached by e-mail at tmetcalfe@duo-gard.com.
