Thursday, March 28, 2013

Daylighting in LEED v4

Scott Preston, Sustainable Building Associate

While technology allows us to artificially light a building’s interior, that doesn’t necessarily mean it’s the best solution. Exposure to daylight has health and well-being benefits, especially indoors, where, according to the EPA, Americans spend approximately 90% of their time indoors. For these reasons, there has been growing demand for a reduction in electric lighting and mechanical cooling through architectural daylighting strategies. Daylight has immense power that can overwhelm any designer with the best intentions. Even the best green buildings designed by world-famous architecture firms have examples of too much glare, overheating, loss of productivity and situations of blinds drawn and lights always on. 

A well-designed daylit building is estimated to reduce lighting energy use by 50%-80%. The 2009 LEED rating system encourages adequate daylighting in schools and commercial buildings by requiring daylight for 75-90% of regularly occupied spaces. In the current system, compliance can be calculated and documented by implementing one of four options: a computer simulation, a prescriptive method (demonstrating compliance of side and top lighting drawn and measured in sections), measurement (demonstrated through records of indoor light measurements) or a combination of the preceding 3 options. 

Daylight modeling software is the best approach to dynamic daylighting metrics.  This approach can predict daylight and glare within a building with incredible accuracy.  In LEED v4, the credit has been updated to address recent innovations in daylight modeling.  Moving forward, the most points will be awarded for a project that implements a more sophisticated and dynamic metric called spatial daylight autonomy or sDA.  This refers to the percentage of the “work plane” that is above 300 lux (28 foot-candles) at least 50% of the time during the spaces occupied hours over the course of the entire year.  This may encourage designers to overglaze the building and therefore the credit requires glare simulations as a counterpoint.  The intent of the credit is to connect building occupants with the outdoors, reinforce circadian rhythms, and reduce the use of electrical lighting by introducing daylight into the space.