• DOI:10.1177/1477153515587613
• Corpus ID: 114386623

Opinion: Climate-based daylighting metrics in LEEDv4 – A fragile progress

@article{Reinhart2015OpinionCD,
  title={Opinion: Climate-based daylighting metrics in LEEDv4 – A fragile progress},
  author={Christoph F. Reinhart},
  journal={Lighting Research \& Technology},
  year={2015},
  volume={47},
  pages={388 - 388}
}

• C. Reinhart
• Published 1 June 2015
• Engineering
• Lighting Research & Technology

The latest version of the Leadership in Energy and Environmental Design accreditation system (LEED v4) introduces the use of a climate-based daylighting metric, spatial daylight autonomy, as a simulation-based compliance path for the daylighting credit. The new LEED criterion is largely based on the IESNA’s recommendation that a point be considered to ‘be daylit’ if the illuminances due to daylight remain above 300 lx for at least half of all occupied hours in the year. LEED v4 further requires that window shades are temporarily closed whenever more than 2% of a space is illuminated by direct sunlight above 1000 lx. This serves as an indicator for glare. Finally, the credit requires that the 1000 lx direct sunlight limit is not exceeded for more than 250 hours a year for more than 10% of a space. In short, the annual solar exposure has to be less than 10%. Does this complicated new process represent progress? In my opinion, it is a step in the right direction but the annual solar exposure limit is too strict. It precludes direct sunlight from entering a space, which promotes ‘dull’ spaces and will leave many good designs deprived of a deserving credit. Unless the credit gets reworded promptly, it is likely that a disenchanted green building design community will rid itself of climate-based daylighting metrics altogether in upcoming LEED versions. It was a bold and laudable move by the IESNA to go beyond the long-held notion that annual daylight simulations are ‘too difficult’ for design teams to conduct during an age when three-dimensional digital versions of buildings are ubiquitous in practice and education. The reality is that most designers do not calculate daylight factors by hand any more (if they ever did). Why should they in the presence of a digital model that is faster and more accurate? Further, in a recent study, spatial daylight autonomy has been shown to be a mostly reliable indicator of occupant perceptions of daylight availability in spaces. The question now is how to meaningfully introduce shading device use into LEED calculations? As an immediate solution – until further research on manual blind control has been completed – I propose that design teams are allowed to apply the 1000 lx direct sunlight criterion only to areas within a space where activities require stringent control of sunlight. These ‘critical visual task areas’ would be desks, white boards, patient beds, etc. Previous studies have found that occupants generally welcome direct sunlight in daylit spaces unless it interferes with the task at hand. Introducing focus areas with special visual requirements but allowing direct sunlight into the rest of the space would facilitate the design of comfortable, well daylit spaces. Any concerns about overheating and increased cooling loads could be addressed by requiring daylighting and energy simulations to use the same schedules for occupancy and shading settings.