MODELLING FOR DAYLIGHT AUTONOMY FOR LEED V 4 – IMPLICATIONS FOR CITIES IN NORTHERN LATITUDES

The daylight performance metrics and requirements in LEED Version 4 are increasingly applied to Canadian projects, either as an explicit target in separate documents defining requirements, such as for publicprivate partnership (P3) projects, or as a target that teams choose to pursue under the LEED Version 4 rating system. The required daylight levels in LEED Version 4 are identical throughout North America, however, the available daylight outside varies significantly based on latitude and local weather. In this research we use DIVA-for-Rhino to model spatial Daylight Autonomy (sDA) levels reached for the same building in 5 Canadian locations and 1 American location. This paper discusses the degree to which daylight performance is affected by latitude and weather in the respective locations, as well as the effect of floor plate depth and glare control devices. The paper includes a proposed alternative compliance path for projects located in northern latitudes to demonstrate compliance to LEED Version 4. The purpose of this alternative path is to incentivize daylight design in locations where occupants would benefit the most from increased access to daylight. This paper focusses on sDA requirements indicated in LEED Version 4; it does not provide an in depth analysis of other daylight requirements in LEED Version 4, such as annual sunlight exposure. INTRODUCTION The benefits of daylighting to human wellbeing are well-documented and in recent years there has been increasing interest in providing this amenity to occupants in high-performance buildings. Access to quality daylight has positive physiological effects as it reinforces our natural circadian rhythm, leading to improved hormone regulation, higher levels of alertness, better digestion and improved sleep quality. For businesses, this translates to tangible benefits such as higher productivity, reduced employee sick time, improved employee morale and lower lighting costs (USGBC, 2013; WELL Building, 2015; Su &Yu, 2014). Daylighting is emphasized in leading green building standards, from the WELL Building standard to the Leadership in Energy and Environmental Design (LEED) rating system, but we also see it being requested on projects not pursuing such certifications. For example, in the authors’ current work in building design and energy performance within the Canadian market, we are seeing an increase in the number of public-private partnership (P3) projects specifying daylight performance thresholds, a testament to the rising importance of this design strategy. LEED Version 4 (LEED v4) and the WELL Building standard both incorporate credits in their respective certification systems to reward buildings that are designed to provide daylit spaces. Both rating systems follow the Illuminating Engineering Society’s metric for measuring daylight sufficiency, where spaces are considered daylit if illuminance levels from daylight reach at least 300 lux for more than 50% of occupied hours (IES, 2012) specified at hours between 8AM and 6Pm. LEED v4 allows three compliance methods to achieve credit for daylighting: illuminance simulation, illuminance site measurement, and sDA and annual sunlight exposure simulation. Projects may earn 2 points using illuminance simulation or up to 3 points by field measuring illuminance levels post construction. While field measurement is fairly simple, credit achievement cannot be determined until the project is built and occupied. The third compliance method, the sDA simulation path, is the fastest and most costeffective (Su & Yu, 2014; Reinhart, 2015) option for design teams, and allows for up to 3 LEED points to be achieved. Moreover, daylight simulation allows immediate feedback during an iterative design process and gives the project team flexibility to evaluate the interaction between daylighting and building energy performance targets. The USGBC is incentivizing the use of sDA in building design by providing more points for this option. However, daylight simulation for sDA is based on climate data, which is new to the industry, so designers are hesitant to adopt it in their projects. Readers should note that while LEED v4 compliance via the sDA simulation path includes a requirement to model Annual Sunlight Exposure (ASE) values as a proxy for potential for visual discomfort resulting from daylighting. ASE simulation is outside the scope of this research paper. Refer to the discussion section where we provide additional commentary on the potential implications for ASE analysis. Through daylight simulation modeling in DIVA-forRhino (DIVA, 2016), this paper models the amount of floor area that can achieve the specified sDA levels in 6 locations in North America and includes the interactive effects of glare control devices. In doing so, this paper includes a rule-of-thumb method to help designers estimate the feasibility of achieving the Daylighting credit in LEED v4, by using a visual tool with several floor plate depths. Our findings are especially significant to cities at more northern latitudes, such as Whitehorse and Inuvik. These cities experience the shortest winter daylight hours, yet it is here where building occupants stand to benefit the most from having well daylit buildings. In this paper, we provide more clarity and guidance to practitioners around daylight design in northern locations, and whether the LEED v4 daylight requirements are even achievable in northern latitudes. We also suggest that different an alternative compliance path in LEED v4 may be needed to incentivize daylighting especially in higher latitude projects where there may be greater occupant benefit from this design strategy. METHODOLOGY Simulations for daylight autonomy levels were run using DIVA-for-Rhino version 3.0 software based on a prototypical office space across 6 locations in North America. A prototypical office space (see Figure 1) was created with windows on one facade, and 4 simulations were run at each location, for the window facing North, East, South and West. Locations were chosen at varying latitudes as indicated in Table 1. As a convenient first indicator of the daylight availability in the selected locations, the table also shows the numbers of bright sunlight hours for each location. There is no source for the sunlight hours which lists all locations; the data for Inuvik and Whitehorse are sourced from Atlas of Canada, 1957, for Edmonton, Vancouver and Toronto from Environment Canada’s website (Canadian Climate Normals, 2015), and for San Francisco from Wikipedia (Sunshine Duration, 2016). As expected, San Francisco receives significantly more sunlight because of its lower latitude; however, although Vancouver is lower in latitude compared to Edmonton, it nonetheless receives less sunlight hours because of its frequently overcast sky condition. Both the LEED v4 reference guide and IES LM-83-12 approved method for sDA simulations state that simulations are to be based on local climate weather files, such as typical meteorological year data. The weather files used for the simulation are CWEC weather data for Canadian locations and TMY3 weather data for San Francisco. The weather files include metered data of observed light levels. There are differences between CWEC and TMY3 data files. For example, CWEC is based on TMY2 methodology, which is based on 30 years of data, while TMY3 files are based on 15 years of data. While there is a difference between CWEC and TMY3 weather files, the respective file types were used at the locations as noted because the intent was to use typical files that would be used by a design team. As a result, the results presented here should be representative of what would be seen by a team in the calculations for LEED v4. Table 1 Locations of Daylight Simulation (based on CWEC and TMY3 data) Location Latitude (°N) Bright Sunlight Hours Inuvik 68 1500 Whitehorse 6