Designing and simulating daylight

Daylight Standards in LEED
As LEED is the most commonly used green building standard in North America, the daylight calculation thresholds it defines play a large part in setting a standard for Canadian projects. There are options to achieve the daylighting credit under Indoor Environment Quality (EQ) credit 8.1, Daylight and Views−Daylight. The following is a summary of daylight requirements and options available for Canadian LEED projects.

LEED Canada NC 1.0

  1. Daylight Factor > two per cent, based on a formula provided in the LEED Reference Guide. The formula can be completed using a spreadsheet analysis.
  2. Simulation Path: Achieve > 250 lux (25 fc) in 75 per cent of regularly occupied spaces in a clear sky condition at noon on June 21.

LEED Canada 2009

  1. Prescriptive Path: 0.15 < VLT x window-to-floor ratio (WFR) < 0.18.
  2. Onsite Measurement: Achieve > 108 lux (10 fc) and < 500 lux (46 fc) in 75 per cent of regularly occupied spaces.
  3. Simulation Path: Achieve > 108 lux and < 500 lux in 75 per cent of regularly occupied spaces in a clear sky condition at 9 a.m. and 3 p.m. on March 21/September 21. (Per LEED Credit Interpretation Request [CIR] 934, the minimum lux target was reduced from 250 to 108 lux.)

LEED v4

  1. Illuminance Simulation: 300 to 3000 lux (27 to 280 fc), 9 a.m. and 3 p.m.
  2. Onsite Measurement: 300 to 3000 lux.
  3. Spatial Daylight Autonomy and Annual Sunlight Exposure.

The simulation path for sDA and ASE is new in LEED v4, and it follows simulation procedures defined in standard IES LM 83-12. The credit threshold is based on 55 per cent of eligible floor area (2 points) and 75 per cent of area (3 points). ‘Eligible floor area’ is defined as the percentage of floor area with > 300 lux for 50 per cent of time annually between 8 a.m. and 6 p.m. Annual sunlight exposure must be < 10 per cent of floor area, and it is defined as the area with > 1000 lux for 250 hours in the year. Simulations are run using typical meteorological year data.

The effect of external fixed shades and internal blinds is handled differently between sDA and ASE. The former includes benefit of both fixed shades and operable blinds, and blinds are assumed to close when >2 per cent of area receives direct sunlight (>1000 lux [92 fc]). The latter includes fixed shades, though not operable blinds. This means that if many spaces have high ASE and associated glare risk, the ASE cannot be reduced with blinds. Rather, ASE must be reduced by building geometry, using fixed shades, or by facing regularly occupied spaces toward orientations that have less direct sunlight.

Daylight autonomy simulation
As the LEED v4 standard is new, there is little understanding in the industry of how challenging the sDA and ASE requirement will be for Canadian projects, particularly those in latitudes that are more northern. To give an indication of possible sDA performance, simulation results are presented here for locations with increasing latitude.

Since sDA is simulated using light levels measured in a typical year, weather and latitude play a part in the available sunlight for daylight modelling (Figure 4). Weather in the prairies allows for significantly more annual bright sunshine than on the West Coast.

Simulations for daylight autonomy levels were run using DIVA-for-Rhino version 3.0 software based on a generic office space across six locations in North America. A prototypical office space (Figure 5) was created with windows on one façade. Four simulations were run at each location, for the window facing north, east, south, and west. Locations were chosen at increasing latitudes as indicated in Figure 6.

These simulations determine daylight levels for a typical office environment with a maximized window size, not including any other specific 
design strategies to improve daylight such as light shelves, light-reflecting blinds, skylights, or clerestory windows.

The simulated office space is 10 m wide by 10 m deep by 2.7 m high (33 x 33 x 9 ft), and located at grade. A depth of 10 m from the window represents four typical workstations each of depth 2.5 m (8 ft). A width of 10 m means there is some effect from light reflecting off walls, but the results are expected to be more representative of an open office than a small, enclosed office. A ceiling height of 2.7 m was chosen to represent a typical office space height. Glazing starts at 0.8 m (2 ½ ft) above finished floor (AFF), representing a sill height at desk level, and continues up to the ceiling. No external fixed shades are included. This office space is simulated four times in each location, with the window rotating to face north, east, south, and west.

LEED v4 requires that glare-control devices be included in sDA simulations. They are to be activated whenever more than two per cent of the analysis points receive direct sunlight, where direct sunlight is defined as the condition when a beam of direct sunlight of more than 1000 lux is received at the analysis point. There is a wide variety of glare-control fabrics, blinds, and shades available on the market, each with unique light transmission properties. In these simulations, blinds reflected all direct sunlight and allowed only 25 per cent of diffuse sunlight into the space—this is the default dynamic shading model used in DIVA v3.0 for LEED sDA simulations. Actual glare-control products on any given project will have different properties.

LEED v4 also requires furnishings be included in sDA simulations, but they were excluded here as the research was focusing on the effect of window orientation and project location. When modelling for compliance with LEED v4, furniture layouts and reflectance characteristics can have a significant effect on daylight performance. For example, tall partitions would prevent daylight from reaching into the space, and conversely, highly reflective work surfaces would allow daylight to reflect more deeply into the space.

Simulations were run to calculate the sDA values at each of the selected locations in Figure 6. The space was simulated with the window facing to the four directions. This first set of simulations included glare controls.

Secondly, simulations were repeated with the same conditions, except without glare controls. This gives an understanding of how much daylight is available at each location and orientation, highlighting the effect of shades on limiting daylight.

Figure 7 shows the sDA values, or percentage of floor area receiving sufficient light annually. The number listed at each orientation indicates the sDA value based on simulating the window facing that orientation. Two sets of values are listed:

  • the shaded numbers indicate simulation results with glare control as per LEED v4 sDA requirements (with blinds); and
  • unshaded numbers represent results without glare control (without blinds).

For spaces with south-facing windows, all locations showed a significant reduction in sDA because of glare control. This means there is a lot of glare and the glare-control device is blocking a lot of light. Consequently, the choice of blind material will have a significant impact on the amount of daylit area. As well, these results suggests use of fixed external shading devices will be an effective strategy to remove glare from direct sun and allow the blinds to stay open longer.

LEED sDA values are similar in all locations (32 to 40 per cent) while the sDA with no shades has a wider range (59 to 99 per cent)—this too indicates the significant impact blinds have on sDA in south-facing spaces.

Continuing to look at spaces with south-facing windows, comparing the sDA without blinds as one moves to more northern latitudes, the largest reduction in sDA occurs in Inuvik, Northwest Territories. Interestingly, Vancouver has a lower value than Whitehorse and Edmonton, which are at a higher latitude and consequently have less annual sun-up time. This is because the sun drops to a lower angle on the horizon, bringing light deeper into the space. As well, Vancouver has a significant amount of cloudy weather reducing the amount of available daylight.

In spaces with north-facing windows, blinds are rarely activated. Inuvik is the location with the largest sDA reduction due to blinds and this is because the town is above the Arctic Circle, with sunlight occasionally coming from the north. The sDA value without blinds generally drops as the latitude increases.

In spaces with windows facing east and west, the sDA values at all locations are within a few percentage points of one another. The lowest sDA values are in Inuvik at 31 and 33 per cent, compared to San Francisco at 50 and 52 per cent. The locations between San Francisco and Inuvik do not follow a clear trend by latitude, which suggests sDA is significantly influenced by the weather.

Generally, there is a noticeable reduction in the sDA daylit area in a typical office floor plate as one moves north. This effect is most obvious for Inuvik, a location beyond the Arctic Circle. In comparison with a project in San Francisco, Canadian projects need shallower floorplates to achieve the same number of LEED daylight points.

For spaces with windows facing south in all geographic locations, there is a lot of glare, so control devices are frequently turned on per IES LM 83-12 requirements. Consequently, the light properties of the glare-control device will be important for design teams to consider. As well, results suggest there is opportunity to increase sDA through use of fixed external shades, so the blinds do not need to be closed as often.

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