Designing and simulating daylight

October 5, 2016

figure-xx-nanaimo-hospital
Images courtesy Stantec

By Marc Trudeau, Architect AIBC, LEED AP
The benefits of daylight are well-documented. For building occupants, it offers a range of positive physiological effects, which, in turn, translates into tangible benefits such as higher productivity, reduced employee sick time, improved employee morale, and lower lighting costs for businesses.

Building occupants, owners, and policy-makers are increasingly aware of these benefits and are asking for buildings to be well-lit by natural light. Daylighting design and simulation is the way to make this happen—to quantifiably understand how light enters a building and to determine how to achieve the project’s daylight goals.

Daylighting is emphasized in green building standards, such as the Leadership in Energy and Environmental Design (LEED) rating system. Daylight targets based on LEED are even being included as requirements for projects not pursuing certification. In this author’s own work, an increase in the Canadian public-private partnership (P3) projects specifying daylight performance thresholds has been seen. These are clear indicators to the rising importance of daylight in design.

LEED v4 allows for credit using Illuminating Engineering Society’s (IES) metric for measuring daylight sufficiency, employing spatial daylight autonomy (sDA) and annual sunlight exposure (ASE). LEED also allows for compliance using illuminance simulation and field measurement.

While the field measurement is relatively straightforward to implement, credit achievement cannot be determined until the project is built and occupied. 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 sDA daylight target is new in LEED v4, though, so it is unclear to many in the building industry how challenging this will be for buildings in Canada. (The article is adapted from the author’s presentation at the 2016 Canada Green Building Council (CaGBC) National Conference, held in Toronto. It includes daylight simulation results as presented at the eSim 2016 Conference, held in Hamilton.)

Benefits and challenges
Daylight and windows offer numerous positive effects for occupant well-being and building performance relating to:

Several research studies have looked at the benefits of daylight to specific building types and occupant groups. In one study, hospital patients in rooms with more sunlight reported less pain and stress and took 22 per cent less analgesic medications, resulting in a 21 per cent reduction in medication costs. (See J.M. Walch et al’s article, “The Effect of Sunlight on Post-operative Analgesic Medication Usage: A Prospective Study of Patients Undergoing Spinal Surgery” in Psychosomatic Medicine (2005).) In another study, comparing office and call-centre workers with best views to those with none, calls were processed six to 12 per cent faster and occupants performed 10 to 25 per cent better on tests of mental function and memory recall. (To read the 2003 report, “A Study of Office Worker Performance and the Indoor Environment: 
CEC PIER,” visit h-m-g.com/projects/daylighting/summaries%20on%20daylighting.htm[1].)

Daylight research studies such as these are challenging to do well because there are many variables and results are often based on subjective human responses. Therefore, the methods and results of these studies need to be considered critically.

Finding relevant and carefully done research is worth the effort because occupant well-being gets to the purpose of why buildings exist. Continuing with the example of medication costs within a hospital, the benefit to patient well-being will be extremely important because it relates to the facility’s fundamental role in improving health. As well, the operational cost benefits to a hospital could 
be substantial.

Balanced against these benefits, daylight and windows also bring design challenges and can cause negative effects:

In the design phase, the benefits and challenges of windows need to be evaluated using design tools and expertise. Daylight consultants can use simulations to quantify glare and the amount of daylight entering a space, using tools such as interior renderings and illuminance analyses. Energy consultants can use energy models to estimate the savings from electric light use and determine the appropriate balance between helpful solar heating and detrimental solar gains. Acoustic consultants can give guidance on strategies to manage sound and room acoustics. Finally, architectural designers must bring all this information together, giving careful thought to the form and materiality of the project to create an inspiring result.

Daylight metrics
There are several metrics for quantifying the amount of daylight in a space.

Illuminance
Measured in lux (foot-candles), illuminance indicates the amount of light falling on a surface. The illuminance value does not depend on surface properties, though surface properties are important to understand how much light is reflected and seen by the eye.

The amount of suitable illuminance varies greatly depending on factors such as age of the viewer and task. For example, an art gallery with a light-sensitive collection may keep light to around 50 lux (4 fc) to reduce degradation, while a workstation in a laboratory requiring precision tasks might require 1500 lux (140 fc). Typical office environments target light levels around 300 lux (27 fc).

Daylight factor
The daylight factor indicates the percentage ratio of indoor daylight illuminance to the outside illuminance. This is calculated at a single point location, and can then be calculated for a room as an average of daylight factors for multiple points in the room. Actual illuminance varies depending on cloud cover and position of the sun—therefore, daylight factor provides an indicator of the amount of natural light in a space, irrespective of weather.

Spatial daylight autonomy
sDA is the percentage area meeting a minimum lux level throughout the year. This metric is calculated using local weather data, and it compares daylight illuminance at a work-plane to a minimum requirement. Annual sunlight exposure, combined with daylight autonomy as defined in IES LM 83-12, Approved Method: IES Spatial Daylight Autonomy and Annual Sunlight Exposure, is a measure of visual discomfort and is the percentage area where the direct sunlight exceeds a maximum illuminance.

Daylight glare probability
The daylight glare probability indicates if the light levels get too high and could cause uncomfortable contrast for occupants within the space.

Simulation tools
At the core of simulation software are the algorithms that calculate light levels and render images. Radiance, developed by the U.S. Lawrence Berkeley National Laboratories (LBNL), is one such software; it is used as part of a number of other front-end software packages to do lighting calculations.

Radiance uses a combination of direct path ray tracing as well as Monte Carlo sampling of other directions where light might come from. The calculations start at a measurement point and trace light back to the emitting sources. Several light paths are considered:

In preparing a daylight model, there are several key steps to the workflow:

1. The model geometry is built using 3D drawing software.

2. Material and surface properties are assigned, including glass visible light transmittance (VLT) and surface reflectance properties (e.g. wall, ceiling, floor, ground, roof).

3. Simulation parameters are set, for example, based on simulation standards such as IES LM 83-12.Considerations include:

Daylight models can be used to provide a number of analytical results and images that are useful to a design team. Some commonly used outputs from daylight simulation software are described below.

Illuminance perspective images
An example illuminance perspective is shown in Figure 1. Renderings such as these show lux illuminance values within a space. Perspective images are useful to visualize how the light will be seen by occupants, and to see what surfaces will be in light or shadow.

Illuminance plan
An illuminance plan is typically used to study illuminance at either a working plane or the floor level. The results shown in Figure 2 are lux levels at two different times of day. The results would be helpful in quantifying what area of the floorplate is above a lux threshold such as for a LEED target, for understanding what workstations are receiving light, and for understanding how deep light travels 
into the room.

Glare probability analysis
Daylight models can be used to estimate likelihood of glare and allow testing of different glare-control devices such as internal shades. In Figure 3, a sample view is shown from the point of view of a person seated at a workstation with an adjacent window. 
The simulation was run to test the likelihood of glare within this field of view. The results without shades (top image) indicate there are times with intolerable glare (red and yellow), particularly from low sun angles in winter. By allowing shades to be closed whenever there is bright light (bottom image), the amount of intolerable glare can be significantly reduced.

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:

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.

figure-8
Figure 8: Simulation results for percentage of floor area achieving spatial daylight autonomy (sDA) targets with and without glare-control devices/blinds.

Design process
The daylighting design process requires co-ordination with the team at key points in the project. The design process follows a series of steps (schematic, design development, and construction documents) where key decisions of increasing levels of detail are made and costed.

The process for energy modelling follows these same design phases. Where there is a design energy target, teams prepare preliminary models at the schematic phase to give guidance on major systems, building geometry, and window area percentage. This means after schematic design, a change such as increasing the amount of window area can significantly penalize the project’s energy performance.

The daylight modelling process needs to follow the design process, as well as co-ordinate with decisions made on the energy model. As indicated in Figure 8, direction is needed on the daylight strategy at schematic design to have meaningful input to the overall project’s daylight performance. By the construction documents phase, only minor changes are able to be made to increase daylight levels.

Design-build and P3 projects are increasingly including daylight targets as part of the project requirements. If the owner and compliance team are serious about achieving these daylight targets, there are several important steps to follow:

These steps are crucial for the owner and compliance team to understand what the daylight target is and how important it is to the client. An aggressive daylight target could require a narrower footprint, increased glazing area, and additional elements for shading and glare control. The team needs to ensure the selected footprint depth can functionally work for the building occupants, can fit on the project site, and can work within overall cost and energy targets. Daylighting can bring great benefits to occupant well-being, and it needs to be fully included in the design process in order for anything other than ‘what usually happens’ to be built.

trudeau_portraitMarc Trudeau, Architect AIBC, LEED AP BD+C, is a building performance consultant at Stantec. He focuses mainly on strategies to achieve energy and sustainability targets for buildings. Trudeau contributes to projects through energy modelling for design assistance and standards compliance, daylighting simulations, and life cycle assesment studies. He has been a LEED accredited professional since 2003, and is a reviewer for Canada Green Building Council (CaGBC) Leadership in Energy and Environmental Design (LEED) submissions. Trudeau can be reached at marc.trudeau@stantec.com[2].

Endnotes:
  1. h-m-g.com/projects/daylighting/summaries%20on%20daylighting.htm: http://h-m-g.com/projects/daylighting/summaries%20on%20daylighting.htm
  2. marc.trudeau@stantec.com: mailto:marc.trudeau@stantec.com

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