Daylight Deconstructed

The first step of the simulation determined whether the blinds are open or closed. The second step of the simulation determines the level of illumination at each point and at each hour after the blinds are in position. For LEED V4, a point must meet a minimum illuminance of 300 lux (28 footcandles) for at least 50% of the year: sDA (300, 50%).
Obtain illuminance levels at each hour using a climate-based simulation. This step is common to all annual metrics using a 10 hour per day “analysis time period extending from 8 am to 6 pm according to local clock time” (IES LM-83-12 Section 2.1.2) . The only difference with sDA is that it measures illuminance for the space with the blinds in their determined position for each hour of the day.

Process illuminance data according to the sDA metric. Each annual metric has a unique way of processing the illuminance data. The sDA metric checks each point on an Illuminance Grid to see if its illuminance is at or above 300 lux for at least 50% of the year during the analysis period with the blinds in operation. With the daily 10 hour analysis period as stated above, “50% represents 1,825 hours per year” (IES LM-83-12 Section 2.2.6).

Below, Figure 1 shows the measured illuminance for one point, and every point on the Illuminance Grid is measured this way.

Figure 1. The orange dots show the hours that the point meets the minimum illuminance threshold of 300 lux. The gray dots show hours that the point does not meet the illuminance threshold.

All of the time points that meet the 300 lux threshold are summed together and are compared to the number of time points that do not meet this illumination threshold (Figure 2).

Figure 2. Total hours of illumination above 300 lux.

The point must meet the minimum threshold for at least 50% (1,825 hours) of the annual analysis period to be counted in the final score (Figure 3).

Figure 3. Since the point meets 300 lux for 53% of the time, it is acceptable.

The final score is the percent of points on the grid that meet the minimum illumination threshold for 50% of the time (Figure 4).

Figure 4. Percentage of points that meet the 50% threshold.

Let’s look at examples of blinds operations. The below image shows a space that has windows with daylight (upper) and view (lower) components.  The daylight and view windows are separated into different window groups since the view window has an overhang above it, while the daylight window is exposed.  This image shows, though, that the groups act independently, namely that the daylight window remains open while the view window is closed.  As a result, some direct sunlight is cast back into the room, but not an excessive amount.

Model from Clanton & Associates

Let’s look at this in more detail, in accordance to measuring grids as prescribed in LM-83.  Below is an Illuminance Grid of this same space with one point that is red, signifying that it is illuminated to more than 1,000 lux (ie it is receiving direct sunlight). This indicates at least one blind group is open. If all blind groups were closed, it would eliminate the red point. But since the sDA metric allows for up 2% of the grid to be hit by direct sun, this one point is permissible and the blinds can stay open

A rendering of the space at the same time reveals that the clerestory blinds are open.

Now let’s see how the Illuminance Grid changes over the course of a day, with and without operable blinds. The image below shows how operable blinds affect illumination levels in a space on a simulation of December 21st. Row A shows a series of renderings of a building without operable blinds. Row B shows a plan view of the Illuminance Grid in a space without operable blinds. Row C shows a series of renderings of operable blinds opening and closing according the sDA metric. Row D shows the same plan view, but this time the space has operable blinds.

Model from Clanton & Associates

An animated simulation of operable blinds in the workspace can be seen below. As the area of direct light increases to over 2% of the total space, blinds close to decrease the area and create a more visually comfortable environment.

Inside and outside views of the space are illustrated for the analysis period on March 21st.

How is exposure to direct light affected by solar angle? The position of the sun in the sky will affect how much light gets into your space. For example, let’s say it’s June 21st and your building is on the equator. The room that you are measuring light in has one window on the southern exposure. The blinds will stay open all day even if it’s sunny, because the sun will pass directly over your building from east to west and never enter the space.

Now take that same building and move it to Boulder, CO (40 degrees north). In the same climate conditions (sunny all day) the blinds will close for a significant portion of the day, because the sun will be at a lower position in the sky, meaning its rays are more horizontal and will penetrate into the space.

Next up, we will discuss how to score the sDA metric.

The first step of the sDA calculation determines whether the blinds are open or closed, depending on how much direct sun gets into the space. Within this step, the windows must first be categorized into groups and then the position of the blinds is determined hourly.

Form Window Groups: Windows must be controlled in groups. A window group is defined as a

group of coplanar windows, with similar shadow patterns from exterior shading and obstructions, and with similar shading device type and operation, which are associated with the same analysis area [Illuminance Grid] (IES LM-83-12 Section 2.2.6).

This means that all windows that are on the same facade with the same external shading devices (overhangs, awnings, lattices, etc.) will be in a group, and that the blinds on every window in this group will open and close together. The blinds operation of each window group is distinct from that of other groups, but every window in the same group will behave the same way.

The image below shows how windows are grouped. In pair A, all the windows are in the same group because they are 1. on the same plane, 2. have the same external shading strategy (which in this case is none), and they all correspond to the same analysis area. In pair B, each window is in its own group because they are all on different planes (facades). Their different orientations will allow light to enter the room in different ways so they will behave independently of each other. In pair C, the top windows make up Group 1 and the windows on the bottom make up Group 2. Even though all the windows are on the same plane, the top windows (Group 1) have no shading and the bottom windows (Group 2) have an awning over them.

Three sets of plan and elevation describing window groups.

Determine the position of the blinds or shades at each hour using the 2% rule: Once window groups are established, the position of their blinds is based on the 2% Rule, which is when “2% or more of the analysis points receive direct sunlight” (IES LM-83-12 Section 2.2.6). If more than 2% of the points on the Illuminance Grid receive direct sun, the blinds will close to bring it below the 2% threshold, and the sDA score will come from the illuminance values with closed blinds instead. Some blinds may stay open, and LightStanza calculates how to simulate with the optimal combination of open and closed blinds to maximize daylight without exceeding the 2% threshold.

The following three images are plan views of an Illuminance Grid with 100 points. In the first image, only two points on the grid are hit by direct sunlight. In other words, there is only 2% direct sunlight in the space, so the blinds don’t have to be drawn. In the second image, 28% of the Illuminance Grid is hit by direct sunlight. Since this is greater than the 2% that the sDA metric allows, this simulation will assume the blinds are closed and blocking the sun. The last image shows the same day and time as the second, now with the blinds closed. This is the data that will be used to score the sDA metric.

Direct Sun Exposure: Plan view of a space at three instances illustrating the two percent rule and blind use for the sDA metric.

These steps are specific to the sDA simulation. In the next post, we will provide examples of operable blinds in action.

Workplane

A workplane is an imaginary plane where work is performed and illumination is specified. The standard height for the workplane is shown in the image below, but there are different methods for choosing how to define workplanes.

Schematic Section

LEED Definition: The USGBC defines the workplane at 30″ above the floor, which is a standard height for table tops. If you are seeking LEED credit you will need to simulate at workplane height, 30″ above the floor.

BREEAM Definition: The Building Research Establishment Environmental Assessment Methodology (BREEAM) uses CIBSE LG10 to define the workplane as the horizontal, vertical or inclined plane in which a visual task lies. The working plane is normally taken as 0.7m above the floor for offices and 0.85m for industry.

Flexible Definition: However, not all furniture has a height of 30″, .7m, or .85m. Before deciding on a workplane height, consider researching the height of the work surfaces (i.e. tables and desks) that will be used in the space for a more accurate analysis. For example, tables in science labs are often 32 to 36 inches tall.

Illuminance Grid

The workplane is a type of Illuminance Grid since it is made up of a grid of points and has the function of collecting light. The spacing of the points on the grid can be changed and will affect the precision and speed of the computation of the results. In other words, densely spaced grids will be more accurate yet more computationally expensive than a sparser point spacing. In the image below, we see how different spacing between points affects the daylight measurement. 2′-0″ spacing is standard for LEED, but choose an appropriate density that makes sense for your project and purpose.

Plan view of Illuminance Grids with different spacing between points. A denser plane of points is more accurate, but more costly to compute.

An Illuminance Grid will typically be defined as a horizontal plane but can be placed to measure light on any plane inside the building. For example, they can be defined to measure light on a vertical surface such as a wall. This function can be useful in assessing situations like daylight hitting artifacts in a museum. Some users may also want to collect light on the ceiling, in which case the Illuminance Grid would be horizontal, and located just below the ceiling, with the active side facing down.

The sDA metric scores a space’s daylighting in conjunction with manual blind operation in a two-step simulation process, which we will briefly outline in this blog post and go over in more detail in following posts. The first determines the position of the blinds (whether they are open or closed) and the second measures daylight levels with the corresponding position of the blinds.

Blind positions are determined by how much direct sun gets into the windows. When a space receives too much direct sun, blinds close in groups until the amount is sufficiently decreased (Step 1).

Step 1. Blind Operations: Blinds, either electronically or manually controlled, contribute greatly to the quality and quantity of light. In the figure above, we see that as the sun’s position changes, different groups of blinds are used to maintain visual comfort in the space. Facades that use dynamic glass or redirect film to control for direct sun do not need blinds.

Realistic illuminance for the space is then calculated using a simulation of the blinds in their determined position from step 1 for each hour of the day. The sDA score is calculated using a formula that takes these raw illuminance values as input (Step 2). The score is the percent of points on the grid that meet the minimum thresholds.

Step 2. Scoring: Annual metrics like sDA determine the level of illumination at each point and at each hour after the blinds are in position., these metrics take thousands of time points of illuminance data comprising potentially millions of light readings and thousands of blind positions and compact them into a single value.

In the next several parts of the sDA series, we will cover blind operations and scoring in detail.

The sDA Metric: Is this a sufficiently daylit space?

Welcome to the reboot of our series on Spatial Daylight Autonomy (sDA), where we will be discussing sDA as a new, annual metric for a more accurate measure of daylight. In Part 1 of our series we will explain what an annual metric is, the difference between how much daylight a space is getting versus the portion of daylight that is usable, and why this distinction is important.
An annual metric is a function of hourly simulation results across an entire year in conjunction with climate analysis. The climate analysis is based on typical meteorological year (TMY) climate data. This type of continuous (and intuitive) analysis of a space can remove the uncertainties found in analyses that only evaluate a single point in time by accounting for hourly and seasonal changes in daylight availability and sun angle. LEED v4 prioritizes annual metrics for daylight credits, which is an improvement over the 2 solar positions simulated for LEED 2009 and the single ratio used for Daylight Factor. Although, LEED v4 does still offer a second option of 2 solar positions, excluding blinds, if sDA scores are too low.

Daylight availability can change by the hour, making annual hourly measurements a more accurate indicator of how much daylight is available to a space compared to point in time metrics.

While there are multiple annual metrics, sDA is the first one to use hourly daylight measures in conjunction with manual blind operation use. As pictured below, blinds are used by building occupants to control glare and maintain visual comfort, so acknowledging occupants’ influence on available daylight is crucial for calculating an accurate score.

Blinds are used by building occupants to control glare.

By not accounting for blind use, annual metrics other than sDA are overestimating the amount of usable light and underestimating energy consumption. The image below shows that even with available daylight, electricity is often used in place to avoid direct sunlight. This not only increases use of electricity, but it also negates window benefits by allowing heating and air conditioning to escape when we’re not using the windows for daylight.

If blinds, along with the fenestration, are not properly simulated, energy consumption can be underestimated and human benefits can be exaggerated in a building’s sDA score.

If your software doesn’t show blinds operating, your score is inaccurate. It is necessary to use a dynamic software simulation that accounts for how occupants may interact with their daylit environment: after all, what do lighting levels mean without considering the people experiencing them?

1. Meet the sDA Metric
2. Overview of sDA Calculation
3. Terminology 
4. Step 1: Blinds Operations
5. Blinds Operations: Examples
6. Step 2: Scoring
7. SDA and Its Relationship to ASE
8. SDA and Its Relationship to LEED
9. Why Your Scores Are Probably Wrong
10. Consequences of Simulating Without Blinds
11. The Debate/Get Involved