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.