Daylighting Calculation Options - LEED

This topic covers the Daylighting calculation options specific to the 2-LEED Simulation type.

LEED Simulation Options

LEED version

Select from the supported versions of LEED:

1-LEED v2 - for NC 2.2 Credit EQ 8.1 (LEED v2)
2-LEED v3 - for Daylighting BD+C: New Construction v3
3-LEED v4 - for Daylighting BD+C: New Construction, Options 1 and 2
4-LEED v4.1 - for EQ Credit, Options 1 and 2

Perform LEED daylight option 1 (annual) simulation

When the LEED version is set to one of 1-LEED v4 or 2-LEED v4.1, check this checkbox if you would like to run an annual sDA + ASE daylighting simulation suitable for assessing credits for Daylighting BD+C: New Construction, Option 1.

Perform LEED daylight option 2 (point in time) simulation

When the LEED version is set to one of 1-LEED v4 or 2-LEED v4.1, check this checkbox if you would like to run a point in time illuminance daylighting simulation suitable for assessing credits for Daylighting BD+C: New Construction, Option 2.

Option 1 (Annual) Options

Dynamic shading method

When running LEED annual daylight simulations, Dynamic blinds are simulated according to the requirements set out in LM-83-12.

For an explanation of the Dynamic shading method options, see the Dynamic shading method article under the General Simulation type.

Override zone occupancy schedule

By default the occupancy schedule used for each zone in the Annual simulations are based on the zone Occupancy schedule defined on the Activity tab, however you can override this to use the same occupancy schedule for each zone by selecting this option and selecting the schedule in the browse control below.

Occupancy schedule

When the above Override zone occupancy schedule option is selected, you can select the occupancy schedule to be used to define when zones are occupied for all zones in the simulation. Values in the schedule greater than 0 indicate occupancy.

The default occupancy schedule provided by DesignBuilder does not include daylight saving time (DST). DST has to be added manually to the schedule. For example for the Northern Hemisphere you can use a schedule like the following:

Schedule:Compact,

8:00 - 18:00 Every day (DST Northern Hemisphere),

Fraction,

Through: 03/30,

For: AllDays,

Until: 08:00, 0,

Until: 18:00, 1,

Until: 24:00, 0,

Through: 10/26,

For: AllDays,

Until: 07:00, 0,

Until: 17:00, 1,

Until: 24:00, 0,

Through: 12/31,

For: AllDays,

Until: 08:00, 0,

Until: 18:00, 1,

Until: 24:00, 0;

Option 2 (Point-in-time) Options

Sky method

There are 2 methods for generating the sky illuminance distribution when running the LEED v2 and v4 the Radiance calculations:

1-Standard sky - the default method where one of the predefined standard sky distributions are applied using the Radiance gensky tool.
2-Perez all weather - an alternative method suitable for all weather types based on the Perez sky clearness (ε) and sky brightness (Δ) parameters and using the Radiance gendaylit tool. There are 3 Perez methods to choose from as described below.

For LEED v4 and v4.1 point in time calculations, the 2-Perez all weather option is always used and this setting is not displayed.

Perez method

There are 4 methods to choose from.

1-Brightness and clearness - if you know the Perez sky clearness (ε) and sky brightness (Δ) parameters of the sky to be analysed choose this option.
2-Direct normal irradiance for when measured direct normal and diffuse horizontal irradiance data is available. This option can be useful since it allows readily available solar data in W/m2 from the EnergyPlus weather file to be used to define sky illuminance. Go to the Simulation tab, navigate to site level and select hourly data to see solar data suitable for the location of your building for any hour of the year. Note that using full spectrum solar data to define illuminance is seen by daylighting purists as less accurate than using measured values of actual illuminance (see option 4).
3-Direct horizontal irradiance for when measured (or calculated) direct horizontal and diffuse horizontal irradiance data is available. Such data can usually be found in EnergyPlus epw files in W/m2.
4-Direct normal illuminance for when measured (or calculated) direct normal and diffuse horizontal illuminance data is available. Such data can often be found in EnergyPlus epw files but DesignBuilder does not provide access to it directly. This is considered to provide the most accurate method if the required illuminance data is available.

The sky properties must be set separately for morning and afternoon using data that depends on the Perez method selection .

Sky clearness (Epsilon)

Perez sky clearness (ε). Enter a value from 1.0 to 12.0.

Available only when using the 2-Perez all weather Sky method and the 1-Brightness and clearness Perez method

See: Perez, R., R. Seals, and J. Michalsky, 1993. All-weather model for sky luminance distribution—Preliminary configuration and validation

Sky brightness (Delta)

Perez sky brightness (Δ). Enter a value from 0.01 to 0.6.

Available only when using the 2-Perez all weather Sky method and the 1-Brightness and clearness Perez method

See: Perez, R., R. Seals, and J. Michalsky, 1993. All-weather model for sky luminance distribution—Preliminary configuration and validation

Direct normal irradiance

The direct normal solar broad spectrum irradiance (in W/m2 or W/ft2). This value can be found from the site level hourly weather data.

Available only when using the 2-Perez all weather Sky method and the 2-Direct normal irradiance Perez method.

Diffuse horizontal irradiance

The diffuse horizontal solar broad spectrum irradiance (in W/m2 or W/ft2). This value can be found from the site level hourly weather data.

Available only when using the 2-Perez all weather Sky method and the 2-Direct normal irradiance or 3-Direct horizontal irradiance Perez method have been selected.

Direct normal illuminance

The direct normal illuminance (visible spectrum) (in lux or fc).

Available only when using the 2-Perez all weather Sky method and the 4-Direct normal illuminance Perez method

Diffuse horizontal illuminance

The diffuse horizontal illuminance (visible spectrum) (in lux or fc).

Available only when using the 2-Perez all weather Sky method and the 4-Direct normal illuminance Perez method

LEED v4 and v4.1 Option 2 Sky Settings

An important part of the LEED v4 Option 2 daylight calculation is to set the sky conditions to be used in the simulations. This must be done using the 2-Perez all weather Sky method. The procedure required for this is described by USGBC:

Calculate illuminance intensity for sun (direct component) and sky (diffuse component) for clear-sky conditions as follows:

Use typical meteorological year data, or an equivalent, for the nearest available weather station.
Select one day within 15 days of September 21 and one day within 15 days of March 21 that represent the clearest sky condition.
Use the average of the hourly value for the two selected days.
Exclude blinds or shades from the model. Include any permanent interior obstructions. Movable furniture and partitions may be excluded.

Method 1 using Direct horizontal irradiance

Based on LEED documentation:

Obtain typical meteorological year data. Use local weather data or TMY weather data files for the nearest city.

To calculate the illuminance intensity for sun (direct component) and sky (diffuse component) in a TMY2 or TMY 3 file, you can export the data into spreadsheet format.

From the TMY, select the day within 15 days of September 21 that has the clearest sky condition (total sky cover at its lowest value) at 9 A.M.

From the TMY, select the day within 15 days of March 21 that has the clearest sky condition at 9 A.M.

Determine the direct horizontal irradiance (W/m2) values at 9 A.M. for the day selected in September and at 9 A.M. for the day selected in March. Average the two values and use the result in the 9 A.M. simulation as the direct horizontal irradiance input. If the file does not explicitly state direct horizontal irradiance, calculate it as follows:

Direct horizontal irradiance = Global horizontal irradiance – Diffuse horizontal irradiance

Determine the diffuse horizontal irradiance (W/m2) values at 9 A.M. for the day selected in September and at 9 A.M. for the day selected in March. Average the two values and use the result in the 9 A.M. simulation as the diffuse horizontal irradiance input.

Repeat procedures 3–6 for 3 P.M.

Choose the 2-Perez all weather Sky method (this is done behind the scenes for LEED v4 and v4.1).

Choose the 3-Direct horizontal irradiance Perez method.

Input the custom values for Direct horizontal irradiance and Diffuse horizontal irradiance in the Calculation options dialog and run the Radiance simulation.

Method 2 using Direct normal irradiance

Another (simpler) procedure to provide the sky condition data within DesignBuilder is as follows:

Click on the Simulation tab and go to Site level to view the site solar radiation data. To focus on this click on the Detailed tab of the Display options panel, scroll down to the Site Weather settings and check only the Direct normal solar and the Diffuse horizontal solar checkboxes. Click on the Save options for future use display options toolbar button and save the settings.
Scroll to view September data and identify the clearest day within 15 days of 21 September. This might be by checking to see the highest value of beam solar radiation. For example 16 September can be seen to be a very clear day:
Change to view 1 day at a time by entering 1 in the Days per page text box and pressing the Update display options toolbar button.
Scroll to 16 September and write down the Direct normal solar and the Diffuse horizontal solar W/m2 values at 9am and 3pm on this day.
Repeat the above process for March to find the Direct normal solar and the Diffuse horizontal solar W/m2 values at 9am and 3pm for the clearest day within 15 days of 21 March.
Calculate average Direct normal solar and the Diffuse horizontal solar values for 9am and 3pm for both the March and September days.
Choose the 2-Perez all weather Sky method (this is done behind the scenes for LEED v4 and v4.1).
Choose the 2-Direct normal irradiance Perez method.
Enter these calculated Direct normal irradiance and Diffuse horizontal irradiance values in the Calculation options dialog and run the Radiance simulation.