DesignBuilder provides calculations and outputs for the following daylighting metrics:
The DesignBuilder Radiance illuminance simulations provide a detailed multi-zone physics-based calculation of illumination levels on the working plane of a building using fixed static external lighting conditions and sky distributions.
Daylight factors are calculated as the ratio of daylight illuminance to that received by an unobstructed horizontal external plane. They are commonly calculated across a grid of points on the working plane using a CIE overcast sky. An average daylight factor can be calculated for a zone as the area-weighted average of the illuminance on each cell on the gridded working plane.
The illuminance on the working plane can be reported instead of daylight factors as described above but without the division by the unobstructed horizontal external plane. Illuminance is used as a key metric in the LEED Daylight Credits Option 2 calculations using a Clear sky.
BREEAM HEA 01 Option 4a is a point in time daylight factor-based calculation method used to check that at least 80% of net lettable floor area in occupied spaces is adequately daylit on the working plane, 0.7m above the floor under a uniform CIE overcast design sky. A zone is adequately daylit if both the following conditions are met:
Average daylight factor is at least 2.0%.
A uniformity ratio of at least 0.3 or a minimum point daylight factor of 0.8%.
Annual climate-based daylight modelling (CBDM) is the prediction of illuminance on the working plane using realistic sun and sky conditions based on standardised hourly climate data. In DesignBuilder, CBDM evaluations are carried out for a full year at a time-step of an hour in order to capture the daily and seasonal dynamics of natural daylight. The Radiance daylighting engine is used in these simulations. You can read more about Radiance and the CBDM metrics output by DesignBuilder below. CBDM is also known as "dynamic daylighting". These metrics summarise the daylight availability over the whole year and throughout a space. Three main dynamic daylighting metrics calculated by DesignBuilder are described below.
Spatial Daylight Autonomy (sDA) assesses whether a space receives sufficient daylight on the working plane. It is expressed as the percentage of occupied time during the year when the minimum work plane illuminance (typically 300 lux) can be maintained by daylight alone. It defines the extent to which a space has enough natural daylight to allow the prescribed activity to be carried out without the need for electric lighting. This is a recommend metric by the Illuminating Engineering Society of North America (IESNA).
Dynamic window shading as specified in IES LM-83-12 is applied. For more specific information on this, see the Dynamic shading method section under Calculation options, 1-General Simulation type.
The most commonly used metric, sDA 300/50%, used in LEED v4 and v4.1 Option 1 daylighting calculations, represents the percentage of the floor area that exceeds 300 lux for at least 50% of the occupied time. Unoccupied periods are ignored.
Annual Sunlight Exposure (ASE) is used alongside sDA to quantify the risk of visual discomfort due to glare. At the zone level, it measures the percentage of the working plane exceeding the threshold of 1000 lux for more than 250 occupied hours per year. For grid points on the working level it measures the number of occupied hours per year the point receives direct sunlight greater than the 1000 lux threshold value.
UDI is the percentage of the occupied time when a target range of illuminance values at a point on the working plane is met by daylight alone. At the zone level, it measures the percentage of the working plane that is within the target illuminance range for more than 50% of the occupied hours. It excludes any times when there is inadequate natural daylight or excessive direct sunlight which would give rise to a risk of glare. UDI therefore not only provides information about useful daylight illuminance, but also on the propensity for excessive levels of daylight that are associated with glare, occupant discomfort and unwanted solar gains. In this way the single UDI metric can be used to summarise the overall daylight performance of a space.
BREEAM annual daylighting calculations are based on an hourly calculation of illuminance on the working plane of each zone of the model. For each hour and each zone, the average and minimum point illuminance are calculated. If the average illuminance for that hour is above the Avg illuminance threshold, this hour passes for the average criteria. If for the hour the minimum point illuminance is above a given Min illuminance threshold, this hour passes the minimum criteria. The number of hours, for which both the average and minimum criteria are met, is compared to the thresholds defined in Display options. If both criteria are satisfied, then the space is considered satisfactory.
The Option 4b simulations are carried out without window shading being applied.
Note: The worst lit point is defined as the same fixed point over the whole year. To identify this, the number of hours in a year each point on the grid receives the minimum illuminance required is calculated. The worst lit point is the one with the fewest hours. Compliance is then based on whether the number of hours at this point exceeds the threshold.
Generally, a right to light refers to the right to receive sufficient light through windows. Right to light generally become an issue when a new development, or proposed development affects the access to light of an adjoining property. Right to light also apply to obstructions caused by trees, hedges etc. DesignBuilder can be used to assess the impact of neighbouring buildings or other obstructions on the windows of an existing building.
Vertical Sky Component is a measure of the amount of sky visible from a given point on the outside face of the window. It is the ratio of the illuminance at that point due to the light received directly from a standard CIE overcast sky to the illuminance on an unobstructed outside horizontal plane under the same sky.
An unobstructed vertical surface will have a VSC of ~40% and 27% is generally considered adequate for new residential developments.
No Sky Line divides areas of the working plane which can receive light directly (i.e. with no bounces) from the sky, from those which cannot.
No more than 20% of floor area should be behind the No Sky Line, i.e. with no view of the sky.
Annual Probable Sunlight Hours is the percentage of direct sunlight hours on the outside face of a window divided by number of hours when sky was clear with sun. Calculations involve counting hours over the year where the sensor on the mid-point of the window receives any direct light and also the hours where the direct normal illuminance (from weather file) is greater than zero and dividing one by the other.
The Average Daylight Factor gives the average illuminance from daylight on the working plane relative to that received by an unobstructed horizontal external plane. It is calculated as the area-weighted average of the illuminance on each cell on the gridded working plane. A single value defines the performance of a zone.
