Average Daylight Factor

The Average Daylight Factor (ADF) is a measure of interior daylight. It can be used to establish whether a room will have a predominantly daylit appearance and if not, it can provide levels below which a room should not fall even if supplementary electric lighting is provided.

What can we do?

ADF values can be calculated for rooms within a proposed development, and checked against the recommended value. Existing and Proposed ADF values can also be calculated for properties which overlook a site. Factors on which the ADF depend are: Vertical Sky Component (VSC) at the face of each window, the Total Window Area, Total Wall Area, Wall Reflectivity and Window Transmission.There are no specific BRE criteria for reduction in ADF if a proposed development were to be implemented, but since the ADF is related to the VSC via the obstruction angle, a reduction in VSC leads to a reduction in ADF.

BRE Criterion

The BRE states that for a predominantly daylit appearance the ADF should be 5% or more if there is no supplementary electric lighting, or 2% or more if there is supplementary electric lighting. There are additional recommendations for dwellings. These are

2.0% - Kitchens
1.5% - Living Rooms
1.0% - Bedrooms

These figures are also recommended in BS 8206 Part 2 1992 entitled 'Code of Practice for Daylighting'.

Above is a false colour image depicting the Daylight Factor values across a series of rooms in a building. It shows the change in Daylight Factor depending upon the proximity of a window. An ADF could be calculated for each room by finding the average value. Below is the same image but as a greyscale, this is a more realistic representation of the apparent light levels.

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