Annex 7.C Daylight factor calculation
Average DF% = (52 x M x W) / A
Where:
A = The sum of the area of all room surfaces (ceiling, floor, walls, doors, windows and roof lights), in m2
M = Correction factor for dirt or ease of cleaning, consisting of:
1.0 for vertical glazing, or
0.8 for sloping glazing, or
0.7 for horizontal glazing
W = Glazed area of windows or roof lights, taking account of framing, in m2. Measure glazed panes or measure window area including frames then multiply by:
0.9 for metal frames (patent glazing), or
0.8 for metal frames (large pane), or
0.7 for timber frames (large pane), or
0.6 for timber frames (smaller pane)
Assume the factors for PVC framed windows are equal to timber.
The angle of visible sky from inside a room has been removed so that the standard is not location dependent.
When designing window apertures it is worth considering that the window area below the working plane does not significantly increase the amount of daylight falling onto the working plane. This is because the light from the lower part of the windows has to bounce off at least two room surfaces before it reaches the working plane.
A room capable of being top lit is the part of the building where roof lights can be used to provide natural daylight, this would include all single-storey buildings or rooms located on the upper most floor of a multi-storey building. Roof lights do not have to be provided where the daylight factor can be achieved from a combination of windows on the vertical plane.