Sizing of a skylight is dependant on the level of illumination required.
The table in ‘Size of Skylights,’ provides guidance for average installations under average related conditions. It also assumes that some other form of lighting, such as borrowed light from a window, contribute to the overall result in the living areas for a satisfactory result.
The figures provided should be adjusted to fine tune the specific installation for optimum results. The sizes or numbers recommended in the Table should be adjusted upwards for:
a) Roof Aspect
b) Longer shafts
c) Low reflectivity shaft surfaces such as timber
d) Glazings that may be shielded by trees or structures
e) Low reflectivity wall surfaces and furnishings
f) Low sun angles in Southern areas
g) Extra high level of illumination requirements
The sizes or numbers in the table can be adjusted downwards for:
a) Rooms where adequate general level of lighting exists and supplementary light is required only in a specific spot such as a work surface.
b) Rooms where low level of lighting is adequate, such as storerooms walk in robes etc.
In general terms, the area of daylighting found to provide adequate lighting levels is related to the floor area of the room and ranges from about 5 percent in the tropic to above 1- percent of the floor area in the southern parts of Australia.
(Note: At the time of publishing this Industry Code of Practice (ICP) in June 2009, a SIA proposal to vary the minimum floor area ratios downwards, has been accepted by the ABCB for inclusion in the BCA 2010)
With the aid of good ventilation, air conditioning or variable shading these areas can be increased substantially for excellent results and many examples exists particularly in commercial situations.
Generally it is better to have more light than not enough as adequate levels of natural lighting will be provided over longer periods of the day reducing the need for artificial lighting. In addition the internal environment of the building will be enhanced as the added brightness is conducive to a feeling of well being and the eye will easily adjust to compensate for the conditions unless the level is grossly excessive.
Acknowledgements: Skylight Industry Association Inc.