Calculation of the openings for natural lighting

Calculating a daylight factor based on your building design includes window/skylight sizes, overhangs/light shelves, glass types, and exterior/interior reflectances can become very complex.

A simple rule of thumb to approximate the daylight factor for day lit spaces using vertical windows is

DF = 0.1 x PG
DF = daylight factor

PG = Percentage of glass to floor area Assuming a 1000 sf office space has 200 sf of windows. The daylight factor would be

DF = 0.1 x (200/1000)
DF = .02 or 2%For more detailed calculation methods which take into account the three major components (light from sky vault, light reflected from exterior surfaces and light reflected from interior surfaces) that make up the daylight factor refer to the references in this guidance.

Requirement: “(excluding all direct sunlight penetration)”Traditional day lighting excludes direct beam sunlight which typically introduces unwanted heat, glare and extreme contrast resulting in increased energy use and uncomfortable conditions. The sun is constantly changing positions (altitude and azimuth) during the day and throughout the seasons. The MOU guidance recognizes that excluding “all” direct sunlight penetration may not be possible or desirable. If the building design incorporates both passive solar heating and day lighting excluding the direct beam would not be desirable since this is the main source of heat. The intention of “all” would be in an office/classroom/business type environment where passive solar heating (direct beam sunlight) would not be part of the design and would lead to poor day lighting. Direct sunlight should not be used when calculating the daylight factor.

Excluding direct beam sunlight starts with good building orientation, placing the long axis of the building facing due north and south. Windows facing north will receive the greatest amount of daylight and can dispense with overhangs and fins for most latitudes


The second best day lighting is from south facing windows which can be protected from direct beam sunlight using fixed overhangs. The least desirable day lighting is from east and west facing windows because of the difficulty of eliminating direct beam sunlight during mornings (East side) and afternoons (West side). The only way to completely omit direct beam sunlight during this time is movable shading devices such as blinds and roller shades which will also block the day lighting. Figure 2 provides solar angles for various latitudes which can be used to size fixed south facing overhangs.


Keep in mind that these solar angles allow for both day lighting and passive solar heating so it will be necessary to divide the window wall into two areas, an upper glass area for continuous day lighting protected by a overhang that prevents both summer and winter direct beam penetration, and a lower area which provides both day lighting and view with light colored blinds or roller shades to control winter direct beam penetration. A more optimum design would make use of light shelves to bounce light further back into the space (see Figure 3).

Recommended daylight factors for interiors
Guidelines for good lighting

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