• It's for daylighting.  How far will the light penetrate into a room if you are using wall windows or skylights.

• DFav=0.2*Window Area/Floor Area

Daylight factor (DF) is the amount of daylight available inside relative to what is available outside at the same time, measured as a percentage on an overcast day. So a DF of 2% means that for every 100 foot-candles measured outside, there are two available inside. In this case, maybe outdoors we have 2,000 fc and inside we measure 40fc. DF accounts for not only the area of glazing, but the depth of the room (because far from the window will be darker), the geometry of the room, and the color of the interior finishes (because white surfaces reflect about 80% of the light that impinges upon them, and brown ones reflect about 20% . . . so a white room will "keep" the light's intensity as it ricochets from the window off the room surfaces). Because the measurement is made on an overcast day, the measurement is independent of climate, time of day, or orientation of the window. In other words, you could pick up your building, drop it at a different latitude and rotate it to face a different direction, and the measurement should remain the same.

A brightly-lit room will have a daylight factor of about 5%, meaning the indoor illuminance, with the lights turned off, is 5% of the illuminance available outside; a dimly-daylit room will have something closer to 0.25% DF.

The "av," part of the DFav of course, indicates that we are estimating the average daylight factor of the room, so by this rule-of-thumb given by the equation, to estimate the DF of your room, take one-fifth the ratio of the window area, as compared to the room floor area, and this returns you the average daylight factor. Note that if this equation returns 0.01, that means you have a daylight factor of 1%.

***While the exam, and most specifications, focus on the average daylight factor, know that the "flaw of averages" problem creeps into this line of reasoning. One can obviously increase the average DF by allowing more light at the window, but that's a counterproductive idea because, paradoxically, more light near the window only makes the room appear to be darker to occupants. Picture a picnic pavilion, with all that aperture area, and ask yourself why it appears dark to the occupants: their eyes can't simultaneously adjust to the intense light at the perimeter and the relative dark of the untreated wood on the overhead plane. The most important, and most overlooked, aspect of proper daylighting is not how much light you bring in, but rather how deep you can bring in the light. . . how bright you can make the DARKEST parts of the room—that’s your goal—not how bright you can make the average! So use the average thinking for the exam, but in your practice, try to move to a MINIMUM DF for your firm's specs.—Michael Ermann, Amber Book creator.