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Daylight Factor: A Metric From The Past?

As diurnal, daytime animals, we humans have evolved (and hopefully still are) to occupy the day, not the night, except for maybe the crepuscular Architects out there burning the candle at both ends!

Our circadian cycles necessitate access to daylight that is proven to impact our mood, focus, sleep, motivation, learning and another 20+ health related aspects. We are spending more time indoors than ever before while our access to the full spectrum of light is now only a fraction of what it has been in the past! In this context, the importance of designing for natural daylight in buildings of all typologies is imperative for good design.

Today, we can design for daylight using a handful (maybe 2 handfuls) of useful metrics, with one metric still dominating many a design discussion: the Daylight Factor.

Having been around for some 50 years now and developed in the world of physical modelling, rule of thumb design and basic equations, this simple metric was fit for purpose for a long time. In the last 25 years, it has slowly been adopted all over the world, being embedded into planning documents, guidelines, legislation and rating tools, it has proved an important backstop for designers in a world less advanced than today.

As simply the ratio of internal illuminance to unobstructed horizontal illuminance, the Daylight Factor metric has many questionable qualities. Based on a CIE overcast sky conditions, it assumes no sun and thus is insensitive to orientation and arguably to the climate of the location you are designing in!

Have a look at the admittedly extreme example below, with fundamentally different daylight hours and solar exposure, the northern (Anchorage) and southern (Florida) orientations are deemed to have the same average Daylight Factors? How can the provision of daylight, with the associated aspects of comfort, really be the same in sunny Florida and the more overcast but extended daylight conditions of Anchorage? 

US – Anchorage – DF Average 11.5 %

US – Florida – DF Average 11.5 %

Given the above, and greater access to radiance based and integrated software programs with the ability to model daylight using real annual climate data, are we not in a position to leave this useful but outdated approach to daylight design in the past?

Many alternatives exist, such as Spatial Daylight Autonomy, which has now been included in the new Green Star Design and As-Built tool under Visual Comfort: Compliance Using Daylight Autonomy. Through this metric, a much more useful assessments of a space can be undertaken that provides a measure of daylight illuminance sufficiency for a given area. For example, a percentage of the floor area that exceeds a specified illuminance level can be modelled, e.g., 300 lux, for a specified amount of annual hours, e.g., 50 percent of the hours from 8:00 am to 6:00 pm.

Being much more useful than the Daylight Factor, this newer solar and orientation sensitive approach to design provides a much more accurate representation of daylight opportunities. What do you think? Are we ready for more sensible design metrics or do we still have a long way to go?


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