Without access to adequate daylight, human circadian rhythms become disturbed. Overwhelming medical evidence suggests that any disruption to our circadian rhythms has highly negative consequences in terms of our health, happiness and well-being.
Depriving the human body of natural light in the daytime and darkness in the night can be biologically disastrous – negatively affecting mood, impairing eyesight and even potentially causing cancer.
Designing buildings that account for natural daylight cycles, reduce the need for artificial lighting and consider the need for visual and thermal comfort is of paramount importance for the well-being of society. To create a space that is optimised for daylight can be a challenging task – accounting for the sun’s ever-changing position, variable weather conditions, building orientation and glare is no easy feat, however, responding to these factors is essential for creating buildings that prioritise human health, comfort and well-being.
There are very few daylighting requirements in existing standards and building regulations that are enforceable by law in any country, nor any holistic consensus on how to assess what constitutes good daylighting design. There are authorities like Building Research Establishment Environmental Assessment Methodology (BREEAM) that state guidelines and recommendations for best practice, however, they tend to stipulate Daylight Factor as the preferred performance indicator, which is fraught with limitations.
Originally devised in the early 20th century, and currently the most widely-used daylight model, Daylight Factor describes the ratio of light outside of a building over the light inside a building, expressed as a percentage – the higher the DF, the more natural light is available in the room. Rooms with an average DF of 2% or more can be considered daylit, but electric lighting may still be needed to perform visual tasks. A room will appear strongly daylit when the average DF is 5% or more, in which case electric lighting will most likely not be used during daytime.
- Simplistic calculation
- Uses a fixed daylight variable that only predicts the worst possible daylight conditions (100% cloud coverage)
- Assumes illuminance levels are always the same across the sky
- Insensitive to building orientation, solar angle and solar intensity
- Produces inaccurate daylight performance data
- Thermal discomfort and glare are common problems found in buildings that use DF as a daylight model
Climate-Based Daylight Modelling (CBDM) is a much more comprehensive model that is slowly being considered as the future standard within the industry. The CBDM concept, introduced around 20 years ago, attempts to deliver a more holistic daylight model based upon Annual Illuminance (AI) data – historical daylight and weather data recorded by meteorological stations across the globe.
CBDM uses hourly daylight and weather variations, recorded over the course of a year in specific locales, to forecast realistic sun and sky conditions in different climates and times of the year. This approach enables CBDM simulations to predict the minimum, maximum and average lux a building can potentially receive while also accounting for many of the variables Daylight Factor fails to consider such as: building orientation, window geometry and transmittance, surface reflections, local topography and solar intensity and angle.
CBDM is generally based upon a set of daylight performance metrics – they include Daylight Autonomy (DA), Useful Daylight Illuminance (UDI), Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE).
Although not every metric is used when simulating a climate based model, it is important to understand what they measure to ensure designers can use the most appropriate combination to simulate a space that is fully optimised for daylight.
Daylight Autonomy (DA)
The extent to which a space has enough natural daylight to allow the prescribed activity to be carried out without the need for electric lighting. It is represented as a percentage of annual daytime hours that a given point in a space is above a specified illumination level (usually 300 lux). DA provides an intuitive insight into how well daylight can penetrate a space and highlights areas within a space that could benefit from responsive lighting control systems.
Useful Daylight Illuminance (UDI)
The annual occurrence of illuminance that is in a range considered “useful” by occupants. The metric uses three ranges to judge the daylight performance of a space.
The light within a space would be considered insufficient without artificial lighting.
UDI Autonomous Range
The daylight within a space is deemed acceptable, and artificial light would not be needed for most of the day.
UDI Exceedance Range
The daylight within a space is regarded as excessive and a source of glare and overheating.
Spatial Daylight Autonomy (sDA)
The percentage of floor space that falls below, within and above a specified level of illuminance. It groups areas of a floor plan and presents users with a single percentage denoting daylight sufficiency.
Annual Sunlight Exposure (ASE)
The number of hours per year a point on the working plane receives direct sunlight greater than a specified threshold value.
Unfortunately, there is still a lot of work to do in bringing more comprehensive daylight standards to the forefront of the industry. While authorities like Leadership in Energy and Environmental Design (LEED) are beginning to incorporate CBDM metrics into their certification, many others are still playing catch up.
The EFA Priority Schools Building Program (PSBP) – a UK government initiative designed to rebuild and refurbish school buildings in the worst condition across the country – is a step in the right direction. PSBP demands that all projects built under the initiative use CBDM as the preferred process for modelling daylight – it did not give an option for using DF.
The initiative states that it will use sDA and UDI as the preferred metrics, and requires all of its school buildings to:
- Achieve a minimum target sDA of 50% for each learning space, sport hall and exam area
- Achieve a minimum target of 80% UDI-a for each learning space, sports hall and exam area
For the time being, only buildings constructed under the PSBP initiative are required to use CBDM in the UK, however, with the benefits of CBDM becoming more and more mainstream, it is only a matter of time before we see more stringent daylight requirements become enforceable.
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