Why is natural light so important in school design?

The reinvigorating effect of natural light and warmth can also be felt on a smaller scale, and in a wide range of environments, from homes and offices, to public buildings, schools and universities. It is perhaps no surprise then, that when a recent study¹ looked at how the physical design of educational buildings affects student performance, one of the significant individual parameters was lighting.

Why daylight?

Several studies have shown that daylight is not only good for children’s overall health and wellbeing, but that it can also significantly improve academic performance.

One such study¹ was conducted by the Sorbonne University using SINPHONIE Study data, covering 13 European countries with a total of 2,387 children participating. It concluded that academic performance can increase by up to 15% when students work in classrooms with larger windows – due both to increased daylight, and a better view to the outside world.

The Clever Classrooms study² conducted by the University of Salford, UK, concluded that good daylight helps to create a sense of physical and mental comfort, its benefits more far-reaching than merely an aid to sight.

Oasis Academy school - Teacher in classroom under Longlight Modular Skylights.

How to design with daylight

While daylight does occasionally need to be supplemented by ample, high quality artificial lighting when outside light levels are too low, where possible we should aim to make daylight the main source of lighting in schools. If it is properly controlled, sunlight is generally welcomed as a source of lighting in buildings throughout Europe.

When windows or skylights face North, the daylight entering a space tends to be softer and more diffused, with subtle changes in light levels and colour texture throughout the day. With other orientations, sunlight enhances the overall brightness of interiors, with specific areas of concentrated light.

The challenge of designing with daylight is particularly evident in deep classrooms, where there is a considerable distance between windows and the back of the room. Here there is often a disparity in light levels – bright near the windows and darker further back.

In situations where the shape or size of classrooms does not allow for adequate light levels throughout, and/or where the possibility of window space is limited, skylights are often the optimum solution. Where there is no direct access to the sky due to constructed floors above, light shafts can be an effective alternative.

One of the collateral benefits of creating openings for daylight is that they also provide us a connection to the outdoors, allowing us to follow changes in the weather, the time of the day and year.

There are many factors to take into account when considering how much daylight will be gained from windows or skylights. These include glazing transmittance, wall thickness, external obstructions, extensions above (e.g. overhangs, balconies) and to the sides (e.g. extension of the building itself), depth of the room, etc. When taking all these factors into account, a skylight typically provides more than twice the amount of daylight than a facade window of equal size.

Self-supporting modular skylights integrated into historic sloping roofs - Music Academy, Hammelburg

Controlling excessive glare

Glare is created when areas that are too bright are located within the field of view, or when the contrast ratio is high. Nevertheless, glare caused by daylight differs from glare caused by electric light sources in terms of the size, complex luminance distribution and acceptance of the users (e.g. people tend to be more tolerant of glare in a daylit environment)⁴.

While high levels of daylight in a classroom provided via large windows and skylights are optimum, they still need to be controlled to avoid the excessive glare that can come from direct sunlight. This issue becomes increasingly acute with today’s widespread use of interactive whiteboards and projectors in classrooms.

One important aspect to consider when controlling glare and contrast is the orientation of the windows. Larger expanses of glazing should ideally be facing north, in order to allow diffused daylight to penetrate as much as possible throughout the day/year.

Another effective solution for glare control is the installation of easily operable opaque blinds to control daylight levels. An alternative to blinds is permanent external shading, but this may still require the use of additional shading devices, depending on individual circumstances and requirements.

Other solar protection devices such as curtains, roller blinds etc., made in textile, film or perforated opaque materials can be used. These will all reduce the occurrence of glare to differing degrees, depending on: optical properties of the material; orientation of the window; geographical location and annual sunshine hours; glazing area and transmittance of the pane and the distance of the user from the light source.

The material properties and levels of glare protection for these type of solar protection devices are defined in European Standard EN 14501 Blinds and shutters, and the European Standard EN 17037 Daylight in Buildings.

Equally important is the issue of contrast. When considering both glare and contrast, the difference between absolute brightness and relative brightness is critical. Consider how high-beam headlights can be blinding at night, while hardly noticeable during the day. Similarly, a strong source of light will cause much less glare and contrast in a generally brightly-lit room compared to, say, a single large window with direct sunlight in an otherwise dimly-lit room.

A rule-of-thumb guideline of the “luminance ratio” between visual task and near surroundings (e.g. adjacent or periphery wall surface) is recommended not to exceed 1:10 within the field of view. Today, and with modern open plan offices, and especially with windows in the visual field, the ratio can be 20:1 or higher between a window and adjacent surfaces. This luminance ratio is an expression of the ratio between the luminance within the central vision and the peripheral vision of the surroundings³.

Windows vs. skylights

Good daylight distribution across a room is often best achieved by using several different sources of daylight, such as a combination of skylights and windows, and this can also ensure reduced levels of glare and contrast. However, let’s not forget the importance of a view: "When we are sitting next to a window, we would rather tolerate a high amount of daylight and enjoy the view outside than draw the blinds down and use artificial lighting.⁴

For spaces where even large areas of window glazing will not allow enough daylight to penetrate, or where their installation is simply not possible – such as very large classrooms, lecture theatres or indoor common areas in the centre of a building – skylights are a great option to consider. Operable skylights strategically located in the overall building design allow plenty of daylight to penetrate in the dark winter months, while also providing fresh air throughout the year, improving indoor air quality and helping regulate temperature.

Dr. Knippenberg College - A future oriented school full of daylight and fresh air in Helmond, the Netherlands

Building standards and light levels

Daylight performance in an interior space depends largely on the availability and properties of daylight at the building’s location (i.e. the prevailing climatic conditions). The European Standard Daylight in buildings (EN 17037) has changed the basis of daylight evaluations. The standard specifies an evaluation method for daylight provision in a space to ensure sufficient levels of daylight throughout the year. To demonstrate compliance with EN 17037, it is necessary to show that a target illuminance level is achieved across 50% of a reference plane for at least half of the yearly daylight hours. In addition, for spaces with vertical or inclined daylight openings, a minimum target illuminance level is also to be achieved across 95% of the reference plane. The standard proposes two methods to assess daylight provision in the interior: 

• Method 1: A calculation method based on daylight factor and yearly daylight availability of diffuse skylight
• Method 2: A calculation method based on the direct prediction of illuminance levels using hourly climate data of diffuse skylight and direct sunlight. 

The target daylight level is based on the provision of an interior illuminance higher or equal to 300 lux, corresponding to the requirement for lighting at work places (see below). The minimum target daylight level is higher or equal to 100 lux. In a space lit only by skylights in a nearly horizontal roof construction, the target daylight level of 300 lux needs to be achieved across 95% of the relevant floor area. The corresponding target daylight factors for each European capital, is available in the standard, but values are higher for countries located in Northern Europe compared to Southern Europe due to the natural decrease in daylight availability.

The absolute light levels that are needed for a particular visual task will depend on the character of the task and the visual environment where it is performed. A European Standard, EN 12464-1: Light and lighting — Lighting of work places — Part 1: Indoor work places, provides information on the indoor light levels applicable for a school environment.

Generally, the following interior light levels in schools are recommended:

• 100 lux for interiors where visual tasks are limited to movement and casual perception, e.g. circulation areas, corridors, etc.
• 500 lux for interiors where school children need to perform their visual tasks and have a feeling of well-being, e.g. auditoriums, lecture halls, practical rooms and laboratories, libraries (reading areas), etc. In classrooms, this should also be the level of light on the blackboard/whiteboard. For classrooms used by young children, the standard state that the requirement can be lowered to 300 lux by dimming. 
• 750-1000 lux for interiors where visual tasks are difficult, requiring small details to be perceived, or if the assumptions differ from the normal visual conditions.

European Standards

EN 12464-1 “Light and lighting — Lighting of work places — Part 1: Part 1: Indoor work places”

EN 14501 “Blinds and shutters - Thermal and visual comfort - Performance characteristics and classification”

EN 17037 “Daylight in Buildings”

Sources

1. Impact of Lighting on School Performance in European Classrooms (2016) C. Maesano and I. Annesi-Maesano, CLIMA 2016, 12th REHVA World Congress 2016, Aalborg
2. Clever Classrooms (2015), Summary report of the HEAD project, University of Salford, Manchester
3. CLEAR Luminance Ratios
4. "Design Innovations for Contemporary Interiors and Civic Art", Luciano Crespi, 2016