Architectural journalist Gideon Sykes looks at the latest advances in daylight modelling and how it can help architects and specifiers determine the right light levels for their projects
Daylighting is the art and science of managing natural light to minimise the use of artificial lighting, reduce carbon emissions, and positively affect the performance, mood and well-being of people occupying a particular space.
Designing, predicting and planning for the impact of daylighting in a space is often misunderstood however. Daylight modelling removes the design mystery and determines the daylight requirements for any building.
It is not a new science, but one which is increasingly important as reduced energy consumption is a continuing major consideration – as well as statutory requirements (such as European workplace directives) and personal wellbeing.
Part of the design
It’s often a mistake not to use daylight modelling as part of the original concept or design. Consequently, it is beneficial that this service is used early to help achieve the best possible results for the client and to form part of the overall building strategy and goals.
For example, the service can help with the documentation required to achieve BREEAM’s health and well-being credit for visual comfort (HEA01) which requires that 80 per cent of the occupied space meets a minimum daylight factor of 2 per cent (3 per cent for ‘exemplary’ level).
It can help architects and specifiers with the following calculations:
- Quantity of light transmitting materials
- Location of light transmitting materials
- Required transmission level of the light transmitting materials
- Glare analysis.
Daylight modelling can help achieve an exact brief from the client – for example the lux level requirement or to address concerns about solar heat gain. It can help quantify daylight transmitting products, determine optimal light transmission and help with positioning on a building to prevent glare issues and provide interior lux levels.
Reports can be looked at from a cost point of view, and recommendations can be made depending on the results. A good example of this is for ‘value engineering’, where daylight modelling examples may show that windows can be reduced in size to save money yet still achieve the desired lux levels. The report could include any or all of the following calculations:
Lux level: Lux is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. For example, sports halls and classrooms would be expected to have lux levels of between 300-400.
Radiance illuminance: Radiance illuminance (measurement of light level) is a snapshot of the ambient lux level (light) at any given time. This tool calculates how effective is daylight penetration at any time of the day or on any day in the year.
Daylight autonomy: Daylight autonomy (DA) is the percentage of the time-in-use that a certain user-defined lux threshold is reached only through the use of daylight. DA is usually given as an annual value but seasonal, monthly and daily calculations can be made. It is the ideal way to achieve optimum natural daylighting conditions for the occupants, predicting when electric lighting may be required and thereby helping to reduce the cost of energy.
Glare pattern analysis: This useful tool calculates luminance (i.e. measurement of glare) within a space. It is used to analyse direct glare or reflected light, such as in sports halls and swimming pools, where it is important to keep balanced light within a space to protect athletes from high contrast light ratios.
The process
Daylight modelling is the way to calculate the most effective daylighting for any type of building. It is calculated using five years’ worth of real world weather files at the exact location of the building as well as information on day, time, position and weather patterns.
In addition, a daylight modelling team will undertake this service using data provided by the architect or client. This comprises an indication of light levels required together with building elevations, floor plans and sections.
It also takes into account proposed internal finishes, which could influence reflectance, the positions of other windows and any external influences, such as tall adjacent buildings or trees.
With this information, the team can look at an individual room or at the overall situation and design the most favourable daylighting solutions.
Case study: Harper Adams Academy
An example of daylight modelling in practice was when Kalwall was specified for the new Weston building at Harper Adams University, near Newport, designed by Michael Hyde Architects. Widely used for cladding and rooflights, the highly insulating system was unique in the way in which it transmits ‘Museum-Quality Daylighting’. Here, the translucent panels have been designed to follow the curve of the laminated timber structure.
One of the project’s key design features was the way in which Kalwall has been used to allow and control the interior daylight, remove glare and shadows, yet maintain light levels and minimise solar heat gain. This was achieved using daylight modelling to predict the illumination levels across the floors. The result is that the privacy of the students is preserved while they benefit from leisure and social areas on two levels.
This roof solution utilises a 0.56 W/m2K U-value panel solution, with only 6 per cent light transmittance. The daylight modelling documented that even with these low transmission figures, lux level requirements were achieved. The main advantage for the client was to bring the G-value figure down to 7 per cent, thereby reducing the solar
heat gain to a tenth of that of a standard low E double glazed unit.
Gideon Sykes is an architectural journalist for Structura UK/Kalwall