Building regulation requirements and traditional engineering practice for daylight calculations is often outdated and unsynchronized with the advance and needs of modern sustainable building design.
State-of-the-art calculation tools provide accurate results on daylight conditions using methods as simple as calculating the useful daylight illuminance. These methods facilitate sustainable building design that also works in practice. This is illustrated with an example where the daylight conditions in an office with solar shading is examined.
The inevitable turn of the building sector to sustainable design techniques has brought daylight analysis to the center of attention. Buildings with an enhanced daylight performance have minimized energy requirements and an improved indoor climate. However, assessing daylight conditions is somewhat adhered to old-fashioned methods, as building regulations and schemes in most countries are not updated to research findings.
Legislation and standards
Daylight Factor is the most widely used method of establishing compliance with building codes and credits within environmental assessment schemes such as BREEAM, DGNB etc. Taking as example the Danish Building Regulation of 2015 the requirement to achieve sufficient daylight conditions in an occupied space is a minimum of 2% daylight factor (DF) covering part of the work plane. This is a typical requirement from Denmark to United Arab Emirates, although the latter almost never experience a standard CIE overcast sky.
As much as the daylight factor method is easy to comprehend and apply, it leaves the designer a lot of space to produce a building with uncomfortable or energy inten- sive daylight conditions. That is because DF takes no account of the building location, façade orientation or varying sky conditions.
Moreover it provides no indication of glare or visual comfort nor is the solar shading taken into account. The latter is of increasing importance in low energy buildings since the solar shading is more often used and is vital for the expected performance of the building. Several examples show a usage of the solar shading for up to 80% of working hours during the summer in order to maintain a satisfying indoor climate.
However, the daylight factor method only evaluates the overcast sky conditions and hence only represents down to 20% of the working hours.
New method - Climate-based daylight metrics
Instead, there are by now several studies1, 2, 3 discussing this exact topic and proving that the introduction of climate-based daylight calculations that rely on hourly meteorological data over the year, form much more accurate and informative, yet simple measures of the daylight conditions in a building compared to the DF and could effectively replace the latter in regulation and scheme requirements.
The climate-based approach uses time varying sky and sun conditions, whilst predicting hourly levels of daylight illuminance. This is fully parallel to standard practice for indoor climate simulation. The superiority of the method is thus evident against the daylight factor approach, which is a single number taking no account of orientation and considering only overcast skies, therefore not being meaningful for climates with predominant sun conditions. Moreover the climatebased approach can take solar shading into account.
Indicative calculation metrics of the climate-based method are e.g. the Daylight Autonomy (DA) and the Useful Daylight Illuminance (UDI).
So far the DA and UDI methods are applied by the UK Education Funding Agency for the evaluation of designs submitted for the Priority Schools Building Programme (PSBP)4. Furthermore a variation of DA, the so-called Spatial Daylight Autonomy (sDA) is used in the environmental ration system LEED v45.
Daylight DA provides the benefit of valuing the contribution of daylight to energy savings; however it is of no value to the occupants’ comfort as it does not reflect on the amount of time of extreme illuminance levels causing discomfort or glare. At the same time, the metric ignores illuminances that are below the threshold, which can still be useful to the building users.
Useful Daylight Illuminance
Useful Daylight Illuminance is more advantageous, because it covers the gaps of DA. The upper and lower threshold of UDI have been defined based on the findings of numerous field studies and surveys in offices indicating that illuminance levels between 100 lux and 2 000 lux are either desirable or tolerable to workers. Hence, UDI informs on how often daylight illuminance is too low, i.e. how often artificial light is needed, how often illuminance is useful to the occupants and how often it is extreme and therefore causes discomfort.
Overall, it relies on a detailed method and it gives value to unconventionally useful illuminance levels plus indicating disturbances, whilst giving an impression on the potential for reduced lighting use.
The full article was published in the REHVA Journal in 2015. For material or further information on the subject, please contact Helle Foldbjerg Rasmussen by mail email@example.com or phone +45 72 14 48 48.