“Daylight Metrics and their Sensibility” by Sophie Stoffer and Kathrine Brejnrod

ABSTRACT

Nowadays there exists considerable knowledge and focus on how daylight increases our physical and mental well-being, as well as our performance. Particular weight is placed on how to bring daylight into buildings in order to reduce the building’s overall energy consumption and to improve the indoor climate within a more complex framework of regulations.

With focus on daylight provisions as well as energy consumption, different daylight metrics have been assessed with the aim to create a basis for recommending a common daylight metric for the European Daylighting Standard. The assessment contains an evaluation of advantages and disadvantages of each metric, including the challenges by moving from a static to a dynamic climate-based daylight metric.

The following metrics are included in the analysis:

- The Daylight Factor (DF)
- The continuous Useful Daylight Illuminance (UDIcon)
- The continuous Daylight Autonomy (DAcon)
- The maximum Daylight
Autonomy (DAmax)

The sensitivity of the various daylight metrics is investigated via a parametric study according to different design alternatives and conditions in order to create an understanding of their sensitivity towards different parametric changes. Changes in different design parameters, as well as changes in orientation and location, are taken into account in the evaluation.

The parametric study is performed with the program iDbuild, which is developed as an early-stage design tool in an integrated design process of low-energy buildings. iDbuild can provide a combination of both energy and daylight conditions and it is an easy assessment tool in a parametric study.

The study showed that the dynamic metrics are very sensitive towards changes in orientation and location as well as façade design. Additionally, it is seen that the sensitivity towards a certain parameter is not the same according to the locations. E.g. the sensitivity towards the different orientations shifts with the location, thus the orientation is seen to influence more in some locations than others. The Daylight Factor on the other hand is a static metric and is therefore neither sensitive towards location nor orientation. When it comes to changes in façade design, the Daylight Factor only shows a minor sensitivity when compared to the dynamic metrics. The analysis of the shifting sensitivity towards locations is an important factor when evaluating a common European daylight metric, and contributes to a more detailed picture of the assessment tools of the variable daylight conditions in Europe. Concerning the energy consumption in combination with the dynamic daylight metrics the correlation is seen to be complex. Both the dynamic daylight metrics and the energy consumption changes in relation to orientations and locations, but the changes in the dynamic daylight metrics does not necessarily show whether the changes will have a positive or negative impact on energy consumption. The correlation between the dynamic metrics and the energy consumption concerning different design parameters is therefore seen to shift when for instance the location shifts. It was found that the dynamic metrics were relatively easily calculated, and a combination of the dynamic metrics UDIcon and DAmax, together with the effect on energy consumption, is preferable when evaluating the daylight conditions from different design options.

Sophie Stoffer and Kathrine Brejnrod is research assistants at Lighting Design Research at the Department of Engineering at Aarhus University in Denmark. Their professional interest is a holistic approach on how day- and electric lighting is integrated in building spaces with focus on the architectural expression, energy performance and occupants’ wellbeing. Their background is Architectural engineering, with specialization in integrated building Design, Aarhus University, Graphical Art at Graphical School in Aarhus and Group and Organization Psychology at University College Copenhagen (UCC).