Daylight analysis in the design process

Ben Storey
Ben Storey October 2, 2020 5 min read

Across the globe, the construction industry faces an enormous and immediate challenge. To accommodate massive urban growth, we must build higher, denser and faster than ever before. At the same time, there is a responsibility to ensure that what is built is both of a high standard and sustainable for future generations. Access to daylight is a critical part of a quality urban environment and as mentioned in part one of this blog post, should be a prime consideration in the early planning stages. Without such assessment, increased urban density can potentially have catastrophic effects on the health and well-being of building occupants.

Daylight analysis is normally performed in the later stages of building design. These analyses are generally carried out by specialized engineers who focus on the performance of individual rooms. Daylight factor is the most common metric for this purpose and many national codes are based on this type of assessment. Daylight factor can only be determined after room geometry and materials have been defined however. While things like room depth and the size and placement of windows have a big effect on daylight factor, the amount of daylight reaching a window is also a critical aspect. And just how much light is available at the window is very much dependent on the amount of obstruction from the surrounding environment. So involving specialized experts at a late stage often means that it is too late to influence one of the most important factors which determine daylight levels: the relationship of a building to its neighbors.

Daylight factor (DF) is a daylight assessment metric that expresses the amount of daylight available inside a room as a percentage of the light available from an unobstructed overcast sky. Results can either be given for a point or across an area.
Credit: Velux

For assessment of daylight factor, light is categorized in three parts: Sky Component (SC), Externally Reflected Component (ERC) and Internally reflected component (IRC). The sky component has by far, the greatest influence on the final result. As such, the degree to which the sky is obstructed is of critical importance to daylight levels inside the room.

There are two established assessment metrics which examine the degree of sky obstruction. These are: Vertical Sky Component and Obstruction Angle. Through the use of these two metrics, Spacemaker provides users with the possibility to analyze the daylight performance of their designs from the very beginning of the design process. Results from these analyses give users important feedback so that the user has the opportunity to change the height and positioning of buildings before they become set in stone. Problem areas can be easily identified and users can work to improve the daylight performance of their designs. The end result being buildings with better daylight access.

The VSC is defined by BRE (Building science center from the UK) as: “Ratio of that part of illuminance, at a point on a given vertical plane, that is received directly from a CIE Standard Overcast Sky, to illuminate on a horizontal plane due to an unobstructed hemisphere of this sky.” (Littlefair, 2011). In other words, the VSC is a measure of how much light reaches the facades as a percentage of the total light available from the same overcast sky. This is particularly useful as the CIE overcast sky is also used in the calculation of Daylight factor.

The Obstruction Angle is a geometric measure commonly used as a rule-of-thumb to approximate daylight access. It measures the height of surrounding objects in terms of the angle from the horizon to the top of the obstructing object. It provides useful information for sure, but as a 2D measure it is insensitive to obstruction from the sides so it is less accurate then VSC. Regardless, a good rule of thumb is that all facade areas with an obstruction angle above 45 degrees should be examined further to ensure adequate daylight access.

Below we see these two metrics as applied in Spacemaker: Vertical Sky Component Analysis (left) and Obstruction Angle Analysis (right)

For daylight access, the importance of the proximity and height of surrounding obstructions cannot be overstated. If ignored in the planning process, conflict with daylight regulations later down the road can lead to costly or suboptimal design solutions. Furthermore, given the vast amount of time we all spend indoors, we passionately believe that the impact of daylight on wellbeing should not be overlooked. The Spacemaker team has worked hard to deliver a platform where multiple design options can be examined in just seconds. This unprecedented access to high-quality daylight analysis in the early design stages is giving architects and urban planners the answers they need, and enabling them to meet the challenge of daylight provision in dense urban environments.

Paul Rogers:

Architect and daylight certification specialist at ACC Glas och Fasadkonsult in Stockholm, Sweden. He is the principal author of two key reports regarding the modernization of Sweden’s daylight regulations and is involved in the development of daylight criteria for the Nordic Svanen, Miljöbyggnad and Breeam-SE certification systems. He has also served on Swedish Standard Institute’s (SIS) national review board for the European daylight standard EN 17037:2018 and helped to formulate the LEED pilot credit for Daylight for Nordic Projects. He is the designated expert daylight for Sweden’s Green Building Council’s and Svanen Nordic label and currently serves on the WELL (IWBI) certification Light Concept Advisory Board. He is a frequent author and lecturer on the subject of daylight and founder of ‘Svensk dagsljusberäkning’ (Swedish daylight calculation) with circa 500 members on LinkedIn.

Magnhild Gjestvang:

Data Scientist and Product Manager at Spacemaker, working on product development and mathematical modelling. She is Product Lead for the team responsible for developing the Sun, Daylight and View analyses in Spacemaker. She holds a master’s in Industrial Economics and Technology Management from the Norwegian University of Science and Technology.


Littlefair, P. (2011). Site layout planning for daylight and sunlight: a guide to good practice (BR209). BREPress.

Velux. (May 2020). Key learnings about daylight performance in a demonstration building and potential outcomes. 

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