Earlier this year, Pager Power began offering a new service to developers in the form of Daylight Sunlight Assessments. This included the development of our own bespoke model that complied with BRE and British Standards guidance to ensure accuracy and compliance with best practices. More information on the guidance can be found here.
This new series of articles looks at the technical terminology used within these assessments, with this specific article exploring the term ‘Vertical Sky Component’ or VSC. In future articles, we will look at additional terms, including ‘Annual Probable Sunlight Hours’ and ‘No Sky Line’.
Definition of Vertical Sky Component
Simplistically, the VSC is the amount of sky (diffuse daylight) visible from a particular window in a property (typically commercial or residential). It is given as a percentage, and it considers both the real-world geometric visibility from a window with a single orientation and any proposed and existing obstructions, which may include buildings and vegetation.
For a vertical window, the amount of sky visible can never exceed 50%, as an observer will never be able to observe the sky that is in the opposite direction to which the outward window is facing. Figure 1 below illustrates this.
Figure 1:Vertical Sky Component diagram.
As illustrated within the figure, the amount of sky observable from a window is much less than 50% in real-world settings due to the geometry and typical level of screening visible from a window.
What does the guidance say?
The BRE 209 guidance acknowledges that the real-world VSC is much lower than one might expect, and therefore the minimum acceptable VSC is stated to be 27%, or 80% of the baseline, whichever is lower.
For example, if the existing baseline VSC was less than 27% (say 26%), but after a building development was built changed to 22%, this would be greater than 80% of the baseline and be deemed an acceptable level of impact (84% of the baseline value).
In many cases, however, it is not as simple as looking at the resultant figures, and geographic context needs to be considered. For example, in city centres with many tall buildings, the VSC of an existing building may never achieve 27% due to the presence of other tall buildings. Within Pager Power’s reports, the setting and context within which a development is built are considered before drawing overall conclusions.
How is the Vertical Sky Component Modelled?
Pager Power’s geometric model considers rays of light from the assessed area of sky to determine the impact of a proposed building development upon available daylight (VSC) considering the interrelationship between the proposed building and those surrounding it. On this basis, a ‘before and after’ assessment is completed to ensure minimum standards are maintained within the existing buildings, as well as the new building itself.
It does this by considering the effects of overshadowing on the access to skylight at the modelled windows. The model considers skylight indicators contained within Appendix A of BRE 209 as a basis for the calculations and modelling results. These skylight indicators contain points in space (in the sky) to which the model calculates whether line of sight is possible from a window. The model automatically excludes all points that are not within line of sight due to the geometry of the window (see Figure 1). The number of points visible from the window considering the baseline and post-construction scenario is then calculated to give an overall percentage. This is completed for every window, both in the proposed development and any relevant building within the assessment area.
The results are then presented visually in the form of a skylight indicator chart (see Figure 2 below) and tabulated for each window. The results outline the VSC for each window in the proposed development and nearby surrounding buildings. Finally, all results are considered alongside one another to determine whether they are acceptable in accordance with BRE 209.
To find out more about how we model daylight sunlight impacts, follow the link here.
Figure 2: VSC (Skylight) modelling output.
Speak to an Expert
If you are interested in finding out more about our daylight sunlight assessments, follow the link here to make an enquiry, or give us a call on +44 (0)1787 319001.
About Pager Power
Pager Power is a dedicated technical consultancy that has been providing independent guidance and advice regarding solar developments, wind farms, and building developments internationally since 2002. Further details about the services we can provide can be found here. Pager Power has completed over 1,400 glint and glare assessments, over 1,000 aviation/radar impact assessments, over 500 television and radio reception surveys, and over 500 telecommunications impact assessments.
Thumbnail image accreditation: Craig Cooper (2018) from Unsplash.com. Last accessed on 24th September 2024. Available at: https://unsplash.com/photos/worms-eye-view-of-buildings-during-daytime-n0Xx7fsWeKM
