What is Vertical Sky Component?
Vertical Sky Component (VSC) is a measure of the quantity of skylight accessible at a point on a vertical surface.
According to the BRE it is defined as the 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 illuminance on a horizontal plane due to an unobstructed hemisphere of this sky.
What is VSC used for?
Within the BRE Guide VSC is principally used to assess the degree to which a proposed development could impact on the levels of skylight available to the accommodation in neighbouring properties.
What do you include in VSC testing?
According to the BRE, VSC testing is intended for rooms in adjoining dwellings where daylight is required, including living rooms, kitchens and bedrooms. Bathrooms, toilets, storerooms, circulation areas and garages need not be analysed. According to the BRE VSC testing may also be applied to any existing non-domestic building where the occupants have a reasonable expectation of daylight. According to the BRE this would normally include schools, hospitals, hotels and hostels, small workshops and some offices.
Where and how is VSC measured?
VSC is measured at a discrete point located at the centre of the window being tested. The discrete point is located on the external plane of the wall associated with the test window.
If a room or room zone is served by more than one window it is permissible to determine the VSC for this room or room zone with reference to the area weighted average of the VSC levels predicted for each of the individual windows. This averaging exercise is only permissible for windows which are located within five meters of each other.
VSC testing is always carried out as a desktop study. While it is possible to calculate VSC using graphic based manual calculations it is more typical nowadays for this testing to be carried out using computer simulations.
Impact testing is carried out by predicting VSC levels at sensitive windows in the neighbouring environment for the existing scenario (with existing buildings on the development site) and then again with the proposed development in place.
What is an acceptable level of VSC?
At the outset it is important to recognise that the maximum theoretical level of VSC is not 100% as one might expect but is a figure closer to 40% VSC. This reduced figure is principally due to the fact that a window in a vertical wall is only capable of “seeing” half of the sky dome.
When applied to the assessment of new developments the BRE advise that if VSC is at least 27% conventional window design will usually give reasonable results.
How is the impact on neighbouring properties determined?
According to the BRE the diffuse daylighting of the existing building will be adversely affected if the VSC measured at the centre of an existing window is less than 27%, and less than 0.8 times its former value.
How should the results of VSC testing be interpreted?
According to the BRE the assessment of impact will depend on a combination of factors and there is no simple rule of thumb that can be applied. The BRE Guide advises that the magnitude of the departure from advisory minimums, the sensitivity of the receptor to loss of light and the existence of mitigating factors are all aspects that need to be considered when attempting to determine the significance of an identified impact.
Beyond the guidance provided in the BRE Guide (which states that “the advice given here is not mandatory”) it is important to recognise that in most planning jurisdictions even significant impacts can be deemed acceptable in circumstances where a countervailing planning objective can be articulated.
Should trees be included in VSC testing?
According to the BRE, for the case of impact assessments, it is usual to ignore the effect of existing trees. The BRE make this recommendation because daylight is at its scarcest and most valuable in winter when most trees will not be in leaf. Having regard to this guidance it is reasonable to assume that the overshadowing effect of trees can be omitted from an impact assessment but that the presence of evergreen trees may be an exception to this rule.
Does VSC analysis have any shortcomings?
VSC is a simple and somewhat abstract measure of natural light. It is for this reason that is a mistake to assume that VSC can provide a meaningful indication of the daylight levels that would be perceived by a human observer within a room.
A critical shortcoming of VSC analysis is that it is insensitive to so many of the important factors that influence the daylighting of internal spaces. These factors include climate, window orientation, window size, the presence of reflected light, the optical properties of window glazing as well as the size and layout of the room a window serves.
Having regard to the above it is instructive to note that a large room, with dark furnishings, served by a small, heavily tinted north facing window, located in an open field in Edinburgh, will have the same VSC as a small room, with light furnishings, served by a large clear south facing window, located in an open field in Marseille.
From the perspective of development management, it may be helpful to regard VSC as a measure which provides an indication of the capacity that exists for windows and their associated accommodation to be served with adequate light from the sky. In this sense VSC can be regarded as similar in nature to the capacity of other services that a development site will need, for example grid and water capacity, etc.
