Sunlight Availability

Sunlight Availability is a measure of the average number of hours of sunlight one would expect to receive at a given position, as a fraction of the unobstructed total number of hours at the same location. The BRE have compiled data sets consisting of a statistical sample of solar positions convolved with local meteorological data. Using these to calculate Sunlight Availability, one would simply calculate the number of solar positions visible from a point, compared to the total number, expressed as a percentage. The diagram below, taken from the BRE report, shows the solar positions, relative to a reference point, used to calculate Sunlight Availability for London (51.5° N).

BRE Criterion

The BRE report states that for windows within a new development, if a point at the centre of a window on the plane of the inside surface of the wall "... can receive more than one quarter of annual probable sunlight hours, including at least 5% of annual probable hours during the winter months between 21 st September and 21 st March, then the room should still receive enough sunlight." For windows in surrounding properties which experience a change in sulight availability, it goes on to say that, "Any reduction in sunlight access below this level should be kept to a minimum. If the available sunlight hours are both less than the amount given and less than 0.8 times their former value, either over the whole year or just during the winter months, then the occupants will notice the loss of sunlight."

Sunlight availability indicator example
 

Sunlight availability can also be represented on a Waldram diagram. The two Waldram Diagrams on the right depict the sun positions for an example situation. The existing building is in green, the proposed development is shown in red.

Existing Proposed  
Summer Winter Total Summer Winter Total Loss %
24
2
26
11
1
12
54

This table shows the results obtained from the two sun position calculations.The existing situation does not meet the BRE levels to be considered adequateley sunlit but it is close, only failing because it receives 2 out of 5 required winter suns. The proposed situation is well below the required levels and also produces a loss of over half the total 'suns' when compared to the existing building.

Looking at the Waldrams again it is obvious that none of the suns are visible above either the existing or proposed buildings. The loss of available sunlight is not due to the height of the proposed development but to the size of the its footprint.

view larger Waldram Diagram
view larger Waldram Diagram

Subjective Assessment

It is often useful to visualise solar paths as viewed from a particular position. The example on the right shows solar paths plotted onto a Waldram Diagram. This provides a snapshot of times and dates showing when a window will receive direct sunlight. It also shows which part of a building is responsible for causing a shadow. Another method of subjective assessment involves producing shadow animations. to compare the existing and proposed scenarios.

view larger Waldram Diagram

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