5 wilson daylight uk
DESCRIPTION
5 Wilson Daylight UKTRANSCRIPT
Daylight, Sunlight, and Planning in the UK
Professor Mike WilsonLEARN
London Metropolitan University
Criteria
The publication of BS 8206 Pt2 in 1992 and BRE 209 'Site Layout' in 1991 saw the average daylight factor
linked to a limiting depth criteria and no sky line introduced as guidance to be used for daylighting in
planning applications for new domestic developments. The availability of sunlight on East,West and South
orientations was also to be considered. The impact on existing buildings is estimated in terms
of the reduction in the vertical sky component and available sunlight
Vertical Sky Component
The vertical sky component is a measure of the light reaching the centre of a window. It is the ratio
between the vertical illuminace on the glazing direct from the sky and the illuminance from an unobstructed sky. For a CIE standard overcast
sky with no obstructions the VSC is 40%. A VSC of 27% is regarded as offering the potential for good daylight. This corresponds to an angle of
obstruction of 270
VSC Criteria
If a new building affects the VSC enjoyed by an existing building such that the VSC falls below 27% and that the reduction is more than 20%
there will be a noticeable loss of daylight
Average Daylight Factor
The average daylight factor formula is given by:
ADF = TΘW/A (1- R2)
where ADF is at 0.85 m above the working planeΘ is the angle of visible sky
W is the window area corrected for glazing bars etcT is the transmission of the glazing
A is the surface area of the room surfaces
Calculation procedure
The Vertical Sky Component ( VSC) may be calculated using a bespoke computer program or
commercially available software such as ECOTECT. The VSC is the converted to an
equivalent angle of visible sky determined using the table in the next slide
Limiting Depth CriteriaL/W + L/H should be less or equal to 2/1-RB;
Where;
L > is the depth or the room from the window to the back wall
W > is the width of the room, measured parallel to the window
H > is the height of the window head above the floor level
RB > is the area weighted average reflectance of the interior surfaces (walls, floor and ceiling) in the half of
the room remote from the window.
All conditions need to be satisfied for good daylighting
The Average Daylight Factor is a measure of the total amount of daylight entering a room through the opening while Limiting Depth Criteria and No
Sky Line consider the distribution of daylight in the space. The Limiting Depth Criteria is determined
by the ratio between the average daylight factor in the front half of the room and the rear half. This should not exceed 3. If a significant area of the
working plane lies beyond the no sky line then the distribution of daylight in the space will look poor.
Sunlight
The Annual Probable Sunlight Hours at a given window should exceed 25% of the total available sunlight hours, at least 5% occuring during the
winter period ( Sept-March)In the case of a new building affecting an existing
building there should be no more than a 20% reduction in probable sunlight hours subject to the
above limitation
Examples of VSC and ADF Calculation
Location Existing VSC Proposed VSC Reduction factor Existing ADF Proposed ADF
13 Westmoreland Rd 31.06 18.96 0.61 2.71 1.86
15 Westmoreland Rd 30.83 18.16 0.59 3.36 2.24
17/19 Westmoreland Rd 30.61 17.98 0.59 3.37 2.25
21 Westmoreland Rd 29.87 18.03 0.66 2.61 1.81
Example of Sunlight Analysis
Location Existing % APSH Proposed %APSHSummer Winter Total Summer Winter Total
13 Westmoreland Rd 31 19 50 26 1 27
15 Westmoreland Rd 37 22 59 32 2 34
17/19 Westmoreland Rd 37 19 56 32 1 33
21 Westmoreland Rd 37 18 55 33 1 34
Inspector's Ruling
..it is a truism that the deeper a building, the more difficult it is to ensure adequate natural lightingto the interior parts, and it is natural light that is the
root of the councils objections....I do not consider it desirable to provide kitchens
that have lttle natural light or outlook....While 'low light' kitchens may be unavoidable ..
such as in a conversion scheme....would not accord with good quality living conditions and high standards of natural light that SP poklicy seeks to
achieve..
Conclusions
● The use of the ADF formula overestimates the actual ADF if the whole window area for full height windows is used. The area of the window below the working plane should be deducted and mulitplied by between 0.1-0.2 befor adding again to window are above working plane
● Distribution criteria ( limiting depth/no sky line) should not be ignored
● Kitchens ( even small kitchens) should not be denied daylight)