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

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

Minimum Requirements

Kitchens 2%

Living Rooms 1.5%

Bedrooms 1%

Angle of Visible Sky

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

Obstruction Angle

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.

No Sky Line

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

Sunlight Availability

Aerial View of SW Aylesbury Estate

Site of New Development

East View

North East View

West View

South West View

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

No Sky Line

120-138 Walworth Rd

Proposed Development

Simulation and Layout of Proposed Apartment

Daylight Factors in Proposed Apartments

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)