How do we design for adequate daylight?

by David Grahame

Typically, as architects, we have a good sense of what a well lit space should look like and how this can be achieved with good positioning of windows and/or roof lights of the right size and type. However, we are often asked to work on projects where a site is affected by nearby buildings, structures or trees, or restrictions on the size and location of openings, that can have an impact on internal levels of daylight. Conversely we may be working on a project where the proposed building or structure might have an impact on a neighbouring property. Both of these might be due to the nature of the site being in a tightly knit urban, or suburban context. We often have to demonstrate that our designs provide for adequate daylight as part of a submission for planning.

So how do we check that our buildings and the spaces around them have good access to daylight and sunlight?

Alongside our experience designing for good levels of daylight and sunlight we can also refer to guidance published in BRE 209 Site Layout Planning for Daylight and Sunlight: A guide to good practice. This was updated in 2022 to reflect the changes to British Standards withdrawing the guidance contained within Lighting for Buildings and replacing it with Daylight in Buildings.

The previous version of the BRE209 guidance referred to a method for measuring internal levels of daylight specifically, termed the Average Daylight Factor (ADF), recommended by the superseded British Standard. The current version, referring to the new standard, recommends using a method termed the Target Daylight Factor. Another method of measurement can also be used whereby hourly illuminance levels can be predicted.

Average daylight factor could be determined using an equation and was measured for kitchens, living rooms and bedrooms based on room and window dimensions, reflectances of interior surfaces, the glass used in the window and obstructions outside the room. The calculation would provide a figure as a percentage and this was used to determine if adequate levels of daylight would be achieved for the intended use.

Target Daylight Factor involves a more in depth assessment and is typically calculated using design software specifically designed for the task. This is a more accurate method for determining internal levels of daylight and whilst the output is ultimately a percentage value over a given area of a room, the method used to reach this can only realistically be derived using a 3D model of the room. The percentage output data is accompanied by a view of the room plan with daylight values distributed across a regularly spaced grid. Using software to determine the distribution of daylight in a room can often be a useful design process in itself, as the specific room shape and distribution of windows on a facade can be adjusted to optimise the results for a well lit room.

The new method of calculation is superior to that of the old one, but despite being superseded by new guidance, the old guidance is still referred to in planning policy by some local authorities, we are therefore sometimes carrying out two sets of calculations.

References

1 - BRE 209 Site Layout Planning for Daylight and Sunlight: A guide to good practice (Paul J Littlefair et al)

2 - BS8206-2: Lighting for Buildings

3 - BS EN 17037: Daylight in Buildings