Dual Aspects, Single Aspects & Internal Daylighting

Often Local Planning Authorities require an internal daylighting analysis to be undertaken for proposed new dwellings, aiming to prove that future occupants will experience high quality naturally lit spaces. The most dominant metric is the Average Daylight Factor (ADF), which is stated in the British Standard 8206-2:2008 Code of Practice for Daylighting.

An emerging approach for assessing internal daylighting is the Median Daylight Factor that is likely to supersede the current BS: 8206-2. The Median Daylight Factor can be derived from a grid-based point Daylight Factor simulation, which is being calculated via the use of new computational tools. The new approach is a more thorough one, compared to the simple ADF, because it is capable of taking into account the design aspects of an internal space, e.g. the difference in daylight quality between a dual aspect room and a single aspect room, or the actual daylight distribution within a space.

The figure and table below illustrates the difference between daylighting performance for dual aspect and single aspect rooms, when assessed under an Average Daylight Factor approach and when assessed under a point Median Daylight Factor approach.

The assessment shows that although both dual and single aspect rooms would appear to meet the current planning ADF requirements, the actual daylight distribution and uniformity within the single aspect space is not ideal. Only the front half of the deep-plan single aspect room will remain well lit, while the rear area will in practice remain dark and dependent upon artificial lighting.

This fact can raise concerns, especially if the new daylighting standard gets adopted in the UK, as the very common single aspect Living Room/Kitchen/Dining room typology will fail to meet the Median Daylight Factor targets. Single aspect is a typology that serves well the architectural functionality in many major housing schemes in the UK and especially in London.

We have found that the Median Daylight Factor approach provides a more realistic assessment of actual daylight performance for internal spaces. Its adoption however would represent a challenge for single aspect rooms. Early stage and well-coordinated design team collaboration is crucial in meeting this challenge.