AECB Daylighting Guidance

Creating a well-lit building is vital for the well-being of people using a building as well as for reducing energy demand and carbon emissions.

Whilst recognising that the amount of daylight available within a building is constrained by the amount of useful daylight outside, the aim of the AECB Daylighting Guidance is to provide adequate daylight between 9am and 5pm for 70% of the year and acceptable daylight 80% of the year between 10am and 2pm.

To satisfy these criteria the Guidance requires you to assess the quality of daylight within each habitable room. The AECB Daylighting Assessment is used to verify that a design meets this Guidance.

To determine the quality of daylight you must evaluate how evenly daylight is distributed within a room (the Daylight Uniformity), and the proportion of daylight within the room relative to outdoor conditions (the Daylight Factor).

Satisfactory Daylight Uniformity is achieved when more than 80% of a habitable room receives natural daylight. Acceptable daylighting is achieved when the specified Daylight Factor is achieved (Appendix 1 in the Daylighting Standard Guidance, describes the Daylight Factor criteria).

To reduce the risk of overheating, high daylight factors (oversized windows) should be avoided. Daylight Uniformity and the Daylight Factor must be calculated in accordance with BS 8206-2 using the AECB Daylight Calculator.

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AECB Daylight Calculator

AECB Daylight Factor is a simple but powerful tool that makes daylight assessment quick and easy.

Daylight Factor helps you:
1) Assess daylight uniformity, so that you can appreciate whether light will be evenly distributed within each room.
2) Determine the daylit area, so that rooms do not appear gloomy.
3) Calculate the daylight factor of a room, so that you can be confident rooms receive and appropriate level of daylight, without being over glazed and prone to excess solar gains.
4) Avoid excess heat loss, because even high performance windows lose 5 to 8 times more heat than a good wall.
5) Demonstrate compliance with the AECB Daylight Standard.

Using Daylight Factor

If you are not a PHPP user don’t worry, or if you’ve not built a PHPP model yet, you can complete a conventional assessment using Daylight Factor. However, if you have a PHPP model you can speed up your workflow by allowing PHribbon to do some of the work – saving time and effort.

Standards

Daylight Factor is ideal for buildings which aspire to the AECB Building Standard or the Passivhaus Standard. An example of the Daylight Factors one-page Verification report is given on the left. A copy of this report should be submitted with your AECB Building Standard documentation.

AECB Daylight Factor is based upon BS 8206-2, and informed by BRE BR209, BRE IP 15/88 and BRE Digest 309.