3D Design Bureau

The third edition of BRE 209, published in 2022, introduced a number of important changes to the way daylight and sunlight assessments are carried out.

The previous edition of the guidance had been in use since 2011. While many of the established methods for assessing impacts on neighbouring properties remained broadly unchanged, the third edition introduced significant changes to the assessment of daylight and sunlight within proposed developments.

Below, we break down some of the key changes introduced by BRE 209 third edition and what they mean in practice.

Internal daylighting in buildings

One of the most significant changes to occur with the publication of the 3^rd edition of the BRE Guidelines was the change of assessment metric for internal daylight within buildings. Up until the release of the BRE 209 (2022), the metric assessed was Average Daylight Factor (ADF). This has changed to illuminance lux levels, otherwise known as spatial daylight autonomy (SDA) or median daylight factor (MDF).

SDA measures the amount of lux levels within a room. For daylight and sunlight assessments, habitable rooms are assessed. Furthermore, target lux level values are taken from the BS-EN 17037 as opposed to the BS 8206 which BRE 209 (2011) referenced. It should be noted that the BRE 209 (2022) also makes reference to the IS-EN 17037 for target values. For full details on this metric of assessment, and more, visit “All you need to know about scheme performance in BRE daylight and sunlight assessments“.

Reflectance values

Reflectance values in daylight and sunlight assessments are the numerical values applied to surfaces that dictate the amount of natural light that is reflected and absorbed by that surface. The darker the material, the lower the numerical value and the less reflective it is.

Within every digital 3D analytical model, surfaces with different finishes should be identified. Then within the assessment process, reflectance values should be assigned to these surfaces. Depending on the reflectance value of the internal and external surfaces of a building, including the site finishes such as paving, grass, etc, the higher or lower the daylight values will be within an assessed room. 

The BRE Guidelines (BRE 209 – 2022) have stated reflectance values that are in line with the lower levels stated in the European standards EN-17037 and Irish adaptation to the European standards, the IS-EN17037. These are to be used unless ‘otherwise specified’ by the design team.

Glass transmittance

Glass transmittance values should be applied to all glazing of assessed units. Under BRE 209 (2022), their properties should be 0.68 unless ‘otherwise specified’ by the design team. The lower the numerical value the less natural light will penetrate into the room.

Averaging of windows in impact assessment

Under BRE 209 (2022), windows that are being averaged out on impact assessments must be weighted. This means where two windows of non-equal size are servicing the same space, this factor must be taken into consideration in the calculations. Furthermore, where windows servicing the same space are more the 5 meters apart, they are not to be averaged and should be tested individually with the level of impact stated for each.

Sunlight exposure (SE)

Sunlight exposure (SE) was introduced as part of the revised approach to assessing sunlight in habitable rooms of a proposed development. All habitable rooms should be tested for SE and levels are given for each room. Under the BRE Guidelines, where one room in a residential unit meets the minimum recommended levels of sunlight exposure, the unit as a whole is considered to have passed. However, the BRE Guidelines does state that the room in question should preferably be a living room.

Trees

The third edition of BRE 209 also provides more detailed guidance on how trees should be considered within daylight and sunlight assessments.

Whilst it does add a layer of complexity to studies, it is important to consider them. Assessments should include existing and proposed deciduous and evergreen trees. For full details on how trees should be included in daylight and sunlight assessments, visit “A professional take on trees for daylight and sunlight assessments“.

If you need advice on your project with regard to daylight and sunlight, get in touch with our team, who will be more than happy to shed more light on your project! For more information on daylight and sunlight assessments, you can also visit the articles below or book a free presentation with the team by contacting us at info@3ddesignbureau.com or +353 1 2880186.

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