A room is considered to have adequate daylight if a certain level of illuminance is achieved across a percentage of the working plane, for 50% of daylight hours.

BS EN17037 defines three levels of recommendation when assessing daylight in interior spaces. The three levels are Minimum, Medium and High.

Each level has a target illuminance [ET] which must be achieved at least 50% of the time, across 50% of the space. In addition, there is a minimum target illuminance [ETM] which should be achieved at least 50% of the time, across 95% of the space. This is a secondary check to ensure that any point of the space [95%] does not drop below this minimum level. Where the space is top lit the percentage area should be 95% for both ET and ETM

BS EN17037:2018 provides two methods for the calculation of daylight provision:

• Method 1: Daylight Factor [hybrid]
• Method 2: CBDM

Our advice is not to use Method 1 as it will significantly underestimate the internal illuminance, or will result in windows/rooflights being incorrectly sized. As an advocate of Climate-based Daylight Modelling, we only recommend using Method 2.

A target daylight factor [DT] is derived from the illuminance targets [100lx, 300lx, 500lx, 750lx], in conjunction with the Median External Diffuse Illuminance [MEDI] for the location. The standard provides the MEDI values for major European cities and the resulting median daylight factor targets required to achieve each recommendation level; Minimum, Medium & High.

Our advice is to not use Method 1, as it may significantly underestimate the illuminance levels, or may result in windows/rooflights being incorrectly sized. 

See the Problems with Method 1 below for more information.

Daylight Factor

Daylight Factor [DF%] is defined as the ratio between the internal and external horizontal illuminance, under CIE overcast sky conditions [CIE Sky types 1 or 16].

Considering that daylight factor calculations use an overcast sky, which is defined as completely cloud covered, means that orientation does not influence the results, which in itself means this method is flawed unless you are only considering north-facing windows. Even then, method 1 is likely to under-estimate the median illuminance.

Importantly, this method does not take into account the spatial distribution of light under varying sky conditions, especially clear skies where direct sunlight would have a huge impact on the distribution and quantity of light within an interior.

In-house Research

Our own in-house research and analysis show that following the DF approach, which is essentially a hybrid solution combining daylight factor [overcast sky] with the median external diffuse illuminance [all skies], results in the under-estimation of the illuminance received within interiors.

As an example, a simple test room has been analysed using Method 1 & 2, in 8 principle orientations. The results showed the following:

Method 1

The Median Daylight Factor was measured to be 1.84%, for all 8 orientations! The London MEDI is 14100lx, giving a Median Illuminance of 259lx.

Method 2

The same room analysed using CBDM gave a median illuminance in the range of 444lx (north) – 897lx (south), with each orientation having different results, as one would expect. This highlights how important orientation is to building daylight performance and can have a huge impact on the internal illuminance levels.

When comparing the two methods, the Method 1 DF approach [259lx] is 42% lower than the north-facing room [444lx] and a massive 72% lower than the south-facing room [897lx], when analysed using Method 2 CBDM.

This example also highlights that there is an approx. 100% increase in the internal median illuminance between the north and south orientated rooms i.e twice as much light during the year!

Comments

Method 1 seems to be a leftover of the old BS8206, albeit a hybrid trapped somewhere being old and new thinking - a halfway house for those not wanting to fully embrace CBDM. There have always been barriers to the adoption of CBDM and many companies prefer to stick with the easier daylight factor approach - maybe CBDM rocks the boat too much! If the focus is on designing energy-efficient, visually and thermally comfortable buildings, then ignoring the local climate in favour of making the analysis easier is in direct conflict with the main objective. Its time to move on from the daylight factor!

In order to as accurately [as possible] predict the expected internal illuminance levels throughout the year, under varying sky conditions, requires Climate-based Daylight Modelling (CBDM).

We often hear comments that CBDM is too difficult, too complex, too time-consuming. Like anything new, there is a learning curve and it is true that CBDM is more complex than a simple Daylight Factor analysis. It should ideally be carried out by a competent professional. Computationally CBDM is more time-consuming, requiring the resulting illuminance from thousands of time-steps through the year to be analysed, but apart from more sophisticated software and greater user knowledge, the underlying components remain the same i.e. a 3D computer model, defined calculation grid points and photometrically accurate software.