Daylight Provision assesses whether sufficient daylight is available across the occupied area of a space for an adequate proportion of the year.

BS EN 17037 allows daylight provision to be assessed using either a daylight factor-based approach or a Climate-Based Daylight Modelling approach.

For climate-based assessment, daylight performance is evaluated using annual weather data and two principal illuminance targets:

Target Illuminance — ET

ET is the principal daylight illuminance target.

For side-lit spaces, the target should be achieved across at least 50% of the reference plane for at least 50% of daylight hours.

Minimum Target Illuminance — ETM

ETM is a lower illuminance threshold intended to ensure that daylight extends across the wider occupied area.

It should be achieved across at least 95% of the reference plane for at least 50% of daylight hours.

For top-lit spaces, both ET and ETM apply across 95% of the reference plane.

BS EN 17037 defines three daylight provision recommendation levels:

Minimum: The baseline recommended daylight performance.

Medium: A higher level of daylight provision for projects seeking improved daylight quality.

High - The highest recommendation level, representing substantially greater daylight availability across the space.

The appropriate target level should be established as part of the project brief and design objectives.

BS EN 17037 recognises two approaches for assessing daylight provision.

Method 1 — Daylight Factor Approach

The daylight factor method is based on a standard overcast sky and converts daylight factor values into equivalent illuminance targets using climate-derived external illuminance data.

Because the underlying internal calculation uses an overcast sky, building orientation has limited influence on the calculated internal daylight distribution.

Method 2 — Climate-Based Daylight Modelling

Climate-Based Daylight Modelling uses annual location-specific weather data to simulate daylight conditions throughout the year.

The method considers changing sky conditions, sun position, building orientation and seasonal variation, providing a much more detailed representation of actual daylight performance.

For this reason, CBDM can be particularly valuable when evaluating orientation, external obstruction, glazing strategy and other design factors.

The two methods recognised by BS EN 17037 can produce materially different results.

In our own comparative analysis of an identical room tested across eight orientations, the daylight factor approach produced the same internal result regardless of orientation.

The climate-based analysis showed substantial differences between north- and south-facing rooms, reflecting the actual effect of orientation and annual climate.

This illustrates one of the main advantages of CBDM: it allows daylight design to respond to the building itself rather than relying on a single static sky condition.

We use daylight modelling to inform the design — not just test it.

Example Comparison — Method 1 vs Method 2

We analysed a simple test room in eight principal orientations using both methods:

Key Findings:

• The Daylight Factor approach underestimated daylight levels by 42% compared to the north-facing CBDM result, and by 72% compared to the south-facing result.

• This simple analysis demonstrates >100% increase in internal daylight levels between north and south orientations — a critical factor entirely overlooked by the hybrid daylight factor approach, which results in the same internal illuminance for all orientations!