What Is Daylight Factor?

The daylight factor (DF) is the ratio of the illuminance at a point inside a room to the simultaneous illuminance on a horizontal plane outside, under an overcast sky, expressed as a percentage. It is dimensionless and independent of the actual outdoor illuminance level, which makes it a stable metric for comparing different design configurations.

A DF of 2% means that at the reference point, the indoor illuminance is 2% of what an unobstructed outdoor location receives at the same time. Under the CIE standard overcast sky model, an outdoor horizontal illuminance of 10,000 lux (a typical diffuse overcast day in Central Europe) would produce 200 lux at a point with DF = 2%.

Why Overcast Sky Conditions?

Daylight factor calculations use an overcast sky model rather than a clear sky because overcast conditions represent the lowest-light scenario that is still widespread. In Poland, overcast or partially overcast conditions occur for a substantial proportion of daylight hours, particularly between October and March. Designing to the overcast sky model ensures adequate light under the conditions where it is most needed.

Clear sky conditions produce higher illuminance but also direct beam radiation, which introduces glare and localized heat gain that require separate analysis.

Components of Daylight Factor

The total daylight factor at a point is the sum of three components:

  • Sky Component (SC): direct light from the visible portion of the sky, seen through the window from the reference point.
  • Externally Reflected Component (ERC): light reflected from external surfaces — the ground, adjacent building facades — that reaches the window opening and then the reference point.
  • Internally Reflected Component (IRC): light that enters the room via the window and is then reflected by internal surfaces (ceiling, walls, floor) to reach the reference point.

In rooms with adequate window size and no significant external obstructions, SC dominates. In rooms with deep floor plans or heavily obstructed facades, IRC becomes a larger fraction of the total.

Simplified Calculation Method (BRE Formula)

For preliminary assessment of single-sided rooms, the Building Research Establishment (BRE) simplified formula provides a reasonable estimate:

DF = (W × T × θ) / (A × (1 − R²))

Where:
W = Net glazing area (m²)
T = Visible light transmittance of glazing (typically 0.6–0.8)
θ = Vertical sky angle from the window centre (degrees)
A = Total internal surface area of the room (m²)
R = Area-weighted average surface reflectance of the room

This formula yields the average daylight factor across the room. It does not resolve the daylight gradient from window to back wall, for which point-by-point calculation or simulation is required.

Minimum Values Under Polish and European Standards

Polish WT regulations do not specify an explicit daylight factor value for most room types. The regulation requires windows meeting a minimum area-to-floor ratio (1:8) and specifies that rooms must be "sufficiently daylit" — a qualitative standard that in practice is assessed against DF targets from professional guidance and the EN 17037 standard.

EN 17037:2018 defines three recommendation levels:

Recommendation Level Target Illuminance Required % of Reference Hours
Minimum 100 lux 50% of occupied hours
Medium 300 lux 50% of occupied hours
High 500 lux 50% of occupied hours
EN 17037 uses a climate-based method tied to actual hourly irradiance data rather than the static overcast sky DF. For Polish locations, reference climate data from Meteonorm or PVGIS can be used. The two methods are complementary; DF is useful for early design screening, while the EN 17037 climate-based method is required for formal compliance checking in some procurement contexts.

Input Data for a Typical Polish Apartment

Applying the simplified formula to a typical Warsaw apartment room:

  • Room: 4 m × 6 m × 2.7 m (floor area 24 m²)
  • One south-facing window: 1.6 m wide × 1.4 m high, net glazed area approximately 2 m²
  • Glazing transmittance: 0.62 (standard low-e double glazing)
  • Vertical sky angle: 40° (low-rise context, no adjacent obstruction)
  • Total internal surface area: approximately 90 m²
  • Average reflectance: 0.50 (white ceiling 0.75, mid-tone walls 0.50, medium floor 0.30)

This produces an average DF in the range of 1.5–2.0%, depending on exact input values — adequate for a living room by most guidance, though the back quarter of the room will receive less than the average.

Simulation Tools

For detailed design work or compliance assessment on complex floor plans, dedicated simulation software provides higher accuracy than the simplified formula. Common tools in Polish professional practice include:

  • DIALux evo — widely used in Poland for daylight and artificial lighting design. Accepts IFC geometry imports from BIM tools.
  • Radiance / DAYSIM — research-grade ray-tracing tools used in academic contexts and high-specification projects.
  • Velux Daylight Visualizer — accessible tool for residential scale, based on validated climate data.

References