The amount of daylight entering and bouncing around a space is broadly a function of the size of each window aperture, the amount of sky visible from each window and the reflectance of internal surfaces. Whilst the inter-relationships between each element is quite complex, it is possible to simplify and formalise them to yield a single room-averaged daylight value. Though not particularly accurate, especially for deep spaces and those closely surrounded by other buildings or vegetation, it does provide a quick and simple guide to the potential for daylighting.
The equation for estimating the average daylight factor is given in the following equation:
- Where
- W = The area of each window (m²),
T = Transmittance of each glazing material (if not known, use 0.8 for clear single glazing or 0.7 for clear double glazing),
Θ = Vertical angle of the sky as seen from the centre of each window (see below),
M = Maintenance factor based on angle of glazing and the cleanliness of its environment (see below),
A = Total internal surface area of the space, including walls, floors, ceilings and windows (m²),
R = Area weighted average reflectance of all surfaces making up A, (use 0.1 as reflectance for glass).
Sky Vertical Angle
It is often necessary to visually estimate the vertical angle of the sky as seen from the centre of a window. This is typically due to variations in either the height of adjacent building / obstructions or shading projections above the window itself. Figure 1 outlines an empirical method for estimating this angle by simply averaging out the heights. It is also possible to calculate this value directly from a window's shading mask.
Glazing Maintenance Factor
The relative cleanliness of glazing panels within a window can have a significant effect on it's overall visible transmittance. To account for this, Table 1 gives a range of values to be used as the maintenance factor for different glazing angles and environments.
| Location of building | Type of work | Inclination of glazing | Maintenance factor |
|---|---|---|---|
| Non-industrial / clean industrial | Non-industrial / clean industrial work | vertical | 0.9 |
| sloping | 0.8 | ||
| horizontal | 0.7 | ||
| Dirty industrial work | vertical | 0.8 | |
| sloping | 0.7 | ||
| horizontal | 0.6 | ||
| Dirty industrial | Non-industrial / clean industrial work | vertical | 0.8 |
| sloping | 0.7 | ||
| horizontal | 0.6 | ||
| Dirty industrial work | vertical | 0.7 | |
| sloping | 0.6 | ||
| horizontal | 0.5 |
Required Window Area
For design purposes, it is possible to re-arrange the above equation to calculate the window area (W) required for a given average daylight factor value:
- Where
- Tavg, Θavg and Mavg are simply the average window values for T, Θ and M as used in the formula at the top of the page.
