Average Daylight Factor

Article 078

Nov. '03

The amount of daylight entering and bouncing around a space is broadly a function of the size of each window apperture, 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 as follows:

DFavg =
S (W T Ø M)

%

A (1 - R2)

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),

Ø = 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).
 
 
 
 

Figure 1 - Calculating the vertical angle of sky visible
from the centre of a window.
 
 

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

 
 

Table 1 - Maintenance factors for various window angles
and working environments.
 
   
 
 

The No-Sky Line
 

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:
 
   
 
 

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:

W =

 DFavg . A (1 - R2)

m2

Tavg . Øavg . Mavg

Where Tavg, Øavg and Mavg are simply the average window values for T, Ø and M as used in the formula at the top of the page.
 
 
 
 

Figure 5 - The different types of point distribution
corresponding to sky types and applications such as the
BRE Vertical Sky Component.

Image created using ECOTECT v5.
 
   
 
 

Related Links

Sky Illuminance
http://www.squ1.com/daylight/sky-illuminance.html
 
 

 

 

 

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