Corrections to calculated daylight factor k1, k2, k3, k4 -- Algorithm 2.13

Components of the daylight factor calculated using the techniques described in the following sections must have their values corrected to allow a more realistic 'real world' daylight factor to be estimated and used for assessment purposes. Four basic correction factors can be identified, which relate to glazing properties and environmental deterioration.

The dirt on glass correction factor, k1, should be applied to the average daylight factor (algorithm 2.12); the glazing bar correction factor, k2, may be used in that algorithm to estimate the net glazed area from the total opening area.

Input

Sky component, Ds
Externally reflected component, De
Internally reflected component, Di

Equation

The corrected daylight factor is

where the factors are
Dirt on glass correction factor, k1
Glazing bar correction factor, k2
Glass correction factor, k3
Decoration deterioration factor, k4

The value of k1 will depend upon levels of air pollution, the slope of the glazing, and the cleaning schedule for the windows. Suggested values of k1 are given in the table below.
Type of location
Vertical glazing
Sloping glazing
Horizontal glazing
Clean
0.9
0.8
0.7
Dirty
0.7
0.6
0.5
Very dirty
0.6
0.5
0.4

The value of k2 will depend upon the extent of glazing framing and glazing bars. The magnitude of k2 is defined by:

The glass correction factor k3 depends upon the daylight transmittance of the glazing. Most of the algorithms in this section have allowed for clear and body-tinted glazing. If the components of the daylight factor have been calculated assuming ordinary clear glass then it is necessary to correct for other glazing materials by applying factor k3, where

The decoration deterioration factor k4 is applied to the internally reflected component of the daylight factor and depends upon the location and use of the building. Suggested values of k4 are given below
Type of location
Type of work
Correction factor
Clean
Clean
0.9
Dirty
Clean
0.8
Clean
Dirty
0.7
Dirty
Dirty
0.6

Source

BRE Digest 310 Part 2 (1)
References

1. Building Research Establishment Estimating daylight in buildings: Part 2 BRE Digest 310 Building Research Establishment (1986)

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