The Code for Sustainable Homes allows credits to be awarded when a number of key rooms have better than the minimum required by the Building Regulations.
One credit is available for each of the following situations:
- Kitchens must achieve a minimum Average Daylight Factor of at least 2%
- All living rooms, dining rooms and studies (including any room designated as a home office under Ene 9 – Home Office) must achieve a minimum Average Daylight Factor of at least 1.5%
- 80% of the working plane in each kitchen, living room, dining room and study (including any room designated as a home office under Ene 9 – Home Office) must receive direct light from the sky
Compliance with all three requirements will result in three Code for Sustainable Homes credits being awarded.
The DF formula described below can be used to model daylighting conditions in any simple rectangular room with a continuous external obstruction or none. For Code for Sustainable Homes credits to be allowed this should be undertaking by a person with sufficient knowledge on the subject. For L-shaped rooms, it is acceptable to divide the room into two sections and calculate the DF for each section based only on the windows present in that section. The DF of both sections can then be averaged to give a final result.
Where external obstructions are of complex geometry and cannot be approximated by a continuous object, it is advisable to use the methodology in Littlefair (1998). Individual trees can be ignored.
More complex room geometries can be modelled using computer simulation software, physical scale modelling or advanced manual calculations.
Where there are two types of room which form part of the same large space, for example, an open-plan kitchen-dining room, calculate as one room as there is no solid partition present to block the distribution of the daylight. Credits will then be awarded on the basis of the DF of the whole space. For example, if the space is used as a kitchen, a living room and a dining room, the same DF will be used when assessing all these areas against the levels set out above.
When two or more windows in a room face different obstructions (e.g. vertical windows and roof lights) or differ in transmittance, the DF should be found separately for each window, and the results summed.
Plotting of the no-sky line or estimating the percentage of the working plane that receives direct light from the sky can be done using the methodology in Annex 3. It must be understood that this methodology will underestimate the actual percentage of the working plane that receives direct light from the sky because obstructions are unlikely to be infinite. Where obstructions are not horizontal, parallel to the window or considered infinite, refer to Littlefair (1998) for a more accurate methodology.
Average daylight factor
The average daylight factor is the average indoor illuminance (from daylight) on the working plane within a room, expressed as a percentage of the simultaneous outdoor illuminance on a horizontal plane under an unobstructed CIE ‘standard overcast sky’. The average daylight factor can be calculated using the following equation:
D=MWuT/(1 – R2))
Where:
- W = total glazed area of windows or roof lights
- A = total area of all the room surfaces (ceiling, floor, walls and windows)
- R = area-weighted average reflectance of the room surfaces
- M = a correction factor for dirt
- T = glass transmission factor
- u = angle of visible sky
Guide values for a typical dwelling with light-coloured walls are as follows (for more accurate values, refer to CIBSE Lighting Guide 10):
R = 0.5
M = 1.0 (vertical glazing that can be cleaned easily)
0.8 (sloping glazing)
0.7 (horizontal glazing)
T = 0.7 (double glazing)
0.6 (double glazing with low emissivity coating)
0.6 (triple glazing)
u = 65? (vertical glazing)
It is advised that this default figure for the angle of visible sky is used with caution; the methodology detailed in the angle of visible sky calculations below must be preferred for more accuracy.
Angle of visible sky
The angle of visible sky u is the angle subtended, in the vertical plane normal to the window, by the visible sky from the centre of the window.
tan(a=) H/D
tan(b)= Tw/Hw
Where:
Hw is the height of the window
Tw is the thickness of the wall
D is the distance from the window to the obstruction
H is the height of the obstruction above the mid-height of the window
And: u = 90 – a – b
No-sky line
The no-sky line divides those areas of the working plane which can receive direct light from the sky, from those which cannot. It is important as it indicates how good the distribution of daylight is in a room. Areas beyond the no-sky line will generally look gloomy.
As an approximation, obstructions that are parallel to the window can be considered infinite. The no sky-line will then be parallel to the window at a distance ’d’ from the window wall, which can be calculated as follows:
d= xh/y
Where:
h = height of the window head above the working plane
y = height of the obstruction above the window head
x = distance from the window to the obstruction
If d is greater than the room depth, then no part of the room lies beyond this no-sky line.
Where results using this methodology do not comply with the requirements, more accurate calculations can be carried out by using a computerised methodology in order for the Code for Sustainable Homes credits to be awarded.
