Getting Window Design Right

An introductory lecture on Planning for Daylight and Sunlight, based largely on a BRE text by Paul Littlefair

Andrew Bairstow


Window design involves finding an appropriate compromise on a lot of conflicting issues:

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It is important to get both the fixed aspects of design and operational details right.
This lecture will concentrate on design for daylight, considering how the other issues impinge on this.

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What is daylight?
Is that part of the solar radiation spectrum with wavelength between 400 and 700nm (total range: 3-14 000nm)

Solar Radiation Spectrum

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Visible proportion of solar radiation reaching the earths surface is significant due to

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Daylight consists of

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Definitions and Units of light

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Why is daylight design important?

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energy efficiency of light

All sources of light, natural and artificial also carry energy. The effectiveness of a light source is affected by the proportion of visible radiation and the response of the eye to the different wavelengths. The term luminous efficacy is the most usual measure to represent this efficiency.

Luminous Efficacy=illuminance/total energy

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Typical luminous efficacies

Tungsten Filament12
Tungsten Halogen18
Compact Fluorescent70
T12 Low Frequency67
T8 Low Frequency77
T8 High Frequency88
T8 Triphosphor High Frequency100
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Luminous Efficacies are reduced by:

Daylight Electric Light down up Bottom top

Steps in designing windows for daylight

It is best if these are done in sequence. Each will be dealt with here after consideration of some of the basics of daylight assessment.

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Daylight design is largely based upon CIE Overcast Sky down up Bottom top

Site planning

For any new development their is a need to plan for sky and sunlight availability for: down up Bottom top

Assessment is based upon

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The illuminance (under an overcast sky) on a horizontal plane will always be greater than that for a vertical surface, since:

The maximum illuminance for a vertical surface (unobscured) is considered to be 40% of that received by a horizontal (unobscured) surface.

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Vertical Sky Component will be less than half of Horizontal Sky Component

HSCVSC down up Bottom top

A continuous obstruction of 25degrees reduces vertical sky component to 27%

Continuous Obstruction down up Bottom top


An L shaped building is planned on a site which comprises one existing building 12m tall.
The two inner faces of the L, are shaded by the adjacent wall as well as the obstruction and need assessment.
The existing building will also need assessment as its access to daylight will be reduced.
Only one face of the new building will be assessed here.

In the absense of more detailed information about glazing (this is an early stage in planning and so window positioning should not be finalised) the assessment should be made for the lowest floor (assuming this is most obstructed) and at a central location of the window wall, 2m up from the ground.

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Plan view of site

example building: plan view down up Bottom top

Elevation of site

example building: elevation down up Bottom top

Assessment uses the Skylight Indicator. Each of 80 crosses represents 0.5% Sky Component

We will be superimposing the obstructions onto the Skylight Indicator. The number of crosses which are not covered will give us a value for the sky component which we can use to check the viabiity of daylighting the rooms on that face.
Why do you think the crosses are arranged in the way they are? In particular, why are they so close near to the centre of the indicator and tail off radially as you get further from the central line

Skylight Indicator

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We need to determine the effective angle of sky which is obstructed by the buildings
Firstly, the horizontal (azimuth) angle can be determined simply by laying the indicator over the building plan (centre at assessment point: baseline along window wall), marking in the corners and joining these to the assessment point.
Can you explain what this shading represents?

Plan overlaid down up Bottom top

Rescale obstructions

However, this shading represents an infitessimally tall sky scraper and we need to remove the shaded area which represents the sky which can be seen above the adjacent buildings.

Considering the first of these obstructions: the external building. Measure or calculate the distance between the assessment point and any point on the obstruction's facing wall (there is a simpler distance to choose than that shown below). Assume transmittance is 0.6 (takes into account dirt build-up)

Measure Distance to Obstruction Face down up Bottom top

Measure the height of the building above the assessment point and divide by the distance derived in the previous step: this is a measure of the elevation angle.

Obstruction Elevation Angle down up Bottom top

Elevation angles above this value will correspond to lower distance:height ratios. Thus we can erase all shaded points with higher ratios.
How many crosses represent obstructed sky? Subtract from the total (80) and divide by 2 to get the Sky Component. Check if it is above 27%, the recommended minimum for good daylighting.

Real Shading down up Bottom top

No-sky assessment.

The no-sky line divides areas which are directly lit from those which must be lit by reflected light.

The area which does not receive direct light from the sky will receive only reflected light (internal and external). Unless the surfaces behind this zone are very reflective, they will need supplementary electric light for most of the hours of use.
The reference Plane in most cases will correspond to desk or table height. No Sky Line down up Bottom top

Where the obstruction is not continuous the No-Sky Area may be reduced

No Sky Area down up Bottom top

In determining the no-sky area prior to defining the location of windows, the ceiling height may be used for the maximum window head height.

Minimum No Sky Area

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It is useful to know the minimum head height which will ensure their is no No-Sky area within each room

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Maximum Depth

A futher indication of the daylight performance of the back area of a room uses the principle of Maximum Depth. This may be calculated for a room, given the area weighted reflectance for the rear half of the room, its height and its width.

Maximum Depth Calculation

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To size windows we need some means of evaluating illuminances within a room. The principle of daylight factor is used:
The Daylight Factor at a point in the room is the ratio of the illuminance at that point to the horizontal illuminance at that point in the absence of any obstruction (including the building).

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The daylight factor will vary from point to point in a room. The average daylight factor provides a useful indication of the average illuminance within the room.

Average Daylight Factor


T = Hemispherical Light Transmittance of the Glazing (range 0.0 to 1.0)
Aw = Area of the window (in m2)
ø = Angle of Visible Sky
A = Total Surface Area
R = Average Area Weighted Reflectance of surfaces (Walls, Floor, Ceiling, Windows)
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Angle of Visible Sky

Taken from the centre of the glazing. For non-continuous obstructions it is more difficult to determine. One way is to use the results for vertical sky components. The following simple rule may be used:
Vertical Sky Component (%)Angle of Visible Sky
Angle of Visible Sky down up Bottom top

Average Daylight Factors give an indication of daylighting performance

The equation may be rearranged to calculate the window area required to achieve a given average daylight factor.

Window Area down up Bottom top

Positioning of windows requires a more complex procedure of calculation and is easiest achieved using a suitable software package such as DAYLIGHT or RELUX. Window positioning should be based upon the distribution of daylight factors that achieves good uniformity, not at the expense of view or aesthetics.

It may be necessary to repeat some of the steps as changes are made to the facade: the addition of shading and definition of windows will affect the Sky Component, No-Sky Areas etc.

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Design Issues
Solar Spectrum
Attenuation of radiation
Luminous efficacy
Typical Luminous Efficacies
Steps in Window Design
Sky Definition
Horizontal and Vertical Sky Components
25o Obstruction
Skylight Indicator
Horizontal Obstruction
Vertical Obstruction
No-Sky Assessment
Maximum Depth
Daylight Factor

Return to Daylight Modelling