07.01 Sunlight

Categories: Lighting

Introduction
It would appear from the evidence available that even the most primitive civilisations recognised how important was the sun as a source of light to the world. Indeed, in some cultures the sun was seen as playing such an important part in life that it became a symbol which was synonymous with life itself. Unfortunately in the world of today it is all too easy to forget about how necessary is sunlight to our well being because the sun is no longer the only practical source of light.

To those sensitive to the world about them the fundamental importance of sunlight is a self evident truth, however, intuitions do not necessarily provide firm foundations for design. Fortunately, there is much evidence that sunlight itself has both beneficial physiological and psychological effects which have a direct impact upon the quality of our lives, even though sunlight is no longer essential for the light we work by. It is also true that utilising natural light as a working illuminant is one of the most effective ways of making use of natural resources and has been an essential aspect of planning buildings in the past. For these reasons it is worthwhile knowing how considerations of sun lighting contribute to the design of buildings.
 

Initial aspects of designing for sunlight
Sun lighting can be a great help in planning a building. It has the advantage that it can be used effectively by the designer and is appreciated by those using and viewing a building. A further advantage of using sun lighting as a primary design criterion is that the basic facts needed for design are simple and have probably been known to readers from their schooldays, i.e.:

- the sun is almost due south at noon,

- the sun rises in the east and sets in the west,

- the sun is higher in the sky in summer than in winter.
 

Although simple, when they are considered in relation to the particular requirements of a design, these facts can be invaluable in helping to sort out the overall planning of a building . Consider the example of a house which is to be built in the temperate British climate. Because of a perceived deficiency in the hours of sunlight, people will generally wish to make the most of whatever sunlight there is and under these circumstances the following points might be considered;

(i)  it is pleasant to awake in the morning with sunlight streaming through the window and therefore as the sun rises in the east it might be worthwhile placing bedroom windows on an easterly aspect,

(ii) kitchens can be rather warm because of all the heat generated in cooking and therefore one should consider placing a kitchen or pantry on a northern facade in order to limit the sunlight further raising the room temperature. Also in a working room where one will be using sharp knives it is preferable to have an even light free from hard shadows across the working surface,

(iii) lounges are mostly used in the afternoon and evening and because they are places of relaxation the presence of sunlight is generally welcomed. By placing the lounge on a southern or western aspect the most can be made of the afternoon and evening sun.

(iv) the low altitude of the sun in winter makes northern slopes less favourable than southern slopes and therefore southern slopes will normally be developed before northern slopes.

Also obstructions to the south are that much more critical and where it is intended to plant trees on the boundary it is better that they are deciduous so that they loose their foliage in winter and thus admit sunlight onto the site.

These points make it quite clear that sun lighting can, and indeed should play an important role in helping to generate the form of an architectural design.

Of course the above suggestions cover only a single aspect of the design and they should not necessarily override all other considerations. Also, the way in which the design is affected by considerations of sun lighting will vary depending on the priority given to different environmental factors by the client. For example, it might well be that a client prefers a splendid view from the kitchen and this might only be available on a western aspect, or a client might be keen on watching television throughout the day and therefore the penetration of sunlight might need to be restricted in order to reduce glare on the television screen.

These last two examples illustrate the fact that even though the basic facts of sun lighting are easily appreciated there is no simple recipe which can ensure a design will satisfy a client's wishes.
 

The need for more detailed design
Although it is simple facts such as have been described that are needed to help plan a building, there is merit in knowing how to predict with a little more accuracy the sun's position at different times of day and year. This is because there are buildings such as museums, art galleries, sports halls and libraries where it is necessary to closely control the sunlight entering the building in order that critical visual tasks are performed under good lighting conditions. Also, in more extreme climates the control of sunlight might be crucial to achieving a comfortable thermal environment without resorting to air conditioning.

There are a number of techniques which can be used to investigate the position of the sun, and although in architectural applications the accuracy required is less than would be expected in astronomical or navigational work, there is a need for information to be readily understood by the designers and clients, and easily applied to design problems. These needs mean that there is no single method which can be recommended above all others and that the particular technique adopted will depend upon the specific circumstances. One method will better allow a client to appreciate a problem whilst another might be needed to ensure the admittance of sunlight at defined times.

An ability to use the more involved techniques will not only facilitate the detailed consideration of sun lighting on the more critical projects, but it also enables designers to predict the position of the sun at latitudes where they have no experience to guide their judgement.

Although a knowledge of the basic astronomical facts is not essential for the successful application of the prediction techniques, they are briefly described in a later section. This is done in order to satisfy the curiosity of those who are inquisitive and make easier the task of explaining the definitions of technical terms.
 

Additional factors in sun lighting design
One feels intuitively that sunlight is an important element in people's lives and there have been attempts to establish just how important by surveying people's responses to sunlight in buildings. However, because of the complexity of our interactions with the environment, these surveys rarely do more than confirm that sunlight is indeed an important factor. In many ways this absence of a clear hierarchy in the priorities of various environmental factors is heartening to designers, for it presents them with the freedom to make decisions according to the circumstances. However in order to fully utilise this freedom the designer must be fully aware of the benefits and problems associated with the different aspects of the environment. In the case of sunlight these benefits can be identified as the following:

(a) the sheer intensity of sunlight provides a sparkle and brilliance that can transform what might otherwise be a dull scene,

(b) the directionality of sunlight means that there is often the added feature of heightened contrasts between areas in sunlight and those in shadow,

(c) that same directionality of light will reveal the texture of building surfaces and the form of a composition,

(d) the movement of the sun across the heavens ensures that it will reveal different aspects of building form at different times of day,

(e) the regular nature of its course across the sky means that it can act as a timekeeper,

(f) the changing nature of sunlight through clouds provides a variety of appearance which gives life to a perhaps otherwise static scene,

(g) a shaft of sunlight piercing through a window can provide a tangible link to the outside world for those cocooned within a building,

(h) the sun also has non visual effects which are of benefit to humans, it is a bactericidal agent against pathogens, is necessary for the synthesis of vitamin D and possibly has other direct physiological effects upon the body,

(i) there is a warmth to the sun which can be utilised to help heat a building and ensure that external areas do not become dank and mouldy.
 

Sunlight used in a measured way is generally beneficial, but excessive exposure can result in problems:

(a) direct sunlight generally makes written material too bright to read,

(b) sunlight can cause excessive direct glare and even when reflected from glass it can be uncomfortably bright,

(c) shadows cast by sunlight can cause mis-perceptions of form and shape,

(d) material in shadow may be difficult if not impossible to see,

(e) excessive insolation will result in buildings overheating,

(f) it is uncomfortably hot to work in sunlight if there is little or no breeze to cool one down,

(g) the heat and UV in sunlight can be harmful to materials and cause a deterioration of their physical properties,

(h) excessive UV can harm living organisms.
 

Simple guidance for sun lighting
In the past guidance was provided in codes of practice for certain building types, such as houses and schools, but these were not enforceable by law and are no longer current. However they are listed here as guidance.

• BS CP 3 1b Sunlight

 

 
 
 

(a)  Domestic buildings
Living rooms: 1 hour a day for 10 months in year from Feb. to Nov.
Kitchen and bedrooms: some insolation if possible

(b) Teaching spaces
Two hours a day throughout the year.

• The 'Sunlight and daylight planning criteria' from the Welsh office and published by the HMSO gives a means of assessing building heights in order to ensure a degree of sunlight availability, but other methods described later are perhaps better used because they demand a greater appreciation of exactly what happens in particular circumstances.

• The codes of practice have been superseded by a Building Research Establishment report, " Site layout planning for daylight and sunlight - a guide to good practice " by P J Littlefair.

Here it is suggested that a building will receive reasonable sunlight if;
(i) at least one main window faces within 90° of due south,
(ii) on this wall, all points on a line 2m above ground level are within 4m sideways of a point which receives at least a quarter of annual probable sunlight hours, including at least 5% during the winter months between 21 September and 21 March;
and a garden will appear adequately sunlit throughout the year if;
(i) less than 40% and preferably no more than 25% of the garden is shaded on 21 March.

There are methods described in the document for assessing levels of sunlight and daylight and these will be considered along with other methods.
 

Controlling sunlight
Once designers have answered for themselves the question of the need for sunlight, there is the problem of how they can satisfy the requirements that have been decided upon. The methods given in the BRE document may be used, along with other methods, but the factors that determine sun lighting within a building each need to be considered for the particular circumstances of a given project.

The following factors are available for controlling sun lighting:
 

Choice of site
In built up areas the availability of building land will be severely limited, but the favoured sites are usually those on southern slopes. For instance in Bath the major crescents and terraces are all on south facing slopes and it is only recently with the desperate shortage of building land that housing has been built on shaded northern slopes such as at Calton gardens, north of the railway station. The effectiveness of building on southern slopes in northern climes is also evident from the use of such slopes to grow vines.
 

Grouping of buildings
Various suggestions have been made by different authors on the effectiveness of grouping buildings in different plan forms but there is no consensus as to an ideal arrangement. However there is a need to be aware of the different ways in which buildings can grouped in order that they can be considered in a rational manner.

It is important to be sensitive to the effects the grouping of buildings have upon their surroundings as well as upon the insolation into the buildings themselves.

Buildings may be grouped together in the following ways:
 

Independent units
Sunlight will not be affected by other buildings and it will be the site and orientation of facades that govern the available sunlight. There is the possibility of making use of each facade.

Shading will not be available from adjacent buildings and if it is required then it might have to be provided by design. This may be done by creating natural shading, e.g. by planting deciduous trees which provide shade in Summer but allow sunlight to pass through branches in Winter, or by introducing brise soleil into the design of the building.

If reflected sunlight is to be used, then there will be a freedom to utilise reflections from the ground or the roof surfaces.
 

Strings
 

There are only two orientations of facade that can be used and the relationship between the lighting on the two facades will depend upon the orientation of the building line. Straight buildings will have facades on opposite faces and if the building axis is north south the facades will not receive sunlight at the same time, whilst an east west axis will create a southern facade which receives good sunlight throughout the year and a northern facade which is shaded for most of the year.

Parallel terraces may both act to shade buildings opposite and also provide an effective means of reflecting sunlight. Quite distant buildings can provide significant reflected light if they are made of a light material such as Bath stone.

Ideas of optimum sunlight penetration can lead to the proposal of a number of parallel rows at some theoretically best orientation and separation. This may lead to a regimentation of overall planning which can limit the range and variety of lighting needed to satisfy the varied needs of individuals and the diverse attitudes of people in general.

A curved string creates its own space and an additional dynamic to its appearance throughout the day, but will require more careful consideration of sunlight penetration in relation to the use of rooms and the design of any shading devices used to control the insolation.
 

Closed strings
 

Curvilinear strings provide a more graded response than those arranged as rectangles which have four clearly defined orientations. The ratio of relative proportions of the building heights and their separation will determine how much light there is to be inter reflected from opposite facades and available to rooms at ground level. This reflected sunlight may considerably increase the light in rooms facing away from the sun. In older cities constant additions to the backs of buildings has meant that buildings often effectively only have a single external face to the light, whilst a renewed fashion to cover the central volume has spawned a plethora of atria.

Buildings are aligned along streets and these may have developed either organically from historical property boundaries or they may be planned into blocks of one sort or another. These might be arranged on some form of grid which could be either rectilinear or radial. Although the radial form gives a more dynamic appearance to the buildings themselves there is more likely to be greater shading of facades.
 

Heights of buildings
Studies by Gropius were used to promote the use of high tower blocks because of an apparent better utilisation of sunlight. The better utilisation was both in terms of a higher degree of insolation into dwellings and greater availability of sunlight at ground level on the land between buildings. A good example of how satisfying the requirements of one design criteria may lead to unsatisfactory conditions in another area.

Limiting the heights of buildings is a principal means of protecting others sunlight. It should be recognised however that there is no legal right to sunlight, as there is to daylight. Clearly a high building which does not extend across the horizon may shade other buildings for some time of day throughout the year but allow sunlight to reach other buildings for at least some time in the day during the winter months, whereas a lower building extending across the horizon will cause no overshadowing at all in summer but may prevent any sunlight reaching other buildings during the winter months when sunlight is so precious.
 

Orientation
The adoption of any one orientation for all circumstances is likely to result in problems, and it is essential that orientation is considered in relation to the actual insolation into the building and not just to the facade. In Germany during the 1920's a consideration of the utilisation of sunlight resulted in the designers of Zeilenbau to orientate dwellings with east-west facing facades. Although the facades of the buildings receive better overall sunlight than a north and south facade taken together, because of the northern latitude, the sunlight during the winter months was at such a large angle of incidence to the facade it did not penetrate into the interiors through the shading caused by the thickness of the walls. This resulted in insufficient sunlight entering into the rooms during the mid winter period.

The grid orientation will affect insolation and the availability of sunlight at ground level.
 

Location of rooms on plan
The particular location on plan adopted for various rooms will depend much upon the latitude of the site. However it is the case that the main difference between east and west facing facades is the time of day when they receive sunlight whilst the main difference between north and south facing facades is the season of the year when they are exposed to sunlight.

Also, because the sun rises and sets in the east and west, it is often those facades which receive sunlight most directly and are therefore the most difficult to protect from excessive solar glare. Correspondingly, at low latitudes it is the south and north facing facades which are the easiest to shade, because the high angle of sunlight during mid day means that even a small projection will shade a window.
 

Fenestration
The window openings themselves are major factors in the quality of the sunlight entering a building as follows;.

Glass ratio
Clearly this determines the total amount of sunlight entering into the building,

Size
A particular glass ratio may achieved by a single large window or a number of small windows. A single large window will result in positions which receive continuous exposure to sunlight for long periods. A number of windows will result in an area receiving a series of successive exposures to sunlight followed by a period of shade.

The pattern of the sun patch will be either a block of light or a series of highlights scattered over the room.

Shape
Vertical windows will produce a series of light and dark patches which move across the room

Horizontal windows will produce a patch of light which does not appear to move as much with time and can cause continuous exposure for some positions.

Sunlight is a parallel beam which can be used to project a shape of light into the space.

Position
High windows will lead to a greater depth of penetration of sunlight into the interior.

Placing windows by the side walls will allow the light for some time in the day to be reflected off the walls, increasing the brightness of the room through inter reflected light but reducing the problems of excessive exposure to direct radiation.

Roof lights when positioned near a north wall will allow sunlight to be reflected off the wall, thus introducing sunlight without some of the problems of cast shadows across working surfaces.

Reveal
The depth of a reveal may considerably affect the penetration of sunlight into an interior. A high reflectance may increase the light entering a window with a deep reveal.

The use of splayed reveals considerably increases the penetration of sunlight through a window, and may provide an intermediate brightness between the bright external view and the dark window wall.

The external appearance of the window will depend upon the apparent depth of the window opening, a reasonable depth to the external reveals will add to the visual texture of the facade.

Glazing
The characteristics of the glazing material will clearly affect the amount and quality of the light entering through the window.

The characteristics of importance are:
- transmittance,
- reflectance,
- absorptance,
- transparency, i.e.- the degree to which parallel beams of light are transmitted without diffusion.
- translucency - the transmission of light where there is a degree of diffusion.
- colour - the degree and manner in which the glazing changes the relative spectral composition of the light.
 

Solar shading
A number of approaches may be adopted for solar shading. Buildings may be oriented so that particular windows receive little direct sunlight or they may be positioned so that they are shaded by other buildings or by natural location or vegetation.

Permanent sun shades may be built into the building form and this is often given the French terminology of - brise soleil.

Controlled shading devices may be constructed either internally or externally. Internal devices suffer from the problem that the solar gain is introduced into the building, whilst external shading devices can be the cause of long term maintenance expense and suffer from unreliability.

It is important to realise that the effects of shading devices are dependent upon their form and shape and not their absolute size. This is because the shading is dependent on angles rather than absolute size..

Any shading device will affect the view out of a window and this may be a crucial factor in favouring one form of shading over another form.

If shading devices are employed they will have a major, if not an overwhelming affect upon the external appearance of a building, and therefore they need to be considered at the outset if they are to be used.
 

New controls
There are a number of novel techniques which may be used for either introducing or excluding sunlight from buildings. These are just listed for the present:
- optic fibres and light guides,
- light reflectors, static or controlled via a heliostat,
- prismatic panels.
 

Probability of sunshine
Clearly in a hot climate with a high probability of sunshine it is likely that the designer will be more interested in shading a building than in facilitating insolation, and in a colder northern climate, where in Winter daylight hours might be quite short, the designer will wish to make the most of whatever sunshine there is. Thus the relative importance ascribed to particular aspects of sunshine will partly depend upon the climate and also the general availability of sunshine.

The availability of sunshine will depend upon:
- the length of time the sun is above the horizon,
- the probability that the sky is sufficiently clear to allow the sunlight to reach the ground undiffused.

This data is not always readily available in a form which is easily understood and when designing for foreign countries it is important to find data about the climate and cloud cover from a local meteorological station or airport. Data for this country is shown in two forms, the average hours of sunlight and the probability of sunshine for selected locations