Solar Shading

This is a subject where expert advice should be sought. There are many different forms of solar shading; each has its own characteristics, advantages and disadvantages, and the architect must be sure of the criteria that should be taken into account when determining the nature of the shading required and whether some form of adjustability is desirable.

The BRE pamphlet 'Solar Shading of Buildings' states that the principal reasons for needing shading are as follows:

1. To reduce the effect of heat gain from the sun

2. To cut down sun glare experienced through the windows

3. The provision of privacy. This will not normally be a requirement, but it may be important in certain circumstances.

1. Reduction of heat gain from the sun

At some times of the year this may be of the greatest importance, but its need will not be permanent; for certain times of the year the heat gain may be welcome.

The problem is most acute on South facing exposures, but there may be special conditions in the building such as abnormally high internal heat gains, or where the building has to be kept at a low temperature.

Since once the heat gain is within the building envelope, it is difficult to control, the shading system which stops the heat from getting in in the first place . . . the external system . . . will be better. When contemplating external shading, it is important to bear in mind the question of structural stability and the need for periodic cleaning.

2. Reduction of sun glare

Glare may result from a direct view of the sun, by reflection from some outside source such as the building opposite a North facing exposure, or by reflection from items inside the building; most noticeably from items which are the object of attention, such as a business machine or computer. Glare, unlike heat, can be controlled easily from within the building.

3. Provision of privacy

This is really the 'net curtain' solution, if it is needed at all. Some form of translucent material which lets through the maximum amount of daylight, but breaks up the internal image seen from the outside. This is less important during the day when the light ouside exceeds that within and there is no disadvantage in placing the diffusing material on the inside of the window. In certain security situations special materials have been developed which in addition to the provision of privacy, capture the shards of glass which occur when a window is broken

Solar shading solutions can broadly be divided into the following three types:

1 External shading

2 Internal shading

3 Alternative glazing

(Note. The BRE pamphlet further lists as a solution the reduction in the area of glazing, comparing this with the loss of daylight associated with the use of some form of tinted 'sun control' glazing. On the basis that the window areas have been calculated correctly in the first place, this must reduce the daylight available, and for this reason has not been included.)

1. External shading

The following methods are available: Overhangs and canopies, light shelves, fixed and movable louvres, shutters, vertical fins, deep window reveals, egg-crate baffles, and roller blinds.

When choosing a method of external shading, the most crucial decision that must be taken is the long-term viability of the hardware involved, associated with the climatic conditions which will be experienced on site; there is also the architect's preoccupation with the exterior appearance of the building with which the former is associated.

Whilst it is best to control the heat gain before it enters the building by external means, any method of external shading can be vulnerable, and the cost and long-term viability of the method employed must be established. Comparisons should be made with internal shading methods, to establish whether the gains in heat control are sufficient to warrant what will initially be expensive, and possibly a long-term maintenance problem.

The following list of shading types are some of the options available, each having their own advantages and disadvantages; although the visual appearance of each type may have more influence with tbe architect as to how he sees the impact on the elevation of the building:

Overhangs and canopies Continental shutters, and awnings Light shelves

Fixed and movable louvres Egg-crate louvres External roller blinds Brise soleil

2. Internal shading

It must be recognized that any form of shading within the building envelope is bound to be less efficient as a control of heat gain than an external device, since the heat which is generated has already entered the building, and is more difficult to extract; however the type of shade will be less vulnerable than that outside, will be easier to maintain and to clean, so that an overall view must take into account all the factors in coming to a decision. If of course it is not possible to control the solar gain sufficiently inside, then other means will have to be adopted.

The most common form of control and one used almost universally in residential building, is the curtain, and provided these are carefully designed, perhaps with a reflective lining to reduce the solar gain when pulled across the opening (whilst at night they can keep out the cold) can be perfectly satisfactory in our temperate climate ... indeed we welcome the sun on all but the exceptional day.

A more flexible form of control is the venetian blind, which has the advantage of adjustability in that it can be raised when not required for sun control, to permit maximum daylight entry.

The demise of this excellent tool has been predicted for many years, but it survives, offering excellent glare control, can be motorized when used in large office projects, can be incorporated within panes of glass to protect it from damage, and specialist versions are available where the tilt of the blades can be varied to enable the top of the blind to reflect light up to the ceiling of a room, whilst the lower blades control the sunlight by reflecting it away from the building. A further advantage of the venetian blind is that the surface design of the horizontal slats can be varied to meet the individual requirements of the building.

The obvious advantage of the venetian blind is that it can and should be raised when not needed for sun control; the problem is that once lowered it tends to be left in the closed position. A procedure should be adopted to ensure that their use is optimized and a simple solution might be for the office cleaners to open the blinds to ensure that each day starts with them open to admit the maximum daylight. Venetian blinds have a lot of life in them yet .

Other types of blind are also available, the vertical hung louvre blind where the louvre slats can be rotated, or moved to one side offer flexibility, provide privacy, and together with roller blinds and those of other materials can provide low-cost solutions in the domestic situation.

The heat gain from the sun can be controlled by the type of glass used, various options being available, First there are the low emissivity glazings, developments in this field continue, and the thermal properties of the glass can now be tailored to give good solar control. Their big advantage is that they admit higher levels of daylight than the original tinted versions, and can control heat loss.

Prismatic glazing panels have also proved useful; these are limited to small panels of prismatic glazing which, when attached to high level rooflights, can allow daylight to enter, but redirects the sunlight on to the ceiling of the space, or excludes it altogether.

Finally there are the high tech glazings already referred to under the glazings available for windows. These include the following:

• Electrochromic and liquid crystal glazings, which can be made to darken on application of an electric current

• Photochromic glass, which darkens when sunlight falls on it

• Thermochromic glass which alters its transmission value on the introduction of heat.

None of the latter is in the mainstream of development, and it is unlikely that these will have a major impact for some years.

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