There is now a large amount of alternative glazing for windows, and it is necessary for the architect, in conjunction with his services consultant, to write a detailed performance specification; this must include the orientation of the window, its thermal and acoustic characteristics, together with its capacity for solar shading. This is of course in addition to the main function of the window which is the admission of daylight and the introduction of the view to outside. Further factors which may need to be taken into account, are whether it is thought desirable to have windows which open or are fixed, and its relationship to ventilation.

But here our concern is with the types of glazing which are available. As already stated the main purpose of a window is for the admission of daylight, and associated with this the view to the exterior.

Glazing types which reduce the impression of daylight significantly, darken both the interior, and the view, whilst the view from the outside towards the building makes the facade look black. It is only when comparisons are made between the view through a clear glass window and one with a modifying glass that reduces the daylight, that the results create disappointment . . . it is true to say that it is human nature to appreciate the natural environment, with all its variations of colour, light and shade.

This is particularly true of residential properties where some form of dark glass has been applied to the facade, giving the impression of a dull day seen from the interior, as compared to the view through a clear window.

There are basically three main types of glazing as follows.

1. Clear glazing

This can be single sheet, double or triple glazed or alternatively a 'thick' glass, but the more sheets or the greater the thickness of glass the more the daylight will be diminished, although the impression of the colour of the exterior will still be perceived as natural.

Clear glass whilst allowing a high transmission of daylight, will at the same time and on certain building facades allow a high transmission of solar radiation. It is this fact that has led to the development of the more high-tech glasses designed to reduce solar gain, with their consequent loss of daylight transmission. Other means such as interplane blinds, located between the panes of glass, may present a solution. These would only need to to be installed on facades subject to solar gain and then only activated when required.

2. Tinted glass

This is of two types: the first where the clear glass is itself modified in such a way as to produce different radiant heat transmission characteristics, therefore the thicker the glass the lower the transmission of daylight, and the greater the control of radiant heat from sunlight.

The second type of glasses are those coated with microscopically thin layers of metallic oxides which reflect the heat away and out of the building. These coatings are applied to the inside layer of glass generally in association with other panes in a sealed double glazed unit as a protection, since on their own they would be vulnerable to damage.

These coated glasses can be designed to have high daylight transmission, due to the very thin layer of reflective material; so that they almost give the appearance of clear glass, and do not suffer from the objections raised by tinted glasses which reduce the daylight significantly. Additionally they do not obstruct the view; however they do have cost implications, and should only be used where the specification demands it. Highly reflective glasses are available, but need to be used with care to avoid the danger of glare to other buildings or motorists.

3. Miscellaneous glazing

A number of different types of glazing are placed in this category, largely because they cannot be lumped together into a single category; they consist of the following:

Patterned glass, wired glass, laminated glasses and glass blocks. Patterned glass

Any number of patterns can be rolled into semi-molten glass, to provide decorative or diffusing sheets for various purposes, though rarely for windows, since their capacity for light transmission will be modified.

Wired glass

A similar process is used for the manufacture of wired glass, where a wire mesh is sandwiched within the thickness of the glass. This used generally in security situations, and sometimes as a protection to vulnerable skylights.

Laminated glasses

Similar methods of manufacture are used for laminating sheets of plastic between sheets of glass, again used for security reasons as resistance to impact. These reduce the transmission of daylight.

In museums where exhibits are exposed to daylight, it will be necessary to control the entry of UV light. This may be done by the use of laminated glasses, where UV absorbing filters can be laminated between the sheets of clear glass.

Glass blocks

These were a popular form of glass wall in the 1930s, having thermal characteristics due to the hollow nature of the blocks, which, because of their structural nature are still in use today for the introduction of daylight into new buildings, but special openings will be required to provide a view.

Finsbury Health Centre. The foyer

High tech glazing

There are a number of glazing types which fall into this category, the most advanced of which are the photovoltaics, where the glass itself is designed to generate electricity from solar radiation on south facing exposures, which can then be used within the building to reduce the energy required for the artificial lighting. Some buildings already use this method, and the UK Government is now putting research money into its further development. (See Doxford International Building Park. Lighting Modern Buildings. Case No 11, pp. 124/5).

Two other types of high tech glass deserve mention, but are not at present economically viable for general use in buildings.

The first are the photochromic glasses, which respond directly to an environmental stimulus (temperature or light) rather like the special sunglasses which are already available which alter their transmission factor depending upon the brightness of the ambient light; alternatively there are the electrochromic glasses designed to respond indirectly by the application of an electrical current which alters their visual and thermal characteristics. These glasses are still at the experimental stage, but are likely to be developed further to a point where they may become viable.

The choice of glazing in a large complex is one of the greatest importance, having implications both on first cost, and the cost in use of the project.

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