A window's thermal performance is largely established according to the glazing material used and its properties. It is possible to divide the various glazing materials according to:
Every glazing material has three properties which define its functioning:
The properties of each glazing material are established according to the ratio between the coefficients, but the following is true in every case.
The coefficients provided by professional literature describe the ratio between the glazing properties for a ray of light striking the glazed surface at right angles. With a change in the striking angle, (the incidence angle) the ratios between the coefficients also change. At an angle of 600 to the normal, the reflectivity coefficient will be close to 1. In other words, most of the radiation striking the glazing will be immediately reflected (like in the case of a mirror). The radiation absorbed within the glazing will lead to a rise of the glazing's own temperature; so that if the surface heats, it acts as a heat radiator that radiates back into the space. The transmissivity coefficient points to the glazing material's ability to transfer energy from one side to the other by radiation.
It is difficult to recommend a general type of glazing that will be appropriate for all cases, as the choice usually depends on factors not connected to energy (opening to a view, design demands, etc.) and because each kind of glass has its advantages and disadvantages. It is important to remember that the functioning of different materials changes according to their angle of placement (vertical, horizontal, sloping) because of the change in the incidence angle at which the sun rays strike the glass and the resulting ratio between the glass properties.
Types of Glazing Materials
Regular Transparent Glass
This is the most common type of glazing used for glazed building openings. The thickness of sheet of glass usually ranges between 3 and 5 mm. This type of glazing permits high proportion of visible light penetration (88-90%) as well as penetration of a large proportion of the solar radiation striking it (77-86%). These properties make it the preferred material for south facing windows and greenhouses. It is important to remember that the thermal resistance coefficient of glass is very low (R = 0.18 W/deg C m2), which makes the glazing a thermally weak point in the structure's envelope.
This consists of two sheets of glass with space in between, sometimes filled with air or other gases, or vacuum. The thickness of both the glass and space are variable. These variations have a certain effect, up to a certain limit, on the percentage of radiation allowed to penetrate and on thermal conductance of the composition (when the thickness of the air space is over 2.5 cm, its marginal effect becomes smaller). The main advantage of this type of cross-section is its ability to reduce heat transfer from one pane to the other, both by conduction and by radiation. Double glazing is more expensive than single glazing but sometimes offers improved performance. Triple glazing (sometimes used) is even more expensive and has a conductance coefficient that is about 20% lower than that of double glazing, but the savings in energy are relatively small compared to its higher cost. For example, calculations performed at the Desert Architecture Unit questioned the economic benefits which could be obtained from double glazing under Israeli conditions.
This type of glazing permits penetration of light (about 80%, depending on thickness), but transmits only a relatively small portion of total solar radiation at the different wavelengths striking it (48-65%). The various industrial products belonging to this group are usually made of two layers of glass with a layer of absorbing material between them or of glass coated with one of the various varnishes. These additions absorb different wavelengths of radiation (from ultraviolet to infrared) and their effectiveness varies. Absorbing glass significantly prevents fading of colors, moderates light penetration into the room and reduces overall radiation. However, absorbing the radiation in the glass will raise its temperature, and its heat will be transferred by convection to the air of the building's interior.
This type of glass reduces both the light and the radiation penetrating it, but a relatively large amount of radiation is absorbed by the glass causing the temperature of the glass surface to rise. In this way the glass itself becomes a source of heat emitted to the room. Dark glass could be a solution for buildings which require maintaining a certain filtration of penetrating daylight while also retaining large openings.
Reflective Glass (mirror)
This material will most significantly reduce penetration of radiation from the reflecting side to the non-reflecting side (penetration of 11-37% of total striking radiation). Such glazing could be used in cases where it is desirable to maintain eye contact with the outside as well as to prevent penetration of radiation (for example, when there is a view on the west side of a building and a western window is not desirable) or in areas where it is hot most days of the year. However, it should be kept in mind that the high level of reflected light could become a nuisance to nearby buildings and to people in adjacent open areas. The reflected radiation could cause a rise in the temperature of areas it strikes (walls of nearby buildings, paved areas) and also glare. Orienting reflective glass towards a road could become a safety hazard.
This material permits penetration of light (about 85%), heats up less than glass and withstands mechanical blows. The material is flexible and it is possible to utilize it for curved surfaces. Available in clear, dark or white.
Double polycarbonate with air space
There are a number of such industrial products. Extruded sheets of polycarbonate are formed as double skin sheets with spacers between them. The various products are less transparent than a single sheet, but the air space increases their thermal resistance. In another configuration of this sheet the internal surface of one of the skins has a saw- tooth section that causes internal reflections inside the material, which results in the ability of the whole sheet to selectively reflect or transmit solar radiation, depending on the incidence angle of the radiation on the sheet.
Corrugated fibre glass
Available in different hues and thickness. In any case, it does not permit eye contact with the outside and its physical deterioration is quite fast. This material is available in clear, dark or white varieties.
They have similar properties to the other plastic materials. Available in clear, dark or white. In most cases acrylic sheets are less fragile than glass but deteriorate quickly because of ultraviolet radiation. In desert areas the deterioration process is accelerated because of abrasion caused by sand and dust.
Recommendation The decision to use double glazing should be based on climatic and economic conditions-the use of doub1e glazing is not justified in all locations. When eye contact with the outside is not necessary, it is recommended that double polycarbonate sheet be used as they are more durable than other plastic materials and provide relatively good insulation. It is possible to use reflective glass on an eastern, western or even northern facade if radiation gains from these orientations are not desired, as long as the reflected radiation does not create a nuisance. The use of dark glazing is not recommended and it is desirable to limit the use of plastic materials to upper openings where deterioration will not become an aesthetic disturbance.