Types of glass used in window glazing
Glass is the peculiar component of a window, because it allows us to look outside and it lets light in. But how many kinds of glass are available?
90% of the glass is produced with the float glass process, where the pane is formed by floating on molten tin bath, obtaining a perfectly regular pane. Standard thickness is 2, 3, 4, 5, 6, 8, 10, 12, 15, 19, 22 and 25 mm. The most important drawback is that when broken, it shatters in big splinters as sharp as razors, making it dangerous for architectural usage.
In order to compensate for this problem, two different processes are available. The first is tempering, obtained by heating and then quickly cooling the glass. The pane is about six times more solid and has internal tensions that cause it to shatter in many small rounded chips. Tempered glass is used, for instance, in glass windows, in glass façades, in glass doors and in rails.
The second one is laminated glass: two panes are joined together with a PVB plastic film, that prevents the creation of sharp pieces and holds chips in place, with no shattering. Car windshields are made of this glass, also used in safety doors and windows. Laminated glass is able to withstand well collisions and breaking attempts. PVB film helps to increase acoustic insulation of the pane.
In order to improve thermal characteristics of glass, it can be treated by coating the surface with special metal oxides that lower thermal transmittance without substantially altering light transmittance. This glass, called low-emitting glass, can appear slightly tinted with a few points of discontinuity, especially on very wide panes.
Solar control glass reduces incoming heat and is useful in hot climates or in all these situations where an excessive amount of infrared rays can cause problems, as in verandas or in glass façades.
So-called selective glass is a kind of low-emitting glass that filters sunrays lowering heat transfer by radiation. They are often joined in a double glazing to achieve a good thermal insulation.
Self-cleaning glass is coated with a very thin layer of titanium dioxide. This substance, when exposed to ultraviolet rays, decomposes organic particles; moreover, it makes the pane hydrophilic: water forms a film on the surface that helps dirt to come off. This kind of glass has to be wet often to avoid that organic matter permanently adheres to the surface.
All kinds of panes here listed are usually combined in a double or triple glazing. They are kept at a distance by a spacer. This spacer contains salts that remove humidity from the gap. It is usually made of aluminium, but can be made of insulating material in order to lower dissipation, or made of steel when extra strength is required.
Air within the gap can be replaced by a noble gas (argon, krypton, xenon) to lower thermal transmittance. Typical glazing combinations are 4/20/4low-emitting (with float glass), 3+3/15/3+3low-emitting or 4+4/15/4+4low-emitting. Thick panes increase acoustic insulation and solidity, but weight goes up as well. An air gap between 15 and 23 mm guarantees the best insulation. If lowered, heat loss for direct transfer increases; if augmented, convective motion builds up in the gap thus causing heat transfer.