The double glazed windows they are fixtures that have not one but two layers of glass usually separated by a layer of air or other gases. The main advantage of this technology is that the layer between the two glasses is a thermal insulator which hinders the transmission of heat from inside to outside of the room in the winter months and, vice versa, from outside to inside in the summer months. This simple physical principle, combined with innovations in the field of materials have perfected the functioning of thisinvention which, later 160 yearsstill allows us to save today energy and isolate ourselves from temperatures and noises of the outside world.
The invention of double glazing and how it works
The first patent record of a double-glazed window was filed in 1865 in the USAby a certain person TD Stetson. The idea resumed, in a single frame and using little space, a concept already used for centuries in many buildings: the use of double windows, one internal and one separate flush with the facade, capable of creating a cavity of similar width to that of the wall.
The greatest commercial success came with the birth of Thermopane® of the Libbey-Owens-Ford Glass Company in 1944: a patented technology in which the double glass was sealed with a glass edge, during production.
Glass is a material that lets heat pass easily, to the point that thermal resistance total of a single glass window, i.e. the difficulty of the heat passing through the frame is mainly due to the two thin layers of air (called “boundary layers“) which are in direct contact with the surface. For this reason, special films or paints are sometimes applied to the glass, which can increase its resistance.
By adding a “air pocket” inside the double glass, we trap more air inside two glassy surfaces and increase the thermal resistance of the window. The air is indeed less conductive than glassi.e. it transmits less heat. This way the heat must pass through a third layer of gas with a path (a width) much greater than the external “boundary layers”. Furthermore, unlike the latter, the internal layer it is not influenced by external factors such as wind or rain, which reduce the thickness of the external layers.
In this way, yes reduces not only that heat loss towards the outside in winter, but also the gain of further heat from the outside towards the inside in the summer months: in short, double glazing retains heat in the home in winter and they don’t let it in from the outside in the summer.
In colder climates it is not uncommon to find triple or quadruple glazing, which they therefore exploit two or three cavities of air further reducing the passage of heat.
Filler gases or vacuum: what is between the two layers of glass
Although air is much less conductive than glass, different gases have better properties: over the decades the search for gases to be used in the cavity has concentrated on gaseous elements such asargon (Ar), krypton (Kr), or sulfur hexafluoride (SF6).
Often the choice of filler gas is a compromise between high thermal resistivity and factors such as resistance to UV rays and a low chemical reactivity, or more simply, the costs: despite the krypton has a lower conductivity (and therefore in theory would be optimal for preventing heat loss), in industry argon is preferred because its production (extraction from the atmosphere) is definitely cheaper.
Sulfur hexafluoride, with excellent characteristics, was progressively abandoned because it is the most potent of greenhouse gases: a ton of SF6 has the same warming effect as 23900 tons of CO2which is why its use is limited by European laws to very few industrial applications.
A further alternative is to create the empty between the two glasses: in this case you reach a low conductivity even with gaps of a few millimetres.
The use of fillers or vacuum is certainly also favored by materials used to seal the two panes: over the decades we have gone from “welded” glass edges, which ensured excellent sealing but with expensive and limited overall dimensions, to metal gaskets and organic materials, designed for minimize losses of the internal content and the passage of humidity from the outside.
The acoustic insulation of double glazing
Double glazing also helps reduce noise pollution in our homes: the sound propagates in the air like a longitudinal wave which “compresses” the gasalso causing the window panes to vibrate.
The best results are achieved with laminated glasswhich have an additional layer of organic polymers such as polyvinyl butyral (PVB): in addition to guaranteeing greater safety in the event of breakage, avoiding the scattering of splinters, these glasses can muffle by more than 45 decibels sounds at higher frequencies, and to a lesser extent those at lower frequencies. In addition to protecting us from the cold, therefore, double glazing can also guarantee us a quieter and more welcoming home.
