Insulation Glass (IG), more commonly known as double glazing (or double-pane, and increasingly triple glazing/pane) are double or triple glass window panes separated by an air or other gas filled space to reduce heat transfer across a part of the building envelope.
Insulation Glass Units are manufactured with glass in range of thickness from 3 mm to 10 mm (1/8″ to 3/8″) or more in special applications. Laminated or tempered glass may also be used as part of the construction. Most units are manufactured with the same thickness of glass used on both panes but special applications such as acoustic attenuation or security may require wide ranges of thicknesses to be incorporated in the same unit.
The glass panes are separated by a “spacer”. A spacer is the piece that separates the two panes of glass in an insulation glass system, and seals the gas space between them. Historically, spacers were made primarily of metal and fiber, which manufacturers thought provided more durability.
However, metal spacers conduct heat (unless the metal is thermally improved), undermining the ability of the IGU to reduce heat flow. It may also result in water or ice forming at the bottom of the sealed unit because of the sharp temperature difference between the window and surrounding air. To reduce heat transfer through the spacer and increase overall thermal performance, manufacturers may make the spacer out of a less-conductive material such as structural foam. A spacer made of aluminum that also contains a highly structural thermal barrier reduces condensation on the glass surface and improves insulation, as measured by the overall U-factor (see Thermal conductivity).
• A spacer that reduces heat flow in glazing configurations may also have characteristics for sound dampening where external noise is an issue.
• Typically, spacers are filled with or contain desiccant to remove moisture trapped in the gas space during manufacturing, thereby lowering the dew point of the gas in that space, and preventing condensation from forming on surface when the outside glass pane temperature falls.
• New technology has emerged to combat the heat loss from traditional spacer bars, including improvements to the structural performance and long-term-durability of improved metal (aluminum with a thermal barrier) and foam spacers.
IGUs are often manufactured on a made to order basis on factory production lines, but standard units are also available. The width and height dimensions, the thickness of the glass panes and the type of glass for each pane as well as the overall thickness of the unit must be supplied to the manufacturer. On the assembly line, spacers of specific thicknesses are cut and assembled into the required overall width and height dimensions and filled with desiccant. On a parallel line, glass panes are cut to size and washed to be optically clear.
An adhesive sealant (polyisobutylene – PIB) is applied to the face of the spacer on each side and the panes pressed against the spacer. If the unit is gas filled, two holes are drilled into the spacer of the assembled unit, lines are attached to draw out the air out of the space and replacing it with the desired gas. The lines are then removed and holes sealed to contain the gas. The more modern technique is to use an online gas filler, which eliminates the need to drill holes in the spacer. The units are then sealed on the edge side using either polysulfide or silicone sealant or similar material to prevent humid outside air from entering the unit. The desiccant will remove traces of humidity from the air space so that no water appears on the inside faces (no condensation) of the glass panes facing the air space during cold weather. Some manufacturers have developed specific processes that combine the spacer and desiccant into a single step application system.
The double glazed window was invented in 1930s, and was commonly available in US in the 1950s under the ThermopaneTM brand name, registered in 1941 by Libbey-Owens-Ford Glass Company. After so many decades, the manufacturing process is well established, though innovation has continued to improve the R factor and other characteristics of windows. The brand name Thermopane has entered the vocabulary of the glazing industry as the genericized trademark for any IGU.
Materials which can be used for double glazing include aluminum, PVC, and wood (timber).
The maximum insulating efficiency of a standard IGU is determined by the thickness of the space. Typically, most sealed units achieve maximum insulating values using a space of 16–19 mm (0.63–0.75 in) when measured at the centre of the IGU.
IGU thickness is a compromise between maximizing insulating value and the ability of the framing system used to carry the unit. Some residential and most commercial glazing systems can accommodate the ideal thickness of a double paned unit. Issues arise with the use of triple glazing to further reduce heat loss in an IGU. The combination of thickness and weight results in units that are too unwieldy for most residential or commercial glazing systems, particularly if these panes are contained in moving frames or sashes.
This trade-off does not apply to Vacuum Insulated Glass (VIG), or evacuated glazing, as heat loss due to convection is eliminated, leaving radiation losses and conduction through the edge seal. These VIG units have most of the air removed from the space between the panes, leaving a nearly-complete vacuum. VIG units which are currently on the market are hermetically sealed along their perimeter with solder glass, that is, a glass frit having a reduced melting point. Such a glass seal is rigid, and will experience increasing stress with increasing temperature differential across the unit. This stress may prevent Vacuum glazing from being used when the temperature differential is too great. One manufacturer provides a recommendation of 35 °C.
Vacuum technology is also used in some non-transparent insulation products called vacuum insulated panels.
An older-established way to improve insulation performance is to replace air in the space with a lower thermal conductivity gas. Gas convective heat transfer is a function of viscosity and specific heat. Monatomic gases such as argon, krypton and xenon are often used since (at normal temperatures) they do not carry heat in rotational modes, resulting in a lower heat capacity than poly-atomic gases. Argon has a thermal conductivity 67% that of air, krypton has about half the conductivity of argon. Krypton and Xenon are very expensive. These gases are used because they are non-toxic, clear, odorless, chemically inert, and commercially available because of their widespread application in industry. Some manufacturers also offer sulfur hexafluoride as an insulating gas, especially to insulate sound. It has only 2/3 the conductivity of argon, but it is stable, inexpensive and dense. However, sulfur hexafluoride is an extremely potent greenhouse gas that contributes to global warming. In Europe, SF6 falls under the F-Gas directive which ban or control its usage for several applications. Since 1 January 2006, SF6 is banned as a tracer gas and in all applications except high-voltage switchgear.
In general, the more effective a fill gas is at its optimum thickness, the thinner the optimum thickness is. For example, the optimum thickness for krypton is lower than for argon, and lower for argon than for air. However, since it is difficult to determine whether the gas in an IGU has become mixed with air at time of manufacture (or becomes mixed with air once installed), many designers prefer to use thicker gaps than would be optimum for the fill gas if it were pure. Argon is commonly used in insulated glazing as it is the most affordable. Krypton, which is considerably more expensive, is not generally used except to produce very thin double glazing units or relatively thin, or extremely high performance triple glazed units. Xenon has found very little application in IGUs because of cost.
Heat insulating properties
The effectiveness of insulated glass can be expressed as an R-value. The higher the R-value, the greater is its resistance to heat transfer. A standard IGU consisting of clear uncoated panes of glass (or lites) with air in the cavity between the lites typically has an R-value of 0.35 K•m2/W.
Using US customary units, a rule of thumb in standard IGU construction is that each change in the component of the IGU results in an increase of 1 R-value to the efficiency of the unit. Adding Argon gas increases the efficiency to about R-3. Using low emissivity glass on surface #2 will add another R-value. Properly designed triple glazed IGUs with low emissivity coatings on surfaces #2 and #4 and filled with argon gas in the cavities result in IG units with R-values as high as R-5. Certain vacuum insulated glass units (VIG) or multi-chambered IG units using coated plastic films result in R-values as high as R-12.5
Additional layers of glazing provide the opportunity for improved insulation. While the standard double glazing is most widely used, triple glazing is not uncommon, and quadruple glazing is produced for very cold environments such as Alaska. Even quintuple glazing (four cavities, five panes) is available – with mid-pane insulation factors equivalent to walls.
Acoustic insulating properties
In some situations the insulation is in reference to noise mitigation. In these circumstances a large air space improves the noise insulation quality or Sound transmission class. Asymmetric double glazing, using different thicknesses of glass rather than the conventional symmetrical systems (equal glass thicknesses used for both lites) will improve the acoustic attenuation properties of the IGU. If standard air spaces are used, sulfur hexafluoride is used to replace or augment an inert gas and improve acoustical attenuation performance.
Other glazing material variations affect acoustics. The most widely used glazing configurations for sound dampening include laminated glass with varied thickness of the interlayer and thickness of the glass. Including a structural, thermally improved aluminum thermal barrier air spacer in the insulating glass can improve acoustical performance by reducing the transmission of exterior noise sources in the fenestration system.
Reviewing the glazing system components, including the air space material used in the insulating glass, can ensure overall sound transmission improvement.