09.04 Gas filled glazing
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Introduction
Multiple glazing units were originally all air-filled, with ordinary glass. However, it was soon realised that heat transfer through these units could be reduced if a better gas could be found (one which suppresses convection better and has a lower thermal conductivity) and the glass surfaces could be coated to give them a lower emissivity.
In a gas-filled unit the air is purged from the unit during manufacture and replaced with a gas such as argon, krypton or sulphur hexafluoride (SF6). These are generally selected to have a greater resistance to heat transfer than air, although SF6 is used because it has better resistance to acoustic transmission. The gas-fill may be used in conjunction with any of the glass options described in Section 09.02 and Section 09.03, to give even lower heat transfer, and the combination of gas-fill with a low-e coating is more usual than just a gas-fill on its own.
Low-emissivity coatings are available as hard coatings, which have a comparatively high emissivity, up to 0.2, or as soft coatings, which have lower emissivities, down to 0.04 - ordinary un-coated glass surfaces have a normal emissivity of about 0.88.
It should be noted that a coating is rarely applied to both panes, as the additional gain from a second coating is rarely worth the additional cost - the use of one soft low-e coating gives a lower U-value than a single hard coating.
Some typical values of double glazing units with various gas-fills are:
Plain glass surfaces
| Fill gas | Density | Thermal conductivity | U-value (W/m2K) | ||
| (kg/m3) | (W/mK) | 4-6-4 mm | 4-12-4 mm | 4-16-4 mm | |
| Air | |||||
| Argon | |||||
| Krypton | |||||
| SF6 | |||||
Single hard low-e coated surface
| Fill gas | Density | Thermal conductivity | U-value (W/m2K) | ||
| (kg/m3) | (W/mK) | 4-6-4 mm | 4-12-4 mm | 4-16-4 mm | |
| Air | |||||
| Argon | |||||
| Krypton | |||||
| SF6 | |||||
Soft low-e coated surface
| Fill gas | Density | Thermal conductivity | U-value (W/m2K) | ||
| (kg/m3) | (W/mK) | 4-6-4 mm | 4-12-4 mm | 4-16-4 mm | |
| Air | |||||
| Argon | |||||
| Krypton | |||||
| SF6 | |||||
These values have been calculated in accordance with BS 6993:Part 1 [1989], and an iterative procedure used to determine the correct heat flow through the gas-space based on environmental temperatures of 20oC and 0oC. The effective emissivity of a plain glass surface is taken as 0.845, that of a hard low-e coating as 0.2, and that of a soft low-e coating as 0.05.
There are two key issues with gas-filled units:
- was the correct amount of gas put into the unit in the first place?
- and:
- will the gas stay in the unit over a period of time?
Quality
Gas-filling is subject to the quality control systems used by the sealed unit manufacturer. In one method of manufacture the sealed unit is assembled and gas is then injected into one opening in the perimeter of the unit whilst air is vented off at another - the process is stopped when the concentration of the fill-gas at the exhaust vent reaches some pre-defined level. However, it is possible, if the injection opening and the exhaust opening are opposite one-another, for gas to pass directly from the injector to the exhaust, or for significant mixing of the fill gas with the air within the unit. An alternative approach to filling is to assemble the unit and then withdraw the air in a vacuum chamber - the fill gas is then introduced to the sealed unit as air is introduced to the chamber outside the unit (thereby equalising the pressure difference across each pane of glass and preventing the unit from imploding or exploding). A third method of manufacture is to assemble the unit using robots in an atmosphere saturated with the fill gas.
Gas filling must be controlled carefully if the units are to be properly filled. There is also a need for reliable non-invasive techniques which can be used to assess the amount and type of the fill-gas in the unit. Customer confidence in gas-filled units will quickly disappear if there are cases of units being supplied without the appropriate fill-gas; at the moment it is only possible to identify the gas with destructive testing techniques, although surface temperature measurements may give some indication of incorrect filling - the customer presently has no assurance that the specified gas-fill is actually provided, other than the word of the supplier.
Durability
The concentration of gas in a unit may also decrease over a period of time, either due to diffusion through the edge seal, or due to a failure of the edge seal. The latter condition will usually lead to interstitial condensation, but the former will only lead to a gradual increase in the U-value and may go undetected for some time.
Double glazing is now reasonably common in the UK and there is a clear set of recognised problems. Principally if the glazing unit is hermetically sealed then there is a need to ensure that the unit is installed in a way that does not impair the durability of the hermetic edge-seal. It is already recognised that wet-glazing can be a problem if the glazing sealant fully encapsulates the edge of the glazing unit - if the glazing sealant comes away from the glazing it may allow water to penetrate by capillary action and stay in close contact with the glazing edge seal, leading to degradation and eventual failure of the edge seal. The probability of failure is however related to the initial quality of the edge seal, and proper examination of the glazing unit edge during manufacture should help to reduce the incidence of this type of failure. The effect of service temperature on the glazing edge seal is also important, as is possible degradation of the polymer sealant materials by ultraviolet.