No-one would consider single glazed PVCu replacement windows and doors these days, in fact it would be in breach of Building Regulations to install them in residential properties now.
The term double glazing originates from the introduction of insulating glass sealed units formed from two sheets of glass, separated by a spacer bar around the edge. The glass unit is then sealed to prevent air from getting in from outside as this would bring in water vapour which can give rise to condensation between the two sheets of glass.
The width of sealed units in uPVC windows
Generally speaking, the wider the gap between the two sheets of glass, the better the insulating properties of the sealed unit are (though exceptionally wide sealed units can encourage convection within the unit, which reduces its effectiveness).
Common consensus within the replacement window industry is that a 28 mm sealed unit (normally consisting of two sheets of 4 mm glass and a 20 mm spacer bar) is the most efficient combination. Sealed units which are narrower than 28 mm are only normally used by installers to save money, or because narrow PVCu or aluminium frames won’t take a 28 mm unit.
Advancements in window technology, particularly in Scandinavia, have involved triple glazing but the benefits for use given the climate in the United Kingdom often do not justify the extra cost.
Another issue with triple glazing is that the extra weight of the glass in each opening sash means that the overall size of sashes needs to be reduced, which could compromise the use of an opening window as an effective fire escape.
Warm edge spacer bars in sealed units
The spacer bar between the two sheets of glass is traditionally extruded aluminium with a silicone desiccant inside (to absorb any water vapour which happens to be in the air inside the factory at the time of manufacture).
Aluminium though, is a good conductor of heat – and cold – and is therefore inefficient at preventing heat loss around the edge of the sealed unit.
A warm edge, or thermal break spacer reduces the heat lost around the perimeter of a sealed unit and some types can provide almost 1,000 times more thermal insulation than aluminium spacer bars, thus improving energy efficiency. With an aluminium spacer bar, the edge of the inner pane of glass within a sealed unit can be quite cold, which can result in condensation around its perimeter; this is vastly reduced by using a warm edge spacer bar.
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