How thermally efficient double glazed sealed units work

Double glazing is essentially just that, two panes of glass separated by a gap, that nowadays is usually between 16mm and 20mm wide. Most modern aluminium double glazed sealed units are 24mm wide and PVCu double glazed sealed units are 28mm wide. These are then fitted into aluminium or PVCu frames, and are held in place by high security glazing beads. Thermally efficient double glazed units are hermetically sealed (i.e. air tight), creating a barrier between the interior and exterior of a property.

The most energy-efficient glass for double glazing is low emissivity (Low-E) glass. This often has an unnoticeable coating of metal oxide, normally on one of the internal panes next to the gap. This lets in light and heat but cuts the amount of heat that can get out.Very efficient windows might use gases such as argon, xenon or krypton in the gap between the sheets of either float, toughened or laminated glass. Spacer bars are set around the inside edges of each sealed unit to keep the two panes of glass apart. For maximum efficiency, ensure you specify pane spacers containing little or no metal — often known as “warm edge” spacers.

What makes one particular sealed unit better than another is the quality and different performance levels of the different material components available in the UK marketplace. The thermal efficiency of the combination of float, toughened or laminated (clear or obscure) glass with a spacer bar and hermetically sealed gas sealed unit can be tested and given a centre-pane U-value. The lower this value, the higher the performance of that particular combination of double glazing.

As an example, Hazlemere Window Company has two standard glass specifications (one Pilkington Glass, one Saint Gobain glass), both of which have centre-pane U-values of 1.2 W/m2K. Both the Pilkington K glass sealed unit and Saint Gobain Planitherm Total + sealed units both have a “softcoat” on the inside of the interior sealed unit, that helps stop heating escaping from inside the home, and an exceptionally clear (non green) outer pane of glass that increases the amount of free heat from the sun that can get into a room, whilst also letting in more light, making the home feel warmer and brighter.

Independent double glazing manufacturers and installers Hazlemere Windows are licensed Pilkington energiKare partners, and also Energy Saving Trust Licensed retailers, as they offer all their replacement window customers high performance double glazed sealed units as standard, thereby helping all their replacement window customers save energy, whilst also reducing their heating bills.


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