Noise and thermal insulation
The energy saving performance of a window is measured by the Window Energy Rating scale. The most energy efficient windows will achieve an A rating on the WER scale but this depends on the combination of the window frame used and all the components within the double glazing window. Improving the Window Energy Rating is dependant on minimising heat lost from the room and maximising use of solar heat energy.
To reduce heat loss, double glazing window components need to be made of materials that are low thermal conductors. Double glazing window with the highest performing energy ratings use a combination of Low E glass, warm edge spacer bars and an argon gas-filled cavity, all within a hermetically sealed double glazed unit.
Low E glass
Glass designed for improving thermal insulation is known as Low E, or low-emissivity glass. Low E glass has a transparent metallic oxide coating which acts almost like a one-way thermal gateway. The coating works by selectively reflecting long wave radiation, characteristic of internal heating sources.
Solar heat gain, which is short-wave energy, can pass through into the room but indoor heating cannot escape to the outside as it is long-wave energy. The coating effectively reflects the room’s radiated heat back in, whilst allowing heat and light from the sun to enter through.
The most efficient double glazed units with offline coated Low E glass can now provide up to three times more insulation than standard double glazing window.
A spacer bar is the inner frame which separates the two glass panes in a double glazing window. Spacer bars are typically made of aluminium, but as a metal this means that heat is conducted and lost through its contact between the two glass panels. The colder outside temperature is transmitted across to the inner pane and room heat is transferred from the inner pane to the outside facing pane. The aluminium spacer bar effectively acts as a thermal bridge.
The thermal bridge effect can be overcome by using a ‘warm edge’ spacer bar made of a low thermally conductive material, such as a glass fibre composite. By using a warm edge spacer bar the energy efficiency of a window can be improved by up to 10%.
The double glazing window’s cavity
The cavity of a double glazing window plays a key role in energy efficiency. As it is not filled with ‘regular’ atmosphere air, it needs to be hermetically sealed, using special primary and secondary leak-proof seals. The unit is filled with either dehydrated air or an inert gas under vacuum conditions.
Heat is transmitted through adjacent materials (whether they be gases, liquids or solids) by convection, conduction or radiation. The heat is effectively lost from a warmer surface, such as a room-facing pane, to a cooler surface, such as an outdoor facing pane. By filling the cavity of a double glazing window with dehydrated air rather than normal air, heat exchange between the two glass panes is greatly reduced. Molecules of dehydrated air are much less mobile than normal air. This means that they transfer much less heat by convection or conduction and therefore have a better thermal insulation.
Inert gases, such as argon, xenon or krypton have an even lower thermal conductivity than dehydrated air. Energy efficiency can be further enhanced by using an inert gas to fill the cavity of the double glazing window.
Contact Diamond Glass for more information on upgrading your windows.
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