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Types of Energy Efficient Windows

woman doing yoga in front of window

The average home may lose 25% or 30% of its heat output through its windows. In fact, a single pane of glass, unprotected, may lose up to ten times more heat than an insulated wall of the same size.

It is no great surprise, therefore, that windows nowadays have an energy rating system similar to that for white goods, such as washing machines and dishwashers. The BFRC (“British Fenestration Rating Council”) operates a system whereby windows are rated “A” to “G” based on the thermal efficiency of the window as a whole. Windows rated “A” are the most energy efficient and those rated “G” the least; a rating of at least “C” is required for “Energy Saving Recommended” certification.

This system takes into account heat losses through the glass and through the frame and allows consumers to make direct comparisons between competing products.

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Types of Energy Efficient Windows

The most familiar form of energy saving window, of course, is the double glazed window. Double glazed windows consist of two individual panes of glass separated by a sealed space containing air – or, more recently, inert argon or krypton gas – and anything up to 20mm wide.

The space between the panes should be at least 10mm for optimal performance, and, if this is the case, double glazing can reduce domestic heat loss by up to 50%, in turn, reducing carbon dioxide (CO2) emissions by over 700kg, and heating bills by over £100 per annum for a typical household. Double glazing can also reduce the propensity for condensation, and reduce the amount of noise transmitted into your home, whilst allowing sunlight to be fully admitted.

Low Emissivity Windows

Some of the latest developments in energy saving technology for windows are low emissivity – “low-e”, for short – films and coatings. These are thin metal coatings applied to a plastic film suspended between the panes of glass in a window, or applied, retrospectively, to the external surfaces.

Low emissivity coatings appear almost invisible to the human eye from the inside looking out, but typically appear slightly mirrored from the outside looking in. Their most important characteristic is that they are “spectrally selective” – in other words, they allow short wave radiation from the Sun to penetrate your home, but do not allow long wave heat radiation to escape in the opposite direction – and can be up to 40% more efficient than conventional double glazed windows as a result.

Energy Saving Window Frames

Window frames conduct heat, and are therefore important in determining the overall energy efficiency of a window. uPVC window frames – especially those with extra insulation – solid, or composite, wood, and fibreglass frames are typically more energy efficient than frames made from aluminium, or other metals, which are very poor thermal insulators. Wood and vinyl are, however, subject to expansion and contraction with temperature, and it is possible to reduce the heat lost through metal frames by inserting an insulating plastic strip – effectively a thermal break – between the inside and outside of the frame.

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