Clear float glass has previously been the major material used for windows in houses. Advances in glazing technology mean that special glass is now available to control heat loss through the window.This low emissivity (low-e) glass has special surface coatings to reflect heat back through the window. The low-e coatings reflect between 40% to 70% of the heat that is normally transmitted through clear glass, while allowing the full amount of light to pass through.This type of glass is now standard under the Building Regulations for most homes in the UK.
The traditional approach to improving the energy efficiency of a window has been to increase the number of layers of glass and air. Double- or triple-glazed windows have insulating air- or gas-filled spaces between each pane. Each layer of glass and the air spaces resists heat flow.The width of the air spaces between the panes is important, air spaces that are too wide have higher U-values and allow too much heat transfer. Highly energy efficient windows are manufactured with inert gases (argon or krypton) in the spaces between the panes because these gases transfer less heat than air.
Window frames are available in a variety of materials and the frame material can be selected for price, maintenance or visual appearance. Any frame material can produce energy efficient windows if designed correctly. For consumers, it is important to get the most energy efficient window from the frame material that is chosen.
New technologies are being used to develop and improve the energy efficiency of windows.Typical of these is the concept of ‘warm edge’ glazing to reduce heat losses at the edges of the glazing unit.
The following factors affect the whole window U-value:
Note: Always ensure that a quoted U-value is for the whole window and not simply for the glass.
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