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Completely obsolete, single glazing now only exists in old buildings and must be replaced as soon as possible by high insulation glazing.
Still used, particularly by turnkey developers, basic double glazing (U < 2.5 to 2.9W/m2K) is technically outdated and only remains on the market to cater to those looking for a low-cost solution.
The norm is currently to use so-called 'high insulation' glazing, the U value of which varies from 1.1 to 1.4W/m2K. It is reasonably priced and can be expected to generate a heat saving of 50% to 60% compared with basic double glazing.
There are also specially manufactured varieties of double and triple glazing which offer even greater insulation, with a U value of as little as 0.56W/m2K. Fitting these special varieties of glazing in a composite frame gives windows an insulation value comparable to that of an insulated external wall.
Formerly called the "K value", the heat transfer coefficient ("U value") indicates the amount of heat that passes from the internal air to the external air through one square metre of a given material, when the temperature difference between the two sides is 1°C. The lower the U value, the greater the insulating capacity of the material.