Triple glazing has been in the news a lot this year and has been generating a lot of comment. Gary Morgan, BFRC Technical Director, adds his expert opinion.
“Two panes of glass are good, but three must be better”
There is some merit in this simplistic statement. Triple glazed windows are capable of achieving much higher levels of thermal performance than double glazed. But the choice of low-e coating and gas fill are critical. For example, consider
two triple glazed IGU’s designed to fit into a 28mm glazing cavity.
4-20-4 Softcoat low-e, argon fill: can achieve a WER ‘A’
4-8-4-8-4 One pane of softcoat low-e, argon fill: can achieve a WER ‘B’
4-8-4-8-4 Two panes of softcoat low-e, argon fill: can achieve a WER ‘A’
In order to realise any benefit from the extra pane of glass, it needs to have a low e coating or the U value actually increases, meaning a thermal efficiency decrease. Even with two panes of low-e coating, there is still no overall benefit in terms
of WER performance. The addition of an extra coated pane can, if the wrong coating is used, lead to a sharp decrease in the solar gain (g) value, which can lead to a window going from an ‘A’ to ‘B’ or even ‘C’
In triple glazing the use of Krypton gas in narrow cavities, or Argon gas in wider glazing cavities, does result in much improved window thermal performance.
“It is only possible to achieve BFRC A+ ratings using triple glazing”
About 80% of A+ windows are triple glazed - but 20% are double-glazed. So it is possible to achieve an ‘A+’ with double glazing using thermally efficient frame systems and good warm edge spacer bars.
“Triple glazing / A+ windows can lead to overheating”
This is incorrect. Is the suggestion that lower window U values result in thermal overheating? This only occurs when the external ambient temperatures are higher than the internal room temperature (a lower U value helps to keep the room cool
by keeping excess heat out). It certainly does not cause overheating.
Or is solar gain supposed to be the culprit? In triple glazed windows solar gain values are at least 10% lower than in equivalent double glazed windows.
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