Triple glazing: some ‘common misconceptions’ corrected

bfrcGary Morgan, BFRC technical director, corrects some ‘common misconceptions’ with regard to triple glazing:

,In response to the assertion that, ‘two panes of glass are good, but three must be better’, Gary Morgan said: “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 IGUs 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’ rating.

,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.

,In response to the assertion that, ‘it is only possible to achieve BFRC A+ ratings using triple glazing’, Morgan said: “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.”

,In response to the assertion that, ‘triple glazing / A+ windows can lead to overheating’, Morgan said: “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.”

,For expert advice on triple glazing contact the BFRC on 020 7403 9200 or email

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