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Is bigger always better - triple better than double?

John Warburton, Windows and Doors Sector Manager, examines the question of whether triple glazing is really better than double.

For years the domestic window market in the UK has developed on the premise that bigger is better.  Double glazing grew from an overall thickness of 20mm to 24mm and finally settled on 28mm even though thermally, with either air or argon cavities, 24mm is the optimum size.  Frame depths grew from nominally 60mm to 70mm for no great technical improvement, just the misconception is that bigger is better.

A newer question is - is 3 better than 2?  Or is triple glazing better than double glazing?  The simple answer is there is no simple answer.  The devil, as always, is in the detail.

To answer the question correctly, we must consider the individual specification of any proposed IGU (Insulated Glass Unit).  The current industry standard double glazed unit is made up of one 4mm pane of “low E” (Low emissivity) coated glass*, one pane of 4mm plain annealed glass and a 20mm cavity of argon gas (make up = 4-20-4).  This will provide a centre pane u-value of 1.1 W/m²K. 

If we introduce a centre pane of plain glass, but keep the overall thickness of the unit at 28mm (ie 4-8-4-8-4), we arrive at a u-value of 1.2 W/m²K – a worse performance than the double glazed unit.  However, if we increase the overall thickness of the triple glazed unit by increasing the cavities between the panes, we begin to improve the unit’s performance.  The optimum thickness is 44mm (4-16-4-16-4), which will give a u-value of 0.9 W/m²K.  We could improve this further by introducing a second “Low E” pane whereby the unit u-value becomes 0.6 W/m²K.

At face value, an improvement in terms of heat loss through glazing from 1.1 W/m²K to 0.6 W/m²K sounds like an improvement worth paying for.  The Window Energy Rating Scheme, widely accepted as the industry standard for judging thermal performance and written into Building Regulations, calculates solar gain as well as heat loss.  This means that the triple glazed unit’s improved heat loss credentials are largely negated by the reduction of warmth created by sunshine.  The solution would be to increase solar gain by using Low Iron glass, which is much more translucent than the traditional variety but can be considerably more expensive.

There is another factor that purveyors of triple glazing must consider.  This issue was discussed by BRE’s John Beasley in an earlier BRE experts article and relates to cracking in units due to thermal stress.  As the centre pane of a triple glazed unit has no air movement around it, because it is encapsulated, it can reach much higher temperatures than would be expected for double glazing.  The solution is, as John pointed out, to use a toughened pane in the centre of the unit, which will inevitably increase cost again.

So the answer to the original question is it is possible to improve thermal performance of IGU by moving to triple glazing. But a new question arises - at what cost?  With energy costs rising by double digit percentages year on year, it is a calculation that should be seriously considered.

*The “low e” coating used in the u-value calculations was Saint Gobain’s Planitherm Total +.