Double Glazing versus Triple Glazing

Is one really better than the other?

  • Double glazing is better than single glazing: FACT
  • Therefore triple glazing must be better than double glazing: FACT…..(well not always)

The key to the differences in performance between double and triple glazing is the selection of glass types, cavity widths, gas filling and overall unit thickness.

Most double glazing in the UK consists of : 4mm clear glass / 20mm cavity / 4mm low e glass

This makes the overall unit thickness 28mm and this is pretty much standard for all UK upvc window manufacturers and tends to mean that most manufacturers will put a triple glazed unit into this 28mm space. Depending how the glazing is made up, the window could actually have worse energy saving properties with a TGU than with the DGU it replaces:

Double Glazed Units

Glass Make Up U-Value Thickness
4mm clear glass/20mm Argon/4mm Planitherm Total+ 1.2 W/m2.K 28mm
4mm clear glass/16mm Ar/4mm Planitherm One 1.0W/m2.K  24mm

Triple Glazed Units

High performance triple glazing requires 2 panes of low e glass, made up as: 4mm clear/4mm low e/4mm low e. It is recommended by the manufacturers that this middle low e pane is toughened to prevent Thermal Stress Cracking.

Glass Make Up U-Value Thickness
4mm clear glass/8mm + Ar/4mm clear glass/8mm + Ar/4mm Plan T+ 1.3 W/m2.K 28mm
4mm clear glass/8mm + Ar/4mm Plan T+/8mm + Ar/4mm Plan T+ 1.0 W/m2.K 28mm
4mm clear glass/8mm + Ar/4mm Plan 1/8mm + Ar/4mm Plan 1 0.9 W/m2.K 28mm
4mm clear glass/10mm + Ar/4mm Plan T+/10mm + Ar/4mm Plan T+ 0.9 W/m2.K  32mm
4mm clear glass/10mm + Ar/4mm Plan 1/10mm + Ar/4mm Plan 1 0.8 W/m2.K 32mm
4mm clear glass/12mm + Ar/4mm Plan T+/12mm + Ar/4mm Plan T+ 0.8 W/m2.K 36mm
4mm clear glass/12mm + Ar/4mm Plan 1/12mm + Ar / 4mm Plan 1 0.7 W/m2.K 36mm

Ar = Argon gas @90% concentration

Looking at these examples, it can be seen that:

  • A TGU can be worse than a DGU
  •  TGUs can be no better than a DGU
  • A 24mm unit can be better than a 28mm unit in both TGU and DGU
  • Wider gas filled cavities giving wider over all unit thickness can achieve significant improvements in lowering U-values
  • Going from a standard 28mm 1.2 W/m2.K DGU to a 36mm TGU with Planitherm One, almost achieves a 50% improvement in performance

The cost of triple glazing dictates that to achieve overall energy savings, correct triple glazing needs to be specified and wider cavities are required.

A major aspect of triple glazing is that the weight of the glass is increased by 50%. Customers need to be assured that the hinges, fixings and other window/door hardware can cope with this increase. Additionally, the centre of gravity of the glazing has moved and this may put additional stress on the hinges and friction stays.

The window may also look less aesthetically pleasing as the bead that holds the glass in place will be narrower and may affect the overall appearance.