Single, Double or Triple? Easy as 1,2,3!

Let’s take some of the jargon out of single, double and triple glazing performance.

There are three main criteria that affect the above:

Firstly, the number of panes of glass
Secondly the distance of air / argon gas filled gap between them,
And lastly the number and type of low E coatings on this glass.

A low E coating means ‘low emissivity’ which is basically a coating applied to the glass that reflects radiant heat back into the room. There are different types and manufactures but we shall refer to it as soft coat from here.

The distance or cavity between panes is important as heat loss will decrease as the cavity increases, peaking at 16 – 20mm.
After this it slightly decreases as air starts to circulate within the cavity.
By filling this cavity with argon gas it also reduces heat loss.

When we talk of heat loss, it is normally quoted as U value.
Please remember the U value quoted for glass can be quite different than the overall U value for a window, but the lower the U value the lower the heat loss.
We will look at some basic calculations later.

U value is a measure of energy lost per square meter of glass, per degree difference in temperature from inside and out. K=kelvin or Celsius for simplicity.
This is expressed as: U=w/m2/k
Here are some values for different glazing. (SC = soft coat)

4mm clear glass single glazed – U=5.8

4mm clear/16 air/4mm clear double glazed – U=2.8

4mm clear/16 cavity argon filled/4mm SC Double glazed – U=1.16

4mm clear/16mm Argon/4mm SC/16mm Argon/4mmSC Triple glazed – U=0.6

As can be seen from the above, there is a huge difference between single glazing and triple glazing with high performance glass – 90% reduction in heat loss.

A point to note from above is that these values are based on 16mm cavities. If the cavity should be reduced in triple glazing down to say 2 x 6mm then the performance in U value terms is worse than double glazing even if two panes of soft coat are used.

See the graph below for further detail:

123-graphic