Low E glass is a form of glass which when added to double glazing allows you to save even more energy.

Typically double glazing using Low-E glass has energy conservation properties as good as normal triple glazing but without the 50% increase in weight.

The advantages of using Low-E glass are:

Better heat insulation
You save money with reduced heating bills
Reduced carbon dioxide emissions into the atmosphere
Reduces condensation on your window frames internally

Installing argon gas within the sealed units instead of air can have even greater energy savings. Argon is an inert gas, which has better thermal properties than dry air. In the UK we express the rate of heat loss in 'U values'. The lower the U value, the greater the thermal insulation and savings on your fuel bill.

Typical U values:
The U value of single clear glass is 5.4
With ordinary double glazing this is improved to 2.6
With Low-E glass the U value is reduced to 1.8
If argon gas is used to fill the air gap, the U value will reduce to 1.6e used 'K' low emmissivity glass, at the time this was just on the
market and is standard now.... triple glazed units used on north facing
elevations