What is Low E Glass?

Low Emissivity (Low E) glass is a new, high performance, double glazing that will retain your valuable heat in the winter and reduce solar gain in the summer. It essentially has very low emission of heat.


U values are used in measuring heat transfer to determine how thermally efficient a product is. The lower the U value the more efficient the product at retaining rather than transferring heat.

The calculation of U Values can be quite complex as it is measured as the amount of heat loss through a 1 square metre of material for every degree difference in temperature either side of the material.  It is indicated in units of watts per square metre per degree Kelvin or W/m2K.  Kelvin is used as the scale of temperature difference, but this is numerically equal to degrees Celsius.

Standard air filled sealed units have a U value of 2.8 which means most of your heat will transfer through the glass to the outside in the winter which of course will lead to higher fuel bills.

Comparison of U Factors (w/m2 K):-

Single glazing U Value = 5.6 w/m2K

Double glazing (with air cavity) U Value = 2.8 w/m2K

Double glazing (with argon gas cavity ) U Value = 2.6 w/m2K

Double glazing, Low E Glass (with air cavity) U Value = 1.8 w/m2K

Double glazing, Low E glass (with argon gas cavity) U Value = 1.5 w/m2K

So for example:- 1 square metre of a standard single glazed window will transmit about 5.6 watts of energy for each degree difference either side of the window, or a U Value of 5.6.  A double glazed window will be significantly better with a U Value of 2.8, i.e. only transmitting 2.8 watts of energy in similar conditions.

Pilkington Tripleshield and Planitherm typically have a U factor of 1.2 with 4mm glass, a 20mm cavity and Argon filled.

Manufactured using a soft metallic coating on the inside of the glass, Low E glass which has a neutral tint when compared to K glass which has a pronounced tint. Triple Shield and Planitherm offer a higher light transmission and a lower solar gain or in other words, helps retain your heat during the winter and helps to keep solar heat out in the summer.

The internal surface temperature of the glass is much higher than standard double glazing so furniture can be placed closer to the glass area without discomfort in the winter which is especially important in conservatories where furniture is almost always closer to the glass area.

Soft coat Low E glass is used as standard throughout continental Europe because of it’s exceptional high performance properties when compared to standard glass.

The diagram below show the percentage transmission of various wavelengths, moving from light (on the left) to heat in the Infra-red (on the right). It can be seen that 6mm clear float glass (Standard) transmits nearly 90% of the heat.


     ----------------------     1/4 ” (6mm)  Clear Float Glass

                            ----------------------     Hi-Trans Low-E Glass

                            ----------------------     Double Silver  Low-E Glass

                            ----------------------     Shading Low-E Glass

                            ----------------------     Solar Reflective Glass

Transmission curve of solar spectrum for different types of glass (unit of wavelength: nm)