A low emissivity glass can specifically enhance the thermal insulation of double glazed units. It achieves this by being positioned in the insulating unit with the coating on the glass surface facing the cavity.
In a double glazed unit, counting the surfaces from the exterior of the building, the coating should be on surface 2 (the inner surface of the outer pane) or surface 3 (the cavity surface of the inner pane).
Low E coatings perform the function of enhancing the thermal insulation (reducing the U value) equally well whether it is positioned on surface 2 or surface 3.
General advice is that the preferred location of the coating is on surface 2 of the unit, but if the glass types used in the unit make it difficult to assemble the unit than surface 3 can be selected.
There is no loss of insulation performance by using it on surface 3 however SHGC performances will vary.
When Viridian LightBridge™ is used the coating is installed on surface 3 of the double glazing unit.