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In a double glazed unit,we have4 surfaces ,ie #1 facing outdoors,#2 the inner surface of the glass on the exterior,#3 the suface of interior glass in contact with air gap and #4 facing the interiors.
The surface on which the low e coating has to be used is determined by which climate you are in,ie whether you prefer a heat gain to offset energy costs for heating(like in western countries) or to minimise heat gain as in hot climates(like middle east)
when low heat gain is preferred the low e coating is done on surface #2.This will allow less heat to pass through during the day.If you have a low e coating on #4 also it will further enhance the insulation properties.
A coating on#3 can improve heat gain.
Manufacturers like PPG have online calculators for calculating the u value of glazed units.
All the EMISSIVE coatings should be used Outside, once the solar rays pass through the Normal glass, their wavelength changes and gets trapped.
