All materials, including windows, emit (or radiate) heat in the form of long-wave, far-infrared energy depending on their temperature. This emission is one of the important components of window heat transfer, so reducing the window’s emittance can greatly improve its insulating properties. Coating a glass surface with a low-e material and facing that coating into the gap between the glazing layers blocks a significant amount of this radiant heat transfer, lowering the total heat flow through the window. When heat or light energy is absorbed by glass, it is either convected away by moving air or reradiated by the glass surface. The ability of a material to radiate energy is called its emissivity.
Verify if you have LowE or Clear glass with the "reflection test"
Hold a lighter, laser, or pen light close to the window and observe the reflections.
In double-paned windows, you'll typically see four reflections of the light source (two from each pane of glass).
If all four reflections are the same color, the glass likely does NOT have a LowE coating.
If one of the reflections has a different color or shade (e.g., slightly greenish, bluish, or clearer), it suggests the presence of a LowE coating.
Make note of which of the flames or reflections is a different color
Since the early 1990s, almost all LowE windows have been manufactured with an insert gas fill between glass panes (typically argon) that also dramatically improves the insulating properties of the glass. So if you find that you have a LowE window, it indicates an overall high-performance window in terms of solar heat gain coefficient (SHGC) and U-factor (heat conduction performance)
Select the correct emissivity value and surface location
If you have confirmed that you have a LowE window, you need to select an appropriate assumption for the location and performance of the low emissivity coating on the window.
The placement of a low-e coating within the air gap of a double-pane window does not affect the U-factor, but it does influence the SHGC. See Figure 1 below for more details.
Emissivity performance level
In northern or heating-dominated climates, it is generally preferable to allow for moderate solar gain in the winter while reducing gain in the summer cooling months. The lower the emissivity value, the lower the SHGC. See Figure 1 for typical values by climate zone.
LowE coating surface location
Using the reflection test, described above, the reflection that is a different color than the others indicates which surface the coating is on. In the example image below, Flame 3 indicates the location of the lowE coating:
In heating-dominated climates, lowE coating on surface #3 is most common to:
Reduce overall SHGC
Maximize winter passive solar gain at the slight expense of less ability to control summer solar gain
In cooling climates, a coating on surface #2 is the optimal location to reduce overall SHGC and therefore reduce cooling loads.
The glass surface numbering convention is as viewed from the exterior to the interior of the home.
Figure 1: Glass surface numbering convention and common emissivity coatings by climate zone