Low E glass is a piece of glass with a nearly invisible coating on one surface. If you can imagine molecular sized mirrors scattered randomly but evenly on the surface of the glass, you’ll be able to understand how it works. Break light into three sections. These are short wave, intermediate wave and long wave. Long wave is also know as “infra-red” and is already heat. As long wave tries to pass through the glass it will most likely hit a “mirror” on the Low E coated surface and bounce back because it is too big to slide between the mirrors. This is how Low E keeps existing heat from escaping through the glass. Short wave is also known as “Ultra Violet” or UV for short. UV is the light you need in order to see, so it is critical that it passes through the glass. The mirrors are far enough apart that enough UV travels through the glass, missing the mirrors because of UV’s small size, that you see comfortably through the coated glass.
Now for the difference between types of Low E glass! Sometimes Low E coatings are referred to because of different manufacturing processes. A “hardcoat” means that the coating was applied to the glass as the glass was being manufactured and was still in a relatively molten state. As a result, the coating is imbedded in the glass. This is what makes it harder than “softcoat” Low E Soft coat is sprayed on the glass after the glass manufacturing process is completed. Because it is on the glass surface without the bonding of hardcoat, manufacturers must use greater care in handling the glass. For the homeowner, these terms are actually useless because they don’t tell you anything about what the glasses do differently!
Now for the difference between types of Low E glass! Call the different Low E types “Solar Gain” and “Solar Shield” and things will start to make sense. In Solar Gain glass, the mirrors are slightly farther apart than in Solar Shield type glass. The result is that the two glasses treat the third size of light wave differently. The third glass is the “intermediate” wavelength. More intermediate wave light can slip between the mirrors in a Solar Gain type Low E than in a Solar Shield, whose mirrors are close enough together to bounce back much of the intermediate light.
Keep in mind that any intermediate light that enters your home will turn into and be reradiated as long wave radiation when it touches your furniture, etc. You might say that it is the warmth on your face when you stand in front of a window receiving sunlight! So, Solar Gain glass benefits more than Solar Shield in utilizing free heat from the sun. On the other hand, Solar Shield glass limits the sun’s heat inside your home and allows your home to be cooler. it all comes down to you. In a Cold climate, generally you have many more heating days than cooling days, so one would tend toward Solar gain glass. However, individuals needs and concerns vary, so you may be more concerned with cooling issues and comfort than with a slight savings on heat. Both Low E glass types are so much better than regular uncoated glass that the difference is not earth shattering!
The differences in the two glass types can be seen in the technical numbers used to evaluate glass and this can sometimes be deceiving. If you look at the insulating value of glass as expressed in either a U-value or an R-value, you will easily see that Solar Shield has a better rating. This is because U and R are ratings of the amount of radiation that passes through a material and limiting more intermediate light from passing through the glass means that the glass means less radiation and a better insulating value. On the other hand, we have a term often presented as the initials SHGC. This stands for solar heat gain coefficient. it is a measure of free solar heat and in this rating, the Solar Gain glass is designed to out perform. In Canada we have a rating system called the E.R. or Energy Rating (described elsewhere in this FAQ). Because SHGC contributes to the E.R. number, identical windows except for Low E type will have a different rating with windows with Solar Gain glass having a significantly higher E.R. beware the salesman who says that one Low E is, “better”. They are different and what is better for one homeowner, may not be better for another!