To answer this, first let’s look at what low-E is.
Low-E is a very thin, almost invisible layer, applied to glass to increase its thermal efficiency. Low-E stands for low emissivity. It is typically applied to glass in either one of two methods; pyrolytic or sputtered. Pyrolytic low-E, also know as a hard coat, involves a process where tin oxide is baked onto the surface of the glass. Forming a covalent bond with the glass, the resulting baked-on pyrolytic or hard coat low-E is extremely durable. Alternatively, sputtered low-E makes use of silver oxide and is referred to as soft coat low-E. Sputtered or soft coat low-E is applied in one, two, and three layers onto the surface of the glass. Unlike pyrolytic low-E, sputtered low-E must be protected from humidity and contact. Thus, sputtered (soft coat) low-E is applied to the interior surface of the glass that faces the sealed air space in a dual or triple glazed unit.
Well for starters, it depends on where you live in the world and what your climate is like. Depending on where you live, low-E is applied to different surfaces on an insulated unit; dual pane and/or triple pane glass.
Low-E is designed to deflect Ultra-Violet and infrared radiation. In colder climates, the low-E is applied to the interior, inner-most surface to deflect heat back into the home, helping to reduce heating costs. In warmer climates, low-E is applied to the interior, outer-most surface to deflect heat out of the home, reducing cooling costs. The diagram to the right illustrates a dual-glazed insulated unit that is best designed for a warmer climate. The low-E II denotes two coatings of low-E applied to the glass.
Choosing windows and doors that have low-E coating(s) on the glass can significantly reduce your energy costs each year. A window or door’s energy efficiency is measured by its U-Value and SHGC ratings, which are directly effected by low-E. When considering and comparing products that best meet your insulating needs, be sure to check these two ratings.