How Low-E Glass Works?

Because of its constantly improving solar and thermal performance, glass is one of the most common and adaptable building materials used today. Passive and solar control low-e coatings are one method for achieving this performance. 

What exactly is Low-E glass?

Understanding the solar energy spectrum, also known as solar energy, is crucial for understanding coatings. Different regions of the solar spectrum are occupied by ultraviolet (UV), visible (visible), and infrared (IR) light; the differences between the three are based on their wavelengths.

1. When assessing glass performance, ultraviolet light, which fades interior materials like fabrics and wall coverings, has wavelengths between 310 and 380 nanometers.

2. The region of the spectrum between wavelengths of roughly 380 and 780 nanometers is occupied by visible light.

3. Starting at wavelengths of 780 nanometers, infrared light (or heat energy) enters a building as heat. The heat emitted by the sun is known as short-wave infrared energy, whereas heat emitted by warm objects has longer wavelengths and is known as long-wave infrared.

To reduce the amount of ultraviolet and infrared light that can pass through glass without reducing the amount of visible light that is transmitted, low-E coatings have been developed. Related product: Double Head Low-E Coating Removing Machine.

When heat or light energy is absorbed by glass, it is either reflected by the glass surface or shifted away by moving air. The term "emissivity" refers to a substance's capacity to emit energy. In general, highly reflective materials have low emissivity, while materials with dull, dark colors have high emissivity. Depending on the emissivity and temperature of their surfaces, all materials, including windows, emit heat in the form of long-wave, infrared energy. One of the key methods for heat transfer through windows is radiant energy. The insulating capabilities of a window are enhanced by lowering the emissivity of one or more of the window glass surfaces. Uncoated glass, for instance, has an emissivity of.84, while Solarban® 70XL glass from Vitro Architectural Glass (formerly PPG glass) has an emissivity of 02.

Low-emissivity (also known as Low-E) glass coatings are useful in this situation. Long-wave infrared energy (or heat) is reflected by the microscopically thin, transparent coating on low-E glass, which is much thinner than human hair. Significant amounts of short-wave solar infrared energy are also reflected by some low-e materials. In the winter, the low-e coating reflects heat inside when interior heat energy tries to escape to the colder outside, minimizing radiant heat loss through the glass. In the summer, the opposite takes place. Low-E glass operates similarly to a thermos, to use a straightforward comparison. The silver lining of a thermos reflects the temperature of the beverage it holds. The constant reflection that takes place and the insulating properties of the space between the thermos' inner and outer shells—which functions similarly to an insulating glass unit—help to keep the temperature stable. The same theory holds for Low-E glass, which is made of incredibly thin layers of silver or other low-emissivity materials. The room is kept warm or cold by the silver low-e coating, which reflects interior temperatures.

Types of Low-e Coatings & Manufacturing Methods

Passive low-e coatings and solar control low-e coatings are the two different kinds of Low-E coatings. Passive low-e coatings are made to increase the amount of solar heat that enters a building or home, giving the impression of "passive" heating and reducing the need for artificial heating. Solar control low-e coatings are intended to reduce the amount of solar heat that enters a building or home to keep it cooler and use less energy for air conditioning.

Two primary production processes—pyrolytic, or "hard coat," and Magnetron Sputter Vacuum Deposition (MSVD), or "soft coat"—are used to make both passive and solar control Low-E glass.

The coating is applied to the glass ribbon while it is being made on the float line in the pyrolytic process, which gained popularity in the early 1970s. The coating then "fuses" to the hot glass surface, forming a solid bond that will last for a very long time during the fabrication of glass. To be shipped to fabricators, the glass is finally cut into stock sheets of various sizes. The coating is applied offline to pre-cut glass in vacuum chambers at room temperature in the MSVD process, which was introduced in the 1980s and has continuously improved in recent decades.

Passive low-e coatings are sometimes associated with the pyrolytic process and solar control low-e coatings with MSVD, but these associations are no longer entirely accurate. This is due to the historical evolution of these coating technologies. Furthermore, performance varies significantly between products and manufacturers (see the table below), but performance data tables are easily accessible, and several online tools can be used to compare all low-e coatings currently on the market.

Coating Location

The first (#1) surface of a standard double panel insulating glass unit faces the outside, and the second (#2) and third (#3) surfaces face one another inside the insulating glass unit and are separated by a peripheral spacer that creates an insulating air space, and the fourth (#4) surface faces the interior directly. Solar control low-e coatings perform best when placed on the surface that is closest to the sun, usually the second surface, whereas passive low-e coatings perform best when placed on the third or fourth surface (furthest away from the sun).

Performance Measures for Low-e Coatings

Insulating glass units' various surfaces receive Low-E coatings. A Low-E coating offers improvements in performance values whether it is used for solar control or passive cooling. The following are used to gauge how well Low-E coated glass performs:

The rating given to a window based on how much heat loss it permits is called the U-value.

The amount of light that passes through a window is measured by its visible light transmittance.

The percentage of incident solar radiation that is admitted through a window, both directly transmitted and absorbed and re-radiated inside, is known as the solar heat gain coefficient. A window transmits less solar heat lowers its solar heat gain coefficient.

The solar heat gain coefficient (SHGC) of a window and its visible light transmittance (VLT) rating are compared to determine its light-to-solar gain. Size, tint, and other aesthetically pleasing characteristics come to mind when considering window designs. Low-E coatings, however, play an equally significant role and have a big impact on a window's overall performance as well as the overall heating, lighting, and cooling costs of a building.