Low-Emissivity (Low-e) Glass

For 60 years, Solarban^® solar control low-e glass has pioneered significant advancements in glass performance. As one of the most trusted and proven solar control, low-e glasses available, Solarban^® glass has been and continues to be specified for countless AIA COTE^® award-winning and LEED^® certified buildings. Learn more about our Solarban^® glass portfolio below:

To understand how glass coatings work, it is essential to first understand the solar energy spectrum. Solar energy, or energy from the sun, is comprised of ultraviolet (UV) light, visible light and infrared (IR) light. Each of these occupies a different part of the solar spectrum, distinguished by their unique wavelengths.

• UV light registers wavelengths of 310-380 nanometers
• Visible light wavelengths range from 380-780 nanometers
• IR light (heat energy) commences at 780 nanometers

When heat or light energy is absorbed by glass, it doesn’t just stay there – it moves in different ways. Some of this energy is carried away by air moving around the glass, while some is re-radiated back into the environment by the glass itself. This process is influenced by a property called emissivity, which measures how well a material can radiate energy.

Materials with high emissivity, like dull or dark-colored surfaces, radiate more energy, while highly reflective materials have low emissivity and radiate less. This is important for materials like windows, as they emit heat in the form of long-wave infrared energy. The amount of heat a window radiates depends on its emissivity and surface temperature. Lower emissivity means the material is better at insulating, making it more efficient at keeping heat in or out. Understanding emissivity helps manufacturers design materials, especially for windows, that improve energy efficiency and insulation in buildings.

This is where low-e coatings come into play, because they’re engineered to precisely manage the transmission of ultraviolet (UV) and infrared (IR) light while maintaining optimal levels of visible light. These coatings are microscopically thin – approximately 500 times finer than a human hair – and are foundational to achieving superior thermal performance in glass applications.

Low-e glass operates on a principle similar to a thermos. A thermos utilizes a silver lining to reflect the temperature of its contents, maintaining it through constant reflection and the insulating air space between its inner and outer shells. In the same way, low-e glass is coated with ultra-thin layers of silver or other low-emissivity materials. This specialized coating reflects indoor temperatures back into the room, effectively maintaining a stable and comfortable interior climate.

Low-e coatings are categorized into two types: passive low-e and solar control low-e.

Passive low-e coatings are engineered to maximize solar heat gain, allowing solar short-wave infrared energy to pass through the glass and help heat a building during the winter. This process creates passive heating, which reduces reliance on artificial heating systems and lowers energy costs.

Solar control low-e coatings are designed to limit solar heat gain by reflecting solar short-wave infrared energy. This functionality is crucial for keeping buildings cooler, especially in warmer climates or seasons, thereby reducing the energy consumption associated with air conditioning.

Passive and solar control low-e coatings are produced through two methods: pyrolytic or “hard coat” and Magnetron Sputter Vacuum Deposition (MSVD) or “soft coat.”

Pyrolytic coatings, introduced in the 1970s, are applied during glass production, bonding to the glass for durability. MSVD coatings, developed in the 1980s, are applied off-line in a vacuum chamber to pre-cut glass. While passive coatings were historically linked to pyrolytic and solar control to MSVD, this is no longer accurate.

In a standard double insulating glass unit (IGU), coatings can be applied to four potential surfaces.

• Surface #1: Faces outdoors
• Surfaces #2 and 3: Face each other inside the IGU, separated by a peripheral spacer creating an insulating air space
• Surface #4: Faces directly indoors

Passive low-e coatings are most effective on the third or fourth surface (farthest from the sun), while solar control low-e coatings work best on the second surface.

Low-e coatings, whether passive or solar control, consistently enhance glazing performance. The effectiveness of glass with low-e coatings is measured using the following key metrics:

• U-Value: Quantifies the rate of heat loss through a window.
• Visible Light Transmittance (VLT): Measures the percentage of visible light passing through the glass.
• Solar Heat Gain Coefficient (SHGC): Represents the fraction of incident solar radiation admitted through a window, including both direct transmission and absorbed/re-radiated heat. A lower SHGC indicates reduced solar heat transmission.
• Light to Solar Gain (LSG): The ratio of VLT to SHGC.

Performance varies across products and manufacturers, but performance data and online comparison tools for low-e coatings are readily available.

Low-e coatings, whether passive or solar control, consistently enhance glazing performance. The effectiveness of glass with low-e coatings is measured using the following key metrics:

• Reduces the need for heating and cooling, leading to lower energy bills and reduced HVAC expenses
• Allows abundant natural light to enter without the associated heat gain
• Improves occupant comfort
• Manages glare and filters out harmful UV rays
• Contributes to lowering carbon emissions

Performance varies across products and manufacturers, but performance data and online comparison tools for low-e coatings are readily available.

Low-e coatings are integral to achieving strict solar and thermal performance objectives. Vitro’s Glass Education Center explains how and why.

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