Low-E glass may appear identical to conventional clear glazing in architectural applications, yet its performance is vastly different. This thermal and solar-control performance comes from extremely thin, highly engineered coatings applied directly to the glass surface. These transparent layers fundamentally alter how the glass handles heat and light.
The different manufacturing methods used to create these coatings directly influence the final durability, optical characteristics, and overall thermal performance of the glass. Understanding how these coatings are manufactured provides architects and specifiers with the technical foundation to accurately evaluate building envelope options, proving that superior glazing performance begins long before the glass reaches the construction site.
What Makes Glass “Low-E”?
Thermal emissivity is a measurement between 0 and 1 defining a surface’s ability to radiate heat. Standard clear glass has a high emissivity of about 0.84, meaning its surface readily emits and absorbs long-wave thermal radiation. Low-E (low-emissivity) coatings reduce this radiative heat exchange, with high-performance soft-coat products capable of reaching emissivity values around 0.02.
Low-E (low-emissivity) coatings reduce this radiative heat exchange, with high-performance soft-coat products capable of reaching emissivity values around 0.02.
These coatings function as selective filters. They act as mirrors to long-wave infrared energy—the radiant heat emitted by interior heaters or outdoor pavement—reducing heat transfer across the glass. At the same time, properly engineered Low-E coatings can maintain high visible-light transmission while selectively limiting portions of the solar infrared spectrum.
By successfully balancing natural illumination with thermal performance, Low-E glazing has become a foundational component in energy-efficient windows, curtain walls, skylights, and glazed façades.
How Is Low-E Glass Made?
Commercial glass manufacturers rely on two primary approaches to apply these selective filters, which dictate the coating’s durability and complexity.
Pyrolytic or Hard-Coat Low-E
In this method, the coating is applied directly to the glass ribbon while it is still hot—typically around 600°C—during the float-glass manufacturing process. These high temperatures cause the coating materials to bond strongly with the glass surface. The resulting coating is incredibly durable and resistant to scratching, making it suitable for single glazing or exposed exterior environments. Because it happens on the production line, this process is commonly referred to as hard-coat or online Low-E.
MSVD or Soft-Coat Low-E
Alternatively, the coating can be applied after the base glass has been manufactured and cooled. The glass passes through a highly controlled vacuum-coating system. Utilizing Magnetron Sputter Vacuum Deposition (MSVD), manufacturers deposit extremely thin, nanometer-scale layers of metal and ceramic with high precision. Conventional high-performance MSVD Low-E coatings are more sensitive to physical damage and environmental exposure than pyrolytic coatings, so they are typically protected inside the sealed cavity of an Insulated Glass Unit (IGU). This process allows for complex layer combinations offering superior thermal and solar performance.
How Magnetron Sputtering Creates a Low-E Coating
The MSVD process is widely used to produce high-performance architectural glass coatings.
Creating Plasma Inside the Vacuum Chamber
In the soft-coat process, the glass moves into a sealed vacuum-coating environment. Specific process gases, commonly including argon, are introduced, and electrical energy ionizes the gas to create a plasma. This highly controlled environment makes exact material deposition possible without introducing unwanted external atmospheric contaminants.
From Sputtering Target to Thin Film
In magnetron sputter vacuum deposition, plasma ions bombard solid sputtering targets containing the specific materials required for the coating. These components provide the source material. As energetic ions bombard their surface, atoms and other material species are ejected and deposited onto the moving glass as an extremely thin, uniform film.
Building the Coating One Layer at a Time
Modern soft-coat Low-E products are engineered as complex multilayer systems rather than a single application of material. As the glass moves through successive vacuum chambers, different layers are deposited, each serving a specific function. Metallic layers provide the primary infrared reflection, while surrounding dielectric (ceramic) layers contribute to adhesion, structural protection, durability, and optical properties. Specialized protective layers are also added to prevent the core materials from oxidizing during heat-treating processes. Together, this sequence of ultra-thin films determines the overall coating performance and the glazing’s ability to selectively filter light and heat.
Modern soft-coat Low-E products are engineered as complex multilayer systems rather than a single application of material.
What Are Low-E Coatings Made Of?
In most advanced soft-coat Low-E systems, the key functional material is an ultra-thin layer of silver. At carefully controlled thicknesses within a multilayer coating stack, silver provides strong infrared reflectance while allowing substantial visible light transmission.
However, a bare silver layer would quickly oxidize and degrade. Therefore, it is supported by surrounding metallic and dielectric layers that physically protect the silver and act as optical tuners to maintain a neutral visual appearance. Dense diffusion barriers and adhesion layers ensure the stack remains intact, especially if the glass undergoes thermal tempering.
Manufacturers categorize these products by their complexity: single-, double-, and triple-silver coating concepts. Using multiple carefully engineered silver layers can increase spectral selectivity, allowing the coating to reject more solar heat while maintaining relatively high visible-light transmission. The finished coating is always engineered as a complete system, with different layers serving different functions, meaning layer configurations vary widely between products.
Why the Coating Process Matters in Architecture
The manufacturing technology behind the coating directly dictates how the building envelope will perform in real-world conditions.
Visible Light and Solar Heat Gain
Glazing efficiency relies heavily on Visible Light Transmittance (VLT)—the percentage of natural daylight permitted through the glass—and the Solar Heat Gain Coefficient (SHGC), which measures the fraction of solar radiation admitted into the building. Architects must constantly balance the desire for natural daylight with the need to block unwanted solar heat to reduce cooling loads. Complex MSVD coatings specifically allow architects to specify a low SHGC while maintaining a high VLT.
Thermal Performance
These coatings also contribute significantly to the glazing’s U-value, which measures overall heat transfer through the glazing assembly. By lowering surface emissivity, Low-E coatings reduce radiant heat transfer and can substantially lower the U-value.
Color, Reflectivity and Façade Appearance
Coating selection is both a performance and design decision. The dielectric layers that tune light transmission also influence exterior reflectivity, subtle glass color, and overall façade appearance. Properly engineered layers ensure stable exterior aesthetics while optimizing interior daylight quality for occupants.
Where the Low-E Coating Sits in an Insulated Glass Unit
Because conventional MSVD soft-coat Low-E coatings are sensitive to oxidation and physical damage, they are normally protected inside the sealed cavity of an Insulated Glass Unit (IGU).
Glazing systems use a standard numbering convention for their surfaces, starting from the outside in. In a double-glazed IGU, Surface #1 is the exterior face, Surface #2 is the inner face of the exterior pane, Surface #3 is the outer face of the interior pane, and Surface #4 is the room-side interior face.
Coating position dramatically influences performance. In warm climates where curtain walls, windows, skylights, and glazed façades must reject solar heat, the coating is typically placed on Surface #2 to reflect heat before it enters the air gap. In heating-dominated climates, passive Low-E products may instead be positioned on Surface #3 to permit more useful solar heat gain while still reducing radiative heat transfer through the IGU.
Practical Specification Guidance
Because every building faces unique environmental pressures, architects should evaluate the complete glazing system rather than selecting Low-E glass based on one performance metric alone.
Specification should begin with the building climate and façade orientation. Warm climates often benefit from solar-control coatings with a low SHGC, commonly positioned on Surface #2, while heating-dominated climates may benefit from passive Low-E configurations with a higher SHGC where useful solar gain is desirable. Evaluate VLT separately based on daylighting requirements. Designers must also weigh hard-coat versus soft-coat requirements; while high-performance soft coats generally offer greater spectral selectivity, hard coats may be preferred for single-glazed or exposed configurations.
The manufacturing technology behind the coating directly dictates how the building envelope will perform in real-world conditions.
Finally, coating appearance and specific glazing configuration must be verified. Surrounding buildings, external shading, and façade orientation can change solar exposure and how the glass appears.
