Does the Position of the Low-E Coating Surface Affect the Performance of Insulated Glass?

Does the Position of the Low-E Coating Surface Affect the Performance of Insulated Glass?

In the field of building energy efficiency, the combination of Low-E glass and insulated glass has become the standard for modern high-performance buildings. This combination significantly enhances the thermal insulation performance of buildings and reduces energy consumption. However, a detail that is often overlooked but crucial is: On which side of the insulated glass cavity is the thin coating of the Low-E glass located? This seemingly minor difference actually has a decisive impact on the overall performance of the glass. The answer is yes: the position of the Low-E glass coating surface not only affects the performance of the insulated glass but is also a core element that must be precisely controlled during the design and production process.

1. First, Let’s Review How Low-E Glass and Insulated Glass Work

To understand the importance of position, we must first understand how they work individually.

1.Core Functions of Low-E Glass:
Low-E glass, or low-emissivity glass, has a nearly invisible coating of metal or metal oxide on its surface. This coating has two key characteristics:

• Reflects Far Infrared Thermal Radiation: It reflects long-wave thermal energy (far-infrared radiation) emitted by objects, much like a mirror reflects light. In winter, it reflects indoor heat back inside, preventing heat loss; in summer, it blocks outdoor heat radiation from entering, reducing heat gain.
• Allows Visible Light Transmission: At the same time, it has high transmittance for visible light, ensuring the glass's daylighting function and transparency.

2.Synergistic Effect of Insulated Glass:

Insulated glass is made of two or more panes of glass bonded together with high-strength, high-airtightness composite adhesives and aluminum alloy frames, with dry air or inert gas (such as argon) filled in between. Its main functions are:

• Reducing Heat Conduction: The intermediate air or gas layer is a poor conductor of heat, effectively blocking heat transfer between the inner and outer panes of glass, thereby improving the insulation (K-value or U-value) performance of the glass.

When Low-E glass is used in insulated glass, a "1+1>2" effect is achieved. The coating of the Low-E glass is responsible for "selectively reflecting" thermal energy, while the structure of the insulated glass is responsible for "blocking" heat conduction, together forming an efficient energy-saving barrier.

2. How Does the Position of the Low-E Coating Surface Affect the Performance of Insulated Glass?

In a standard double-pane insulated glass unit, there are four surfaces: counting from the outdoor side to the indoor side, they are the #1 surface (outer surface of the outdoor-side glass), #2 surface (inner surface of the outdoor-side glass), #3 surface (outer surface of the indoor-side glass), and #4 surface (inner surface of the indoor-side glass). The coating layer of the Low-E glass is typically located on the #2 or #3 surface. The difference between these two positions leads to significant variations in performance.

Key Point 1: Coating on the #2 Surface (Facing the Gas Cavity on the Outdoor Side)

This configuration typically focuses more on the shading performance of the building and is suitable for areas with hot summers where blocking solar heat is a priority.

• Thermal Insulation (Shading) Performance: When the Low-E glass coating is on the #2 surface, it encounters incoming short-wave solar radiation earlier. The coating reflects most of the far-infrared portion of solar heat, preventing it from entering the interior. At the same time, it effectively blocks indoor heat from radiating outward, but its main advantage lies in its excellent Shading Coefficient (SC) and lower Solar Heat Gain Coefficient (SHGC).
• Thermal Insulation (U-value) Performance: The thermal insulation performance remains good, but compared to the #3 surface, it is slightly less effective at retaining indoor heat in winter.
• Applicable Scenarios: Large curtain wall buildings, areas with severe western sun exposure, and southern regions where air conditioning cooling is the primary need.
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Key Point 2: Coating on the #3 Surface (Facing the Gas Cavity on the Indoor Side)

This configuration typically focuses more on the thermal insulation performance of the building and is suitable for cold winter regions where maximizing the retention of indoor heat is essential.

• Thermal Insulation (U-value) Performance: When the Low-E glass coating is on the #3 surface, it is closer to the indoor environment. In winter, far-infrared thermal radiation generated by indoor objects and heating systems is efficiently reflected back indoors upon contacting the glass, like putting a "thermal coat" on the building, significantly reducing heat loss through the glass. This is the classic configuration for achieving the best thermal insulation performance (lowest U-value).
• Thermal Insulation (Shading) Performance: It also provides thermal insulation, but solar heat must first pass through the outer pane of glass and the air layer before being reflected by the coating. Some heat is already absorbed and convected by the air layer, so its shading effect is slightly lower than the #2 surface configuration.
• Applicable Scenarios: Severe cold and cold northern regions, residential windows, and any buildings with high requirements for winter thermal insulation.

Simple Comparison Summary:

3. What Are the Consequences of Incorrect Position Selection?

If the position of the Low-E glass coating in the insulated glass is chosen incorrectly, it may not only fail to achieve the expected energy-saving goals but could even be counterproductive.

• Case 1: Misuse of #2 Surface Configuration in Northern Buildings. If insulated glass with the Low-E glass coating on the #2 surface is used in a project in Harbin, although it works well in summer, its thermal insulation performance is insufficient to effectively prevent indoor heat from escaping during the long winter. This leads to a sharp increase in building heating energy consumption, noticeable "cold radiation" near the glass indoors, and even potential condensation on the interior surface of the glass due to low surface temperatures, affecting living comfort and building lifespan.
• Case 2: Misuse of #3 Surface Configuration in Southern Buildings. In an office building in Guangzhou, if insulated glass with the Low-E glass coating on the #3 surface is mistakenly used, its relatively high solar heat gain capability allows significant solar heat to enter the interior, greatly increasing the cooling load on the air conditioning system and causing electricity bills to soar, contrary to the original intention of energy-efficient design.

Therefore, accurately selecting the position of the Low-E glass coating in the insulated glass based on the climatic conditions of the building's location and energy efficiency design goals is the cornerstone for ensuring the performance of the building envelope meets standards.

Therefore, accurately selecting the position of the Low-E glass coating in the insulated glass based on the climatic conditions of the building's location and energy efficiency design goals is the cornerstone for ensuring the performance of the building envelope meets standards.

4. How to Determine and Choose? Professional Advice

For ordinary consumers or project managers, how can they ensure the position of the Low-E glass coating in the insulated glass is correct?

1. "Match Test" (Simple Identification): At night, shine a flashlight or bring a lit match close to the glass. Observe the reflections in the glass; usually, four reflected images will be visible. One image will have a different color from the other three (possibly slightly colored, like light blue or gray). That unique image comes from the Low-E glass coating surface. By observing the relative position of that image to the flashlight/match, one can roughly determine on which side the coating is located.
2. Trust Professional Labels and Specifications: Reputable insulated glass manufacturers will clearly mark the coating surface position of the Low-E glass on the product label or spacer bar (e.g., "Coating on #2" or "Coating on #3"). This technical parameter should also be clearly stated in the procurement contract.
3. Follow the Climate-Oriented Principle:

• Severe Cold/Cold Regions: Prioritize insulated glass with the Low-E glass coating on the #3 surface, focusing on thermal insulation.
• Hot Summer/Cold Winter Regions: A balance between thermal insulation and shading is needed. The choice can be based on building orientation and primary needs. Typically, insulated glass with the Low-E glass coating on the #3 surface is recommended, adjusting the glass's light transmittance to assist in heat gain control. For areas with extremely high shading requirements, the #2 surface can also be considered.
• Hot Regions: Prioritize insulated glass with the Low-E glass coating on the #2 surface, and consider double-silver or even triple-silver Low-E glass to maximize shading and insulation effects.

Conclusion

The combination of Low-E glass and insulated glass is a testament to the wisdom of modern building energy efficiency technology. However, this magical coating cannot be placed arbitrarily. Its position acts like a precision switch, directly regulating the flow and intensity of heat, profoundly affecting the final thermal insulation, shading, and even daylighting performance of the insulated glass. Therefore, whether designers, developers, or end-users, it is essential to fully recognize the importance of the Low-E glass coating surface position. Making the correct choice based on scientific principles and actual needs ensures that every pane of glass is used to its fullest potential, truly contributing to a green, comfortable, and low-carbon built environment.