20 questions to understand Low-E (coated/low radiation) insulating glass. ①

Low-E coating + insulating glass in home improvement doors and windows is a combination of 1 + 1 > 2. It has a significant improvement in the thermal insulation performance of doors and windows (glass) and has a high-cost performance. However, Low-E insulating glass is not popular, and consumers are easily confused when purchasing, such as "Why is Low-E glass useful? What kind of home is suitable for it? How useful is it? What are the precautions for use?" In this article, Jinan LIJIANG Glass has compiled 20 essential questions and answers about Low-E insulating glass from daily questions with customers, hoping to help you. 

^{Picture 1 Urban high-rise Low-E insulating glass building is widely used} 

The directory is as follows:

1. What is Low-E glass
2. What are offline & online Low-E glass
3. The difference between the use and performance of offline and online Low-E glass
4. What is Low-Emissivity & what is the surface emissivity of glass & is online Low-E considered Low-Emissivity glass?
5. How does Low-E glass play the role of heat insulation and heat preservation
6. What is the total solar infrared thermal transmittance (gIR)
7. What are double silver and triple silver Low-E Glass
8. Why do door and window merchants often give priority to recommending Low-E insulating glass
9. Why is there no Low-E laminated glass?
10. Is Low-E glass also useful for rooms that do not face the sun?
11. How to consider the light transmittance of Low-E glass
12. The color of Low-E glass
13. Is Low-E glass UV resistant?
14. The service life of insulating Low-E glass
15. What kinds of glass building is suitable for Low-E glass?
16. How should I choose Low-E glass?
17. How to choose between three-glass two-chamber and Low-E insulating glass
18. Low-E insulating glass, which side of the film layer is suitable
19. Selection of interval thickness of Low-E insulating glass hollow layer
20. Simple identification method of Low-E glass 

1. What is Low-E glass 

Low-E glass, short for Low Emissty GIass, is a type of coated glass (sunlight control, low radiation, conductive glass) and a member of building energy-saving glass. The process principle of Low-E glass is that the surface of the glass substrate is coated with a film layer composed of multi-layer metals (silver, copper, tin, etc.) or other compounds with low radiation properties, and the emissivity of the glass surface is reduced by the coating layer, thereby reducing the emissivity of the glass surface. Realize energy-saving effect. 

^{Picture 2 Schematic diagram of Low-E glass coating (offline)}

2. What are offline & online Low-E glass 

According to the difference in the preparation process, Low-E glass is divided into two types: offline and online Low-E glass. Offline Low-E is prepared by "physical method": the solid material is transferred to the glass surface and deposited into a thin film by a plasma gas formed by a negative high voltage and a working gas in a vacuum environment. Since the coating process is done on a separate coated glass production line, it is called "offline Low-E glass". On-line Low-E produced by "chemical method": It is a Low-E film plated by chemical vapor spraying technology in the high-temperature area of glass-forming on the production line for manufacturing float glass. 

^{Picture 3 Schematic diagram of the online Low-e glass production process}

3. The difference between the use and performance of offline and online Low-E glass 

The difference in film layer and process makes the use and performance of the offline and online Low-E glass different:

Offline Low-E Glass Features: 

Offline Low-E is the silver film (silver coating, silver ions), that is, single silver, double silver and triple silver that we often hear from merchants are all offline Low-E glass;

Offline Low-E film is afraid of grinding and corrosion, and metallic silver ions are afraid of oxidation, so it is used in combination with insulating glass (the film is placed inside the insulating glass to prevent oxidation, and now new technologies have also appeared in silver-free offline Low-E. Use with insulating glass, but less); 

Offline Low-E glass has better thermal insulation performance, and the emissivity of the glass surface is lower than 0.15; 

There is much offline Low-E glass used in home decoration. The glass deep processing factory will purchase the original tempered offline Low-E glass for processing, and will also customize the tempered offline Low-E glass; 

Online Low-E Glass Features: 

The online Low-E film is non-silver (such as tin oxide), and is equivalent to being integrated with glass. The film layer is resistant to high temperature, corrosion, and abrasion. All can be directly tempered and can be used directly in a single piece; 

The thermal insulation performance of online Low-E glass is relatively poor, and the emissivity of the glass surface is about 0.17-0.22; 

Due to factors such as convenience, online Low-E film is also used in glass deep processing plants; 

^{Picture 4 Offline Low-E glass blanks in glass factories} 

4. What is Low-Emissivity & what is the surface emissivity of glass & does online Low-E count as Low-Emissivity glass? 

The above chapters have improved the emissivity many times, so what are low emissivity, surface emissivity, emissivity lower than 0.15, and emissivity higher than 0.15?

The so-called radiation rate: 

The international standard "Coated Glass: Low-E Coated Glass" defines: emissivity, i.e. hemispheric emissivity, as the ratio of the radiation emittance of a radiating object to the radiation emittance of a Planck radiator at the same temperature; 

The so-called low radiation: 

Physics classifies all objects with an emissivity below 0.15 (also written as 15 %) as Low-Emission objects; 

The so-called glass surface emissivity: 

The surface emissivity of glass refers to the emissivity of the glass hemisphere, which is a measure of the ability of the glass surface to absorb radiant energy and then radiate energy outwards (it can be seen that water will only release water vapor after being burned to 100°); The lower the emissivity, the less heat the object absorbs and the less heat it emits. 

Does online Low-E count as Low-E glass: The emissivity of offline Low-E glass is lower than 0.15, which is undoubtedly the "orthodox" Low-E glass; The general emissivity of ordinary glass is 0.84, and the emissivity of online Low-E glass is about 0.17-0.22, so strictly speaking, online Low-E glass is not Low-Emissivity glass, so it has another exclusive term "K glass" ; However, because the online Low-E emissivity is also much lower than that of ordinary glass, it is also called Low-E Low-E glass most of the time; 

^{Picture 5 The Introduction of Online Low-E Glass} 

"The lower the emissivity of Low-E glass, the less heat the object absorbs, and the less heat it emits to the outside"  how to save energy? Let's take a look at how it plays the role of heat insulation and heat preservation. 

5. How does Low-E glass play the role of heat insulation and heat preservation 

In the natural environment, the radiation energy related to glass energy saving has two forms: solar radiation and far-infrared radiation. 

^{Picture 6 Schematic diagram of heat transfer through the glass} 

Sun radiation: 

Solar radiation energy is sunlight, which can be divided into ultraviolet radiation, visible light radiation, and infrared radiation; Ultraviolet radiation, the energy accounts for about 3% of the total energy of solar radiation, the wavelength is 0.3 ~ 0.38 μm (read as microns, wavelength unit), ultraviolet rays are invisible to the human eye;

Visible light radiation, the energy accounts for about 47% of the total solar radiation energy, but it is not the main carrier of thermal energy, the wavelength is 0.38 ~ 0.78 μm, visible light is visible to the human eye (red, orange, yellow, green, blue, blue, purple); Infrared radiation, whose energy accounts for about 50% of the total energy of solar radiation, is the main carrier of thermal energy, with a wavelength of 0.78 ~ 2.5 μm, infrared radiation is invisible to the human eye, but can feel the heat; 

Far infrared radiation: 

Far-infrared radiation energy is the thermal radiation emitted by any object with temperature; Its wavelength is greater than 2.5 μm, and the radiation wavelength range is generally 2.5 ~ 50 μm. The higher the temperature, the thermal radiation emitted by the object, the hotter the human body feels; In summer, the outdoor temperature is high, and the atmosphere, roads, buildings, and other objects are heated by solar radiation (heated by the sun), and the heat released to the outside world is far-infrared radiation, which is introduced into the room; In winter, the indoor temperature is high, and heating, electrical appliances, and the human body release far-infrared radiation to the outside world, which is introduced into the outdoors; 

^{Picture 7 The Solar radiation spectrum curve and The Far-infrared radiation spectrum curve}

To evaluate the thermal insulation performance of glass, the solar heat gain coefficient SHGC is widely used in China: The solar heat gain coefficient (SHGC) refers to the ratio of the total solar radiation energy through the glass (including the energy directly irradiated by the sun through the glass, and the energy of secondary radiation to the room after the glass absorbs heat) to the incident solar radiation energy; In layman's terms, if a piece of glass reflects more sunlight (ie, solar radiation, the same below), less solar radiation comes in, or less heat absorbed by the glass makes less secondary heat transfer indoors, and the more insulated it will be; The solar heat gain coefficient SHGC is the total solar transmittance g, and another shading coefficient (Sc) for evaluating thermal insulation parameters can be converted from the total solar transmittance (g) value (g = 0.87 Sc); 

^{Picture 8 Insulation performance: solar heat gain coefficient (SHGC)} 

The low emissivity of Low-E glass is the key to its superior thermal insulation properties: 

When solar radiation irradiates on the glass surface, the sum of reflected, absorbed, and transmitted by the glass is equal to solar radiation, that is, glass reflectance + absorption + transmittance = 100%; 

And because when the glass reaches the equilibrium state, how much is absorbed and how much is radiated outward, so there are: absorption rate = radiation rate; 

Low-E glass reduces indoor secondary heat transfer through low absorption (low emissivity), and high reflection of solar radiation reduces direct solar radiation heat transfer (below ▽), which greatly improves the thermal insulation performance parameters of glass; 

^{Picture 9 Ordinary white glass has high transmittance to solar radiation with a wavelength of 0.3 ~ 2.5 μm} 

^{Picture 10 Low-E glass reduces the transmittance of sunlight, especially the transmittance of infrared radiation with a wavelength of 0.78 ~ 2.5 μm has a cliff-like decrease} 

^{Table 1 Low-E glass performance parameters provided by glass manufacturers}

By the way, Jinan LIJIANG Glass Provides the Low-E glass inspection tools.

To evaluate the thermal insulation performance of glass, there is also a parameter index, that is, the heat transfer coefficient of glass (K value, U value, K, U value all refer to the heat transfer coefficient, there is a little difference, consumers can understand it as the same): 

The heat transfer coefficient of glass (K value) refers to the heat passing through a unit area of doors, windows, or glass curtain walls in a unit of time (without considering the influence of glass edges) when the ambient temperature difference between the two sides of the glass is 1 K under steady-state conditions; 

Generally speaking, if a piece of glass absorbs less heat to reduce the secondary heat transfer in the room, or if it reflects more far-infrared radiation and absorbs less far-infrared radiation, it will be more thermally insulated; 

The low emissivity of Low-E glass also plays a role in thermal insulation: 

Low absorption of solar radiation (low emissivity) to reduce indoor secondary heat transfer; 

The thermal energy emitted by far-infrared radiation generally has a wavelength range of 2.5 ~ 50 μm, while the far-infrared radiation of glass is only transmitted in the low-wavelength frequency band. Heat transfer through the glass through radiation (convection and radiation heat transfer by itself into a high-temperature object); 

Low-E glass has a low absorption rate (low emissivity), and far-infrared radiation is not transmitted, which means that most of the far-infrared radiation is reflected (so Low-E glass can also be used to keep warm in rooms without sunlight in winter. Function Oh, reflecting indoor far-infrared radiation); 

it is known that the absorption rate (radiance rate) of ordinary white glass is 0.87, and the transmittance of far-infrared radiation with a wavelength greater than 4.5 μm is 0, so it absorbs more far-infrared radiation heat and transmits more heat. 

^{Picture 11 Low-E glass has low emissivity Function, which can reflect most of the far-infrared radiation}

6. What is the total solar infrared thermal transmittance (gIR) 

Compared with other energy-saving glass (insulating glass, three-glass two-cavity, etc.), Low-E glass has a big difference: 

it has high visible light transmittance to solar radiation, low infrared radiation transmittance, and other The glass of both is consistent. For the impact of building energy saving, the thermal effect of visible light is very low, and the thermal energy of infrared radiation is the main one. This means that using the solar heat gain coefficient (SHGC, or shading coefficient Sc) to evaluate other glass is very objective, and it is unbiased to evaluate Low-E glass (such as a Low-E insulating glass and a three-glass two-chamber solar panel). The heat gain coefficient is the same, but the thermal insulation effect of Low-E insulating glass is better in actual use). 

^{Picture 12 The solar spectral transmittance curve of transparent glass, etc., its spectral transmittance in the visible light region (wavelength is 0.38 ~ 0.78 μm) and infrared heat region (wavelength is 0.78 ~ 2.5 μm) is equivalent} 

^{Picture 13 The solar spectrum transmittance curves of different Low-E glasses have high transmittance of visible light and low transmittance of infrared radiation} 

Therefore, to more accurately measure the thermal insulation performance of all glass, especially Low-E glass, there is also a measurement parameter - the total solar infrared thermal transmittance (gIR): 

The total transmittance of solar infrared heat energy, that is, in the near-infrared band of the solar spectrum from 780 to 2500 nm, is the sum of the solar radiation intensity directly passing through the glass and the part of the glass that absorbs solar energy through a secondary heat transfer and the incident solar radiation in this wavelength range ratio of strength;

^{Table 2 The lower the total transmittance of solar infrared heat energy, the stronger the thermal insulation performance.} 

7. What is Double Silver Glass and Triple Silver Low-E Glass?

Multi-silver Low-E Glass is mentioned here, mainly because the "Total Solar Infrared Thermal Transmittance (gIR)" in the previous chapter not only reflects the difference between Low-E glass and non-Low-E glass - the solar heat gain coefficient (SHGC, or shading coefficient Sc) is the same, but the actual thermal insulation effect of Low-E glass is better.

It also explains why multi-silver Low-E glass has better thermal insulation properties:
Ordinary offline Low-E only contains single-layer silver film, and double silver and triple silver can be directly understood as an upgraded version of single-silver Low-E (superimposed silver film layer);

Polysilver can block more solar infrared radiation from passing through, which means stronger thermal insulation (whether it is the solar heat gain coefficient SHGC, the shading coefficient Sc, or the total solar infrared heat transmittance gIR);

The emissivity of multi-silver is lower, so the heat transfer coefficient also improves (not as obvious as the improvement of thermal insulation performance);

^{Picture 14 The multi-silver Low-E glass structure diagram}

^{Table 3 Simple performance comparison of single, double, and triple silver}

The first 7 questions of this article mainly introduce the theoretical concept of the complex performance of Low-E Glass; the next 13 questions will mainly introduce the application of Low-E Glass.

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