Introduction: Since the introduction of the Insulating Glass Unit (IGU), window components have been steadily developing to improve the thermal performance of the house. Special editor Scott Gibson (Scott Gibson) introduced the progress of IGU design, from the invention and application of low-emissivity coatings to the development of glass windows other than double glazing, suspension films and different types of insulating gases, and the future Understanding of technology.
Andersen Windows introduced welded insulated glass panels in 1952, which is very important. Consumers can buy components that combine two pieces of glass and a layer of insulation in a single product. For countless homeowners, Andersen’s commercial release meant an end to the tedious work of riot windows. More importantly, in the past 70 years, the beginning of the industry has repeatedly improved the thermal performance of windows.
Multi-pane Insulating Glass Window (IGU) combines metal coating and inert gas filling components to make the house more comfortable and reduce heating and cooling costs. By adjusting the characteristics of low-emissivity (low-e) coatings and selectively applying them, glass manufacturers can customize IGUs for specific needs and climates. But even with the best paint and gas, glass manufacturers are still struggling hard.
Compared with the exterior walls of high-performance houses, the best glass will make insulators inferior. For example, the wall in an energy-efficient house is rated at R-40, while the U-factor of a high-quality three-pane window may be 0.15, which is only equivalent to R-6.6. The International Energy Conservation Law of 2018 requires that even in the coldest areas of the country, the minimum U coefficient of windows is only 0.32, which is approximately R-3.
At the same time, work on new technologies continues, and these new technologies can enable better windows to be used more widely. Innovative technologies include a three-pane design with an ultra-thin central pane, a suspended film unit with up to eight inner layers, a vacuum insulation unit with a glass center insulation potential exceeding R-19, and a vacuum insulation that is almost as thin as a single pane Unit cup.
For all the advantages of Andersen welding insulating glass, it has some limitations. The introduction of low-emissivity coatings in 1982 was another major step forward. Steve Urich, director of the National Window Decoration Rating Board program, said that the exact formulations of these coatings vary from manufacturer to manufacturer, but they are all microscopic thin layers of metal that reflect radiant energy back to its source. -Inside or outside the window.
There are two coating methods, called hard coating and soft coating. Hard coating applications (also known as pyrolytic coatings) date back to the late 1990s and are still in use. In the manufacture of glass, the coating is applied to the surface of the glass-essentially baked into the surface. Can’t be scraped off. A soft coating (also called sputter coating) is used in the vacuum deposition chamber. They are not as strong as hard coatings and cannot be exposed to the air, so manufacturers only apply them to the surface to be sealed. When a low-emissivity coating is applied to a surface facing the room, it will be a hard coating. A soft coat is more effective in controlling solar heat. Cardinal Glass Technical Marketing Director Jim Larsen (Jim Larsen) said that the emissivity coefficient may drop to 0.015, which means that more than 98% of the radiant energy is reflected.
Despite the inherent difficulties in applying a uniform metal layer with a thickness of only 2500 nanometers, manufacturers have become increasingly adept at manipulating low-emissivity coatings to control the amount of heat and light passing through the glass. Larson said that in the multilayer low-emissivity coating, the anti-reflection and silver layer limit the absorption of solar heat (infrared light) while maintaining as much visible light as possible.
“We are studying the physics of light,” Larson said. “These are precision optical filters, and the thickness of each layer is critical to maintaining the color balance of the coating.”
The components of the low-e coating are just one factor. The other is where they are applied. The Low-e coating reflects radiant energy back to its source. In this way, if the outer surface of the glass is coated, the radiant energy from the sun will be reflected back to the outside, thereby minimizing the heat absorption inside the windows and inside the house. Similarly, the low-radiation coating applied to the side of the multi-pane unit facing the room will reflect the radiant energy generated inside the house back into the room. In winter, this feature will help the house retain heat.
Advanced low-emissivity coatings have steadily reduced the U-factor in IGU, from 0.6 or 0.65 for the original Andersen panel to 0.35 in the early 1980s. It was not until the late 1980s that the inert gas argon was added, which provided another tool that glass manufacturers could use and reduced the U factor to about 0.3. Argon is heavier than air and can better resist convection in the center of the window seal. Larson said that the conductivity of argon is also lower than that of air, which can reduce conduction and increase the thermal performance of the glass center by about 20%.
With it, the manufacturer pushes the dual-pane window to its maximum potential. It consists of two 1⁄8 inch panes. Glass, a 1⁄2 inch space filled with argon gas, and a low-emissivity coating added to the side of the glass room. The U factor drops to about 0.25 or lower.
The triple-glazed window is the next jumping point. Conventional components are three pieces of 1⁄8 inch. Glass and two 1⁄2 inch spaces, each cavity has a low-emissivity coating. The additional gas and the ability to use low-emissivity coatings on more surfaces greatly improves performance. The downside is that windows are usually too heavy for double-hung sashes that usually slide up and down. Glass is 50% heavier than double glazing and 1-3⁄8 inches. Thick. These IGUs cannot fit within 3⁄4 inches. Glass bags with standard window frames.
This unfortunate reality pushes manufacturers to windows that replace the inner glass layer (suspended film windows) with thin polymer sheets. Southwall Technologies has become a representative of the industry with its hot mirror film, making it possible to produce three-layer or even four-layer glazing with the same weight as a double glazing unit. However, it is easy for the window unit to seal leaks around the glass window, thereby allowing insulating gas to escape and allowing moisture to enter the interior. The window seal failure made by Hurd has become a widely publicized nightmare in the industry. However, the hot mirror film now owned by Eastman Chemical Company is still a viable option in multi-pane windows and is still used by manufacturers such as Alpen High Performance Products.
Alpen CEO Brad Begin said of the Hurd tragedy: “The entire industry is indeed under dark circles, causing some manufacturers to break away from the suspension film.” “The process is not that difficult, but if If you don’t do a good job or don’t pay attention to quality, such as any window, any type of IG, then you are destined to suffer too much premature failure on site.
Today, the hot mirror film is produced by a joint venture between DuPont and Teijin, and then shipped to Eastman, where the low-emissivity coating is obtained in the vapor deposition chamber, and then sent to the manufacturer for conversion to IGU. Begin says that once the film and glass layers are assembled, they are placed in an oven and baked at 205°F for 45 minutes. The film shrinks and tensions itself around the gasket at the end of the unit, making it largely invisible.
As long as it is kept sealed, the window unit should be no problem. Despite doubts about the suspended film IGU, Begin said that Alpen provided 13,000 units for a New York City Empire State Building project nine years ago, but has not received any reports of failure.
The latest glass design also allows manufacturers to start using k, which is an inert gas that has better insulating properties than argon. According to Dr. Charlie Curcija, a researcher at Lawrence Berkeley National Laboratory, the optimal gap is 7 mm (about 1⁄4 inch), which is half that of argon. rypto is not very suitable for 1⁄2 inch IGU. The gap between the glass plates, but it turns out that this method is very useful in glass windows where the internal distance between the glass plates or the suspended film is smaller than this distance.
Kensington (Kensington) is one of the companies selling suspended film windows. The company provides k-filled hot mirror units with R-values ​​of up to R-10 in the center of the glass. However, no company fully accepts suspended membrane technology like LiteZone Glass Inc. of Canada. LiteZoneGlass Inc. is a company that sells IGU with a glass center R value of 19.6. how is it? By making the thickness of the unit 7.6 inches.
The company’s chief executive officer Greg Clarahan said that five years have passed since the development of IGU, and it was put into production in November 2019. He said the company’s goals are two: to make IGUs with “extremely high” insulation values, and to make them strong enough to sustain the life of the building. The designer accepted the need for thicker glass units to improve the thermal performance of the vulnerable edges of the IGU.
“The thickness of the glass unit is essential to improve the thermal performance of the overall window, make the temperature inside the glass more uniform and the heat transfer in the entire assembly (including the edges and the frame) more uniform.” said.
However, the thicker IGU presents problems. The thickest unit produced by LiteZone has eight suspended films between two pieces of glass. If all these spaces are sealed, there will be a pressure difference problem, so LiteZone designed the unit using what Clarahan calls a “pressure balance duct”. It is a small vent tube that can balance the air pressure in all chambers with the air outside the device. Clarahan said that the drying chamber built into the tube prevents water vapor from accumulating inside the equipment and can be used effectively for at least 60 years.
The company added another feature. Instead of using heat to shrink the film inside the device, they designed a gasket for the edge of the device that keeps the film suspended under the action of tiny springs. Clarahan said that because the film is not heated, the stress is less. The windows also showed excellent sound attenuation.
Suspended film is a way to reduce the weight of multi-pane IGUs. Curcija described another product called “Thin Triple,” which has attracted widespread attention in the industry. It consists of an ultra-thin glass layer of 0.7 mm to 1.1 mm (0.027 inches and 0.04 inches) between two outer layers of 3 mm glass (0.118 inches). Using k-filling, the device can be packed into a 3⁄4-inch wide glass bag, the same as a traditional double-pane device.
Curcija said that the thin triplet has just begun to take a place in the United States, and its market share is now less than 1%. When they were first commercialized more than a decade ago, these devices faced an arduous battle for market acceptance due to their high manufacturing prices. Only Corning produces the ultra-thin glass that the design relies on, at a price of $8 to $10 per square foot. In addition, the price of k is expensive, about 100 times the price of argon.
According to Kursia, in the past five years, two things have happened. First, other glass companies began to float thin glass using a conventional process, which was to make standard window glass on a bed of molten tin. This can reduce the cost to about 50 cents per square foot, which is equivalent to ordinary glass. The surge in interest in LED lighting has prompted an increase in xenon production, and it turns out that k is a by-product of this process. The current price is about a quarter of what it used to be, and the overall premium for a thin three-layer triple is about $2 per square foot of a conventional double-glazed IGU.
Curcija said: “With a thin three-tier rack, you can increase to R-10, so if you consider a premium of $2 per square foot, it is a very good price compared to R-4 at a reasonable price. A big leap.” Therefore, Curcija expects the commercial interest of Mie IGU to increase. Andersen has used it for its Windows commercial renewal line. Ply Gem, the largest window manufacturer in the United States, also seems interested. Even Alpen continues to promote the advantages of suspended film windows and has discovered the potential advantages of triple film devices.
Mark Montgomery, senior vice president of US window marketing at Ply Gem, said the company currently produces 1-in-1 products. And 7⁄8 inch triplets. “We are experimenting with 3⁄4-in. He wrote in an email. “But (we) can currently achieve higher levels of performance. ”
Don’t seek batch conversion to thin triples right away. But Begin said that the thin glass center layer is easier to process than the suspended film, has the potential to speed up production, and allows the use of warm-edge gaskets to replace the stronger stainless steel gaskets required by some suspended film IGUs.
The last point is crucial. The suspended film that shrinks in the oven will exert considerable tension on the peripheral gasket, which will break the seal, but the thin glass does not have to be stretched, thereby reducing the problem.
Curcija said: “In the final analysis, both technologies provide the same things, but in terms of durability and quality, glass is better than film.”
However, the three-layer sheet drawn by Larsen is not so optimistic. Cardinals are manufacturing some of these IGUs, but their cost is about twice that of traditional three-in-one glass, and the ultra-thin glass in the center of the module has a high breakage rate. This forced the cardinal to use a 1.6mm center layer instead.
“The concept of this thin glass is half the strength,” Larsen said. “Will you buy half-strength glass and expect to use it in the same size as dual-strength glass? No. It’s just that our handling breakage rate is much higher.”
He added that weight-loss triplets also face other obstacles. A big reason is that the thin glass is too thin to be tempered, which is a heat treatment to increase strength. Tempered glass is an important part of the market, accounting for 40% of Cardinal’s total IGU sales.
Finally, there is the problem of rypto gas filling. Larson said that Lawrence Berkeley Labs’ cost estimates are too low, and the industry has done a poor job of providing enough natural gas for IGU. To be effective, 90% of the sealed internal space should be filled with gas, but the industry’s standard practice focuses on production speed rather than actual results, and the gas filling rate in products on the market may be as low as 20%.
“There is a lot of interest in this,” Larson said of the weight-loss trio. “What happens if you only get a 20% fill level on these windows? It’s not R-8 glass, but R-4 glass. This is the same as when using dual-pane low-e. You have everything I didn’t get it.”
Both argon and k gas are better insulators than air, but no filling gas (vacuum) will greatly improve the thermal efficiency, and the R value potential is between 10 and 14 (U coefficient from 0.1 to 0.07). Curcija said the thickness of the unit is as thin as single-pane glass.
A Japanese manufacturer called Nippon Sheet Glass (NSG) is already producing vacuum insulating glass (VIG) devices. According to Curcija, Chinese manufacturers and Guardian Glass of the United States have also begun manufacturing R-10 VIG devices. (We tried to contact the Guardian but did not get a response.)
There are technical challenges. First, a fully evacuated core pulls the two outer layers of glass together. To prevent this, the manufacturer inserted tiny spacers between the glass to prevent the layers from collapsing. These tiny pillars are separated from each other by a distance of 1 inch to 2 inches, forming a space of about 50 microns. If you look closely, you can see that they are a weak matrix.
Manufacturers also struggle with how to create a completely reliable edge seal. If it fails, vacuuming fails, and the window is essentially garbage. Curcija says these devices can be sealed around the edges with molten glass instead of tape or adhesive on inflatable IGUs. The trick is to develop a compound that is soft enough to melt at a temperature that won’t damage the low-E coating on the glass. Since the heat transfer of the entire device is limited to the pillar separating the two glass plates, the maximum R value should be 20.
Curcija said that the equipment to manufacture the VIG device is expensive and the process is not as fast as the production of ordinary glass. Despite the potential advantages of such new technologies, the fundamental resistance of the construction industry to stricter energy and building codes will slow progress.
Larson said that in terms of U-factor, VIG devices may be a game changer, but one problem that window manufacturers must overcome is heat loss at the edge of the window. It would be an improvement if VIG could be embedded in a stronger frame with better thermal performance, but they would never replace the industry standard double-pane, inflatable Low-e device.
Kyle Sword, Pilkington’s North American business development manager, said that as a subsidiary of NSG, Pilkington has produced a series of VIG units called Spacia, which have been used in residential and commercial applications in the United States. The device comes in a variety of configurations, including devices that are only 1⁄4 inch thick. They consist of an outer layer of low-e glass, a 0.2mm vacuum space and an inner layer of transparent float glass. A spacer with a diameter of 0.5 mm separates the two pieces of glass. The thickness of the Super Spacia version is 10.2 mm (about 0.40 inches), and the U coefficient of the glass center is 0.11 (R-9).
Sword wrote in an email: “Most of the sales of our VIG department went into existing buildings.” “Most of them are for commercial use, but we have also completed a variety of residential buildings. This product It can be purchased from the market and ordered in custom sizes.” Sword said that a company called Heirloom Windows uses vacuum units in its windows, which are designed to look like original windows in historical buildings. “I have talked with many residential window companies that can use our products,” Sword wrote. “However, the IGU currently used by most residential window companies today is about 1 inch thick, so its window design and extrusion molding can accommodate thicker windows.”
Sword said that the cost of VIG is about $14 to $15 per square foot, compared to $8 to $10 per square foot for a standard 1-inch thick IGU.
Another possibility is to use aerogel to make windows. Aerogel is a material invented in 1931. It is made by extracting liquid into the gel and replacing it with gas. The result is a nearly weightless solid with a very high R value. Larsen said that its application prospects on glass are broad, with the potential for better thermal performance than three-layer or vacuum IGU. The problem is its optical quality-it is not completely transparent.
More promising technologies are about to emerge, but they all have a stumbling block: higher costs. Without stricter energy regulations requiring better performance, certain technologies will be temporarily unavailable. Montgomery said: “We have worked closely with many companies adopting new glass technology,”-”paints, thermal/optical/electric dense coatings and [vacuum insulation glass]. Although all of these enhance the performance of the window, but the current The cost structure will limit adoption in the residential market.”
The thermal performance of the IGU is different from the thermal performance of the entire window. This article focuses on IGU, but usually when comparing the performance levels of windows, especially on the stickers of the National Window Frame Rating Board and the manufacturer’s website, you will find a “whole window” rating, which takes into account the IGU and window frame performance. As a unit. The performance of the entire window is always lower than the glass center grade of IGU. To understand the performance and complete window of IGU, you need to understand the following three terms:
The U factor measures the rate of heat transfer through the material. The U factor is the reciprocal of the R value. To obtain the equivalent R value, divide the U factor by 1. A lower U factor means higher heat flow resistance and better thermal performance. It is always desirable to have a low U coefficient.
The solar heat gain coefficient (SHGC) passes through the solar radiation part of the glass. SHGC is a number between 0 (no transmission) and 1 (unlimited transmission). It is recommended to use low SHGC windows in hotter, sunny areas of the country to take heat out of the house and reduce cooling costs.
Visible light transmittance (VT) The proportion of visible light passing through the glass is also a number between 0 and 1. The larger the number, the higher the light transmittance. This level is usually surprisingly low, but this is because the entire window level includes the frame.
When the sun shines through the window, the light will warm the surface inside the house, and the indoor temperature will rise. It was a good thing in a cold winter in Maine. On a hot summer day in Texas, there are not so many. Low solar heat gain coefficient (SHGC) windows help minimize heat transfer through the IGU. One way for manufacturers to make low SHGC is to use low-emissivity coatings. These transparent metal coatings are designed to block ultraviolet rays, allow visible light to pass through and control infrared rays to suit the house and its climate. This is not only a question of using the right type of low-emissivity coating, but also its application location. Although there is no information on the application standards for low-emissivity coatings, and the standards differ between manufacturers and coating types, the following are common examples.
The best way to minimize the solar heat gained through windows is to cover them with overhangs and other shading devices. In hot climates, it is also a good idea to choose lower SHGC windows with low-emissivity coatings. Windows for cool climates usually have a low-emissivity coating on the inner surface of the outer glass-two surfaces in a double-pane window, two and four surfaces in a three-pane window.
If your house is located in a colder part of the country and you want to provide some winter heating through passive solar heat harvesting, you want to use a low-emissivity coating on the outer surface of the inner glass (the third Layer surface) window, and display three and five surfaces on a three-pane window). Choosing a coated window at this location will not only get more solar heat, but the window will also help prevent radiant heat from inside the house.
There is twice as much insulating gas. The standard dual pane IGU has two 1⁄8 inch panes. Glass, argon filled 1⁄2 inch. Air space and low-emissivity coating on at least one surface. In order to improve the performance of the double pane glass, the manufacturer added another piece of glass, which created an additional cavity for the insulating gas. The standard three-pane window has three 1⁄8-inch windows. Glass, 2 1⁄2 inch gas-filled spaces, and low-E coating in each cavity. These are three examples of three-pane windows from domestic manufacturers. U factor and SHGC are the levels of the entire window.
The ecoSmart window of Great Lakes Window (Ply Gem Company) contains polyurethane foam insulation in a PVC frame. You can order windows with double-pane or triple-pane glass and argon or K gas. Other options include low-emissivity coatings and thin-film coatings called Easy-Clean. The U factor ranges from 0.14 to 0.20, and the SHGC ranges from 0.14 to 0.25.
Sierra Pacific Windows is a vertically integrated company. According to the company, the extruded aluminum exterior is covered with a wooden structure of Ponderosa pine or Douglas pine, which comes from its own sustainable forestry initiative. The Aspen unit shown here has 2-1⁄4-inch thick window sashes and supports 1-3⁄8-inch thick three-layer IGU. The U value ranges from 0.13 to 0.18, and the SHGC ranges from 0.16 to 0.36.
Martin’s Ultimate Double Hung G2 window has an aluminum extruded exterior wall and an unfinished pine interior. The exterior finish of the window is a high-performance PVDF fluoropolymer coating, shown here in Cascade Blue. The triple-glazed window sash is filled with argon or air, and its U factor is as low as 0.25, and the range of SHGC is from 0.25 to 0.28.
If the three-pane window has a disadvantage, it is the weight of the IGU. Some manufacturers have made three-pane double-hung windows work, but more often, three-pane IGUs are limited to fixed, side-open and tilt/turn window operations. Suspended film is one of the methods used by manufacturers to produce IGU with three-layer glass performance with a lighter weight.
Make the triad easy to manage. Alpen offers a hot mirror film IGU, which is configured with two gas-filled chambers with a 0.16 U factor and 0.24 to 0.51 SHGC, and a structure with four gas-filled chambers, which has a 0.05 U factor, range From SHGC is 0.22 to 0.38. Using thin films instead of other glass can reduce weight and volume.
Breaking the limit, LiteZone Glass makes the thickness of the IGU reach 7-1⁄2 inches, and can hang up to eight layers of film. You won’t find this type of glass in standard double-hung window panes, but in fixed windows, the extra thickness will increase the R-value at the center of the glass to 19.6. The space between the film layers is filled with air and connected to a pressure equalizing pipe.
The thinnest IGU profile can be found on the VIG unit or vacuum insulated glass unit. The insulation effect of vacuum on IGU is better than that of air or two kinds of gases commonly used for isolation, and the space between windows can be as small as a few millimeters. Vacuum also attempts to crash the equipment, so these VIG equipment must be designed to resist this force.
Pilkington’s Spacia is a VIG device with a thickness of only 6 mm, which is why the company chose it as an option for historical preservation projects. According to company literature, VIG provides “the thermal performance of traditional double glazing with the same thickness as double glazing”. Spacia’s U factor ranges from 0.12 to 0.25, and SHGC ranges from 0.46 to 0.66.
Pilkington’s VIG device has an outer glass plate coated with a low-emissivity coating, and an inner glass plate is transparent float glass. In order to prevent the 0.2mm vacuum space from collapsing, the inner glass and the outer glass are separated by a 1⁄2mm spacer. The protective cover covers the holes that draw air from the device and stays in place for the life of the window.
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