New Methods to Make R-5 Affordable

Asymmetric triple-pane construction and advanced gas filling techniques provide new alternatives to improve performance numbers
By Mike McHugh, Integrated Automation Systems
July 30, 2012
FEATURE ARTICLE | Operations, Energy Efficiency, Methods & Techniques

Conventional wisdom says R-5 windows are too costly to manufacture. This assumption is largely based on experience employing older technologies for fabricating high-performing triple IG units.

Several recent developments challenge the assumption that R-5 cannot be reached affordably. One is the addition of 4th surface low-E glass coatings in dual lite units. Another is the introduction of automation that can dramatically reduce the cost—and raise the quality—of triple IG units.

This article focuses on two other alternatives. First is the use of asymmetric triples—IG units with two airspace cavities of two different thicknesses. The second is the use of more intelligent gas-filling techniques that can dramatically reduce the cost of using krypton to reduce U-values.

 Fig. 1

Why Asymmetric Matters
To better understand the benefits of using two different airspace widths in an IG unit, it is important to understand the relationship between airspace thickness, gas type and center of glass (COG) U-value. Fig. 1 depicts the following significant data:

  • The optimal airspace thickness for an air filled IG is close to ½ inch with a COG U-value of approximately 0.29. If the airspace thickness is decreased to ¼ inch, the U-value is diminished to only 0.42.
  • The optimal airspace for 90 percent argon is close to .45 inch, with a COG U-value of 0.24. If the airspace thickness is decreased to ¼ inch, the U-value is diminished to 0.33.
  • The optimal airspace for 90 percent krypton is close to 0.3 inch with a center of glass U-value of 0.22. If the airspace thickness is decreased to ¼ inch, the U-value achieved is still 0.24.

If there is a mixture of 80 percent krypton and 20 percent argon, the U-value curve is equal to or slightly better than the 90 percent krypton. Although it is not depicted on the graph, a mixture of 80 percent krypton, 15 percent argon and five percent air is almost exactly the same as 90 percent krypton.

Once the above information is digested and then connected with the fact that many sash designs are capable of glazing up to a 1-inch thick IG unit, it makes sense to optimize the proper airspace with the proper gas while taking into consideration total U-value and cost as well as ease to manufacture. One example of this kind of optimization would be an asymmetric unit made of three lites of 2.5 mm clear with a double silver coating on surface #2 and #5. In airspace 1—0.48 inch thick—a mixture of 90 percent argon and 10 percent air is used. In airspace 2—¼ inch thick— 80 percent krypton, 15 percent argon and 5 percent air are used. Using standard simulation technique, such a unit in a generic vinyl window is calculated to have a 0.203 U-value and a rounded U-value of 0.20 (true R-5). The gas cost would be $5.94 or $0.59 per square foot.

Even though much less krypton is used in this unit, matching the right airspace with the proper gas yields a center-of-glass U-value rating of approximately 0.13. This is accomplished with a gas cost reduction of more than 85 percent compared to a symmetrical IG with traditional-filled 90 percent krypton in both airspaces.

Also of note, muntin bars reduce the window U-value if there is less than a ⅛ inch gap between the grid and glass. If a muntin is needed in this window, the wider airspace with argon gas provides an opportunity to insert it with no thermal penalty. This design also avoids the nearly impossible task of inserting 90 percent krypton into an airspace that contains muntins.

Finally, the asymmetric design also enhances sound insulation.

More Intelligent Gas Filling
Intelligent gas filling requires computer controlled equipment, capable of automatically filling argon or krypton or custom mixes of argon and krypton. To understand the dramatic benefits of improved gas filling techniques, it is first necessary to understand how gas filling is performed at most window manufacturing operations. Most conventional krypton filling processes insert krypton into the bottom of a unit in a way that pushes the lighter air out of an exhaust port in the top. At this exhaust port, there is typically a sensor that is measuring the percentage of oxygen in the exhaust.

A manufacturer claiming 90 percent krypton in its units will typically set its machinery up to reach a target of 95 percent. This allows some room for error while still meeting the posted U-value on the National Fenestration Rating Council label.

Since oxygen is approximately 20 percent of the earth’s atmosphere, conventional systems will continue to fill krypton until 1 percent oxygen is observed in the effluent to reach the above goal. This method correctly allows the manufacturer to infer that if 95 percent of the oxygen in the unit is displaced, there will be 95 percent krypton in its place.

There are two problems with this method. First, when the process registers 2 percent oxygen in the effluent, it is continuing to pump krypton gas into the unit even though 80 percent of the effluent is the expensive krypton gas the manufacturer does not want to waste.

 Fig. 2

Second, most oxygen and TCD sensors have significant delays between sensing the presence of a gas and indicating the proper output of what it is sensing. This delay is responsible for additional consumption because the amount is not immediately registered. This is why the Fig. 2 shows no krypton waste until after 3 liters of consumption, even though the cavity only holds 2.45 liters.

Fig. 2 depicts the relationship between percentage of gas sensed in the exhaust and the amount of krypton consumed. It is noteworthy that nearly 6.5 liters of krypton is consumed to get the sensor to 1 percent oxygen in the exhaust. This represents more than a 100 percent loss in this type of process.

Fig. 3 represents how krypton is lost when comparing the amount of liters consumed to the percentage of krypton that remains in the unit. This chart shows that to reach a percentage fill rate in the high 90s, the amount of krypton wasted becomes very large. It also shows that very little krypton is lost—less than 3 percent—until the krypton fill level exceeds 85 percent.

Intelligent gas filling drastically reduces cost because it does not rely on “exhaust sensing” because it knows the requirements of a unit and is proactively capable of dosing the required gas amount. Since it is not sensing exhaust gasses, the inherent delay incorporated into sensors is not a factor.

It is also known that 80 percent krypton injected into an argon environment performs equally to a 90 percent krypton /10 percent air mix. As the waste associated with producing an 80 percent krypton fill is less than 3 percent, the reduction in krypton used overall to reach a targeted U-value is dramatic. Smart filling technologies enable manufacturers to achieve the same U-value using 0.8 liters of krypton for each liter of required airspace compared to 2 liters of krypton for each liter of required airspace with traditional systems. This is a savings of more than 60 percent in krypton.

With asymmetric IG, a manufacturer can save even more. There are many ways to use intelligent gas filling technology to cost effectively achieve targeted results. Table 1 presents a range of examples.

Precise, computer-controlled gas filling allows the manufacturer to engineer the exact U-value and cost for different product lines and for different regions. A manufacturer can reduce krypton percentages to achieve a targeted U-value that is engineered for different products or Energy Star zones. There are many options using the above combinations and no wrong answers.

If there is a wrong answer, it is that R-5 windows are not affordable. With a gas cost premium of less than $1.00 per square foot of glazing, the industry can provide solutions to the rising energy costs, as well as CO2 reductions.

Sample IG Unit Configurations and Performance Levels

3/4-inchAirspace 1 FillAirspace 1 ThicknessAirspace 2 FillAirspace 2 ThicknessExact U-ValueRounded U-valueGas CostGas Cost per sq. ft of Glass
Dual 60/Clear100% air0.563 inchNANA0.3320.33BaseBase
Dual 60/Clear90% argon/10% air0.563 inchNANA0.2960.30$0.08$0.01
Dual 60/60090% argon/10% air0.563 inchNANA0.2850.26$0.08$0.01
Dual 60/60040% krypton/55% argon/5%air 0.563 inchNANA0.2540.25$6.66$0.67
7/8-inchAirspace 1 FillAirspace 1 ThicknessAirspace 2 FillAirspace 2 ThicknessExact U-ValueRounded U-valueGas CostGas Cost per sq. ft of Glass
Dual 60/Clear100% air0.687 inchNANA0.3390.34BaseBase
Dual 60/Clear90% argon/10% air0.687 inchNANA0.3020.30$0.10$0.01
Triple 60/Clear/6085% argon/10% krypton/5% air 0.292 inch85% argon/10% krypton/5% air 0.292 inch0.2300.23$1.75$0.17
Triple 60/Clear/6075% argon/20% krypton/5%air 0.292 inch75% argon/20% krypton/5%air 0.292 inch0.2240.22$3.46$0.35
Triple 60/Clear/6055% argon/40% krypton/5% air 0.292 inch55% argon/40% krypton/5% air 0.292 inch0.2130.21S6.87S0.69
Triple 60/Clear/6035% argon/60% krypton/5% air 0.292 inch35% argon/60% krypton/5% air 0.292 inch0.2030.20$10.28$1.03
Triple 60/Clear/6015% argon/80% krypton/5% air 0.292 inch15% argon/80% krypton/5% air 0.292 inch0.1940.19$13.70$1.37
1-inchAirspace 1 FillAirspace 1 ThicknessAirspace 2 FillAirspace 2 ThicknessExact U-ValueRounded U-valueGas CostGas Cost per sq. ft of Glass
Triple 60/Clear/6090% argon/10% air0.354 inch90% argon/10% air0.354 inch0.2200.22$0.05Base
Triple 60/Clear/6085% argon/10% krypton/5% air 0.354 inch85% argon/10% krypton/5% air 0.354 inch0.2130.21$2.13$0.21
Triple Assymetric A 60/Clear/6090% argon/10% air0.479 inch15% argon/80% krypton/5% air 0.250 inch0.2030.20$5.94$0.59
Triple Assymetric B 60/Clear/6075% argon/20% krypton/5%air 0.417 inch15% argon/80% krypton/5% air 0.313 inch0.1940.19$8.25$0.82
Triple Assymetric B 60/Clear/60075% argon/20% krypton/5%air 0.417 inch15% argon/80% krypton/5% air 0.313 inch0.1840.18$8.25$0.82
Table 1–U-values are for a generic vinyl window using standard NFRC simulation and calculation procedures. Glass used is Sungate 60 and Sungate 600 from PPG Industries




Mike McHugh is the president of Integrated Automation Systems, based in Aurora, Ohio.  The company manufactures the OptiGas and FastGas intelligent gas filling systems, as well as the ThermalCert and ThermalCheck thermal verification systems. He can be reached at A technical paper with details on the various ways to hit thermal performance targets for windows with different ¾-, ⅞- and 1-inch insulating glass configurations is available at