Elevating Efficiency: Solutions for Window Performance

The Bottom Line: High-altitude locations present unique challenges for maintaining energy efficiency in window systems, requiring advanced argon retention technologies, such as bagged capillary tubes and pre-equalized IGUs, to meet stringent energy codes while optimizing thermal performance and minimizing IGU thickness and weight.

The vista from a mountainside retreat can be breathtaking in more ways than one as window wall systems and large openings continue to be a dominate design trend.

However, the view from high altitudes presents a unique challenge when it comes to complying with regional energy codes.

This is where current and emerging argon “retention” technologies come in. Whether it’s “bagging” a capillary tube or using the latest pre-equalized insulating glass units, any sharp elevation change from fabrication will require some form of pressure management, ensuring that the energy efficiency of buildings remains within the increasingly stringent energy specs.

Failure to use these technologies means that the thermal performance of an IGU cannot (i.e., shouldn’t) be calculated using argon and may very well leave your window system outside of the parameters set by the region.

Navigating the thin air of high-performance glass

Take, for example, a ¼-inch double-glazed IGU with a common low-emissivity coating on surface #2: without argon the U-value is 0.287 Btu/hr*ft²*F and with argon is the U-value is 0.239 Btu/hr*ft²*F. Argon contributes to a 16.73% increase in overall efficiency. This is a significant advantage in a real-world scenario.

Excluding any performance trade-offs, keeping strictly prescriptive, and assuming a window frame will arbitrarily increase the U-value of the window assembly, here are several instances where ensuring argon is retained means keeping with a double-glazed IGU versus moving to a triple-glazed IGU. For the purposes of this example, we’ll say the window assembly is 0.03 Btu/hr*ft²*F.

Because of argon, a double-glazed IGU can be used, even in the Northern zone of the newest Energy Star 7 program.

You get the picture—argon is important, but it’s not always easy to just fill up the window. 

The location where these IGUs are fabricated is important. For example, atmospheric pressure (excluding weather) at sea level is 14.7 PSI. At 2,000 feet above sea level the atmospheric pressure is 13.7 PSI (7% loss), 5,000 feet is 12.2 PSI (17% loss) and 8,000 feet is 10.9 PSI (25% loss). Thus, an IGU fabricated at sea level will attempt to “bow out” as the pressure inside can be up to 25%+ greater than the outside. The opposite is also true for fabricators at elevation.

Where the unit is fabricated has major impacts on concavity, flatness and primary seal resilience.

The issue with capillaries

The main way to alleviate concavity or convexity, and the risks that come with it, was to use capillary tubes (breather tubes). These are tried and true. The issue, however, resides in venting to the atmosphere.

How much argon is lost? In the field, this is hard to measure, if ever. This is why IGUs that undergo large elevation changes have traditionally been calculated with air opposed to argon. But as we can see in the previous diagram, performance codes are tightening the reins on capillary tube technology.

The benefits of current technology

There are a couple of options on the market, which allows one to calculate the thermal performance inclusive of argon.

• Bagged capillary tubes—Using a patented mylar bag attached to the capillary tube allows the capillary tube to inhale-exhale argon as the IGU goes through elevation changes in transport. The tube is then crimped off at the final installation site.
• Pre-equalized IGUs—A pre-equalized IGU is sealed at the plant with a pre-determined pressure appropriate for the final installation location. Often this means the glass is concave-convex leaving the facility and arrives fully flat at the final location.

Both options have pros and cons illustrated in the chart above.

In many cases, pre-equalized units offer peace of mind that the unit is fully sealed and allows for rapid install. However, shipping routes, temperature and onsite availability must be considered. If the IGU dwells in a yard much below its rated altitude and it is cold, the glass may touch due to further reduction in air density. Bagged capillary tubes are a great solution but carry some added risk that accompanies all capillary tube installation and may require dwell time at site.

As a window manufacturer, both choices provide optimal thermal performance. Being aware of each option’s strengths and weaknesses will help navigate expectations from the homeowner, architect and window dealer.

Charting the course for an elevated world

The path to sustainable design at higher altitudes is one carved with deliberate choices and technical innovations. The desire to keep sightlines, window thickness and weight to a minimum often means relying on double-glazed IGUs. Including argon in the thermal performance calculation is pivotal in maintaining code compliance.

“The City of Aspen is a great example of a local government with a very aggressive energy code pushing high-performance windows. They recognize the beneficial impact of including argon and the current technology needed to implement it,” says Thomas Culp of Birch Point Consulting. “They want to help enable the window manufacturers, dealers and builders by saying, ‘Hey yes, you can use these new products, and we accept your thermal performance calculations because of this.’”

High altitude may be synonymous with low oxygen, but in the world of architectural glass, it’s where ideas and technologies need to breathe the most. Together with window manufacturers, leading fabricators are paving the way for better thermal performance of the overall window system.