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High Elevation Problems Jeopardize Gas-Fill Windows

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In December 1998, Hurd MillWORK agreed to a $5.3 million class-action settlement relating to claims about R-value in gas-fill windows that were shipped with breather tubes installed. Andersen Windows devotes a full page in this year’s residential product catalog to “High Altitude Glass Considerations.” The window industry is struggling with the issue of pressure equalization in sealed insulated glazing when windows are transported over or installed at higher elevations—and some building scientists are concerned that the ultimate loser could well be energy performance.


The “ear popping” that you feel in an airplane or driving in the mountains is evidence that air pressure changes with elevation; as you go up, the air gets less dense. Your ears “pop” as the pressure across your ear drum equalizes by way of your Eustachian tubes—small tubes that connect your inner ears to your throat (that’s why swallowing tends to help). Windows have to deal with the same physics. If a sealed insulated-glass window is manufactured at sea level and installed at 5,000 feet, where the air is less dense, the window glass will bow out. In some cases the glass may even break or the seals fail. Even when the windows are installed at the same elevation where they were made, if they were shipped over a high-elevation pass (across the Rockies to the West Coast, for example), the glass could break or the seals fail en route. For this reason, manufacturers commonly install small hollow tubes that allow the interpane space to equilibrate with the outside air pressure. But what about windows that are filled with a special low-conductivity gas? Will that gas be lost as the windows equilibrate to the outside air? That’s the heart of this story.

Understanding Pressure Within Insulated Glass Units

Glass Deflection with Change in Temperature or Elevation


1. 36” x 36” (910 x 910 mm) IG unit with 3⁄32” (2.4 mm) glass

The impact of elevation change—and other factors that affect the pressure inside an insulated-glass (IG) unit—can be dramatic. Randi Ernst, president of FDR Design, Inc. (a producer of equipment for gas-filling IG units) and a recognized expert on glazing performance, says that “manufacturers generally get concerned with elevation gains of 2,000 feet (610 m) or more and elevation losses of as little as a 1,000 feet (305 m).” Atmospheric pressure decreases by about one pound per square inch (6,900 N/m2) for an elevation rise of 2,000 feet (610 m). For a 2’ by 4’ (0.6 m x 1.2 m) window, that 2,000’ elevation rise puts an additional 1,150 pounds (5,120 N) of force on each piece of glass in the IG unit! Small elevation changes can cause modest glass deflection, resulting in slight visual distortion and a drop in thermal performance due to changes in the interpane space. With large elevation changes, the pressure may be great enough to cause seal or glass failure.

Barometric pressure and temperature also play a role in this. A fluctuation of barometric pressure between an intense storm (low-pressure system) and a following clear day can be equivalent to 2,000 feet (610 m) of elevation change. Similarly, if windows are hot when sealed, it is equivalent to manufacturing them at a higher elevation. A rough rule of thumb is that an 18°F (10°C) increase in temperature is equivalent to manufacturing an IG unit at 1,000 feet (305 m) higher.

The susceptibility of an IG unit to glass deflection, seal failure, and breakage from pressure differences also depends on the unit’s characteristics. Thicker glass that is less flexible is more likely to break or cause the seals to blow out. Long, narrow glazing units are more likely to fail than ones that are closer to square. Tempered glass is far less prone to breakage. And seal quality determines how well the seal will hold. Glass deflection for a moderate-sized, sealed IG unit at reduced temperature and various elevation changes is shown in the table below.

Equalizing Pressure With Breather Tubes



The stainless steel capillary tube at the top of this penny measures just 0.031 inches (0.789 mm) in diameter.

Photo: FDR Design, Inc. Photo: FDR Design, Inc.

Many window manufacturers install some type of breather tube in windows to allow for pressure equalization. At one time, short, fairly thick breather tubes with 18” (3 mm) openings were installed. These are seldom used today because they were difficult to install and had to be crimped shut to prevent the IG unit from fogging (the most common evidence of seal failure). Today, most tubes are much longer (12”—300 mm) and thinner (0.021”—0.53 mm) and usually called capillary tubes. Because capillary tubes are so thin, gases flow through very slowly, and some experts argue that they can be left open without resulting in fogging. There is a myth that tubes can be sized to allow smaller air molecules to pass through but not larger molecules like water vapor. “The idea that any manufacturer has access to stainless steel vent tubes that are sized or designed to accomplish this selective gas movement is complete folly,” says Dr. Ken Abate, the new Director of Research, Development and Quality at Hurd Millwork. “I tell all of our staff and distributors to call the person’s bluff when they hear this and demand proof, because it just is not possible.” In fact, notes Ernst, a water molecule is actually smaller in diameter (2.8 angstroms) than the nitrogen molecules (3.6 angstroms) that comprise roughly 78% of our air. When windows are filled with a special low-conductivity gas, such as argon or krypton, it is clear that those gases will be lost over time if capillary tubes are left open. It is also likely that water vapor will gradually enter the IG unit, resulting in fogging if enough moisture gets in. Just how long the IG unit can survive before fogging depends on the amount of water vapor in the outside air and the quantity of desiccant in the glazing spacer. (Desiccants are commonly put in glazing spacers to absorb water vapor that leaks into IG units.) According to Ernst, “if any tube is left open connecting the sealed insulated glazing unit and the environment, then pumping action—from barometric pressure changes, wind loading, or events as common as a slammed door in the house—will result in all of the gas fill in the unit eventually being replaced with air. And this does not even include a discussion of the partial pressure differential of argon gas inside and outside the window.” Clearly, capillary tubes left open pose some serious questions regarding the service life of the sealed insulated glass—whether or not argon or krypton is used.

Hurd Millwork’s Woes

Hurd, like many window manufacturers, commonly installed capillary tubes in their gas-filled units when they knew that the destination involved significant elevation changes—with Hurd this included Mountain and Western states. The Hurd process reportedly called for the tubes to be taped closed during transport, temporarily opened for pressure equalization upon arrival, and then permanently crimped shut during installation to prevent further gas exchange. The amount of gas exchanged in this process is small and has little effect on the relative concentration of gas fill—or the risk of water vapor intrusion. If, however, the tubes were left open and not crimped shut, complete loss of the gas fill would occur over time, and the energy performance of the window would drop. The heart of Hurd’s legal problem lay in the company’s claim of higher thermal performance associated with gas fill and the need for action outside of the company’s control to ensure this performance.

According to a reliable source, Hurd settled the case because the legal costs were threatening the company’s very survival, and there was (and still is) no easy way to measure (and therefore prove) the gas-fill content of windows in the field. The fear is that the Hurd case will discourage innovative thermal improvements in the industry. “The fenestration industry shuddered,” says Ernst. “Not only are high-performance units in danger, but any sealed units may represent a large liability. There simply is not enough research and development in window technologies to support the increasing demands placed by building codes and government energy-efficiency initiatives,” he told EBN. Without the R&D support, new technologies may be hitting the market before enough investment is made in verification and modification.

Finding Solutions

Window manufacturers are faced with some daunting challenges relating to maintaining tight seals in IG units shipped to different elevations.

Preventing fogging is key with all IG units, and containing low-conductivity gas fills is necessary with most high-performance windows. Here are some options:

1.Do nothing. Although there are a number of large national window manufacturers distributing to higher elevation areas or transporting product over high elevations, there are many smaller companies for which elevation change may not be an issue. These companies may be comfortable simply sealing their IG units at the factory—with or without gas fill. End of story.

2.Selectively install tubes. Andersen worked out in great detail which of their units require pressure equalization and at which elevations. Tempered glass can be selected to avoid use of tubes in many of their units up to 10,000 feet (3,000 m) but tubes are required in all units destined for over 10,000 feet. These “custom” units with the tubes would presumably be accompanied with a different energy label, given that Andersen explicitly states in the Product Guide on page 209 that the tubes “will result in a less effective insulating glass.” According to Bipen Shah, Program Manager for Engineering at the National Fenestration Rating Council (NFRC), “if a window unit leaves the point of manufacture with any device that permits exchange between the glazing unit and the environment, then no credit for the argon’s contribution to thermal performance may be taken.”

3.Install capillary tubes that need to be crimped closed during installation. This was Hurd’s seemingly reasonable solution. For two reasons, however, no manufacturers currently take this approach: 1) the NFRC policy prevents them from taking any rating value from the gas fill, and 2) the liability risk is just too great—at least for U.S. manufacturers. (Leftover funds from the Hurd settlement—reportedly some $50,000 —are just now being donated to two local Habitat for Humanity affiliates on the West Coast, per terms of the settlement.)

4.Install tubes in all units and drop gas fills. Window companies supplying national distributors or “big box” retailers could install permanently-open capillary tubes in all units and eliminate gas fills altogether. Indeed, there may already be a shift in this direction, according to one industry analyst who wished to remain anonymous. This would result in significant losses in energy performance and leave unresolved the issue of moisture entry into IG units.

5.Install a high-tech closed-tube system. Some building scientists EBN spoke with referred to proprietary systems that handle elevation and associated pressure change with a bladder or a liquid reservoir. “We investigated a bladder system and found it either unworkable or impractical or both,” says Steven Thwaites of Thermotech Windows in Ottawa, Ontario. Bernie Herron, a technical service engineer for Cardinal IG, one of the largest producers of sealed insulated glass in the U.S., says the company is “not aware of any successful [closed-system] mechanism. There have been efforts, but the range of pressures the unit encounters makes the point and range of activation a real problem.”

6.Go to regional manufacturing. By regionally producing IG units, manufacturers could produce windows that were pressurized to the local atmospheric conditions. Carl Wagus of the American Architectural Manufacturer Association (AAMA) indicated that the elevation issue has some manufacturers seriously considering this option. (Weathershield already has manufacturing facilities in both Wisconsin and Utah.) Only IG units would need to be manufactured regionally; frames and sashes could still be produced centrally.

7.Pre-pressurize IG units for the intended destination. Randi Ernst points out that if manufacturers knew where windows were going, they could adjust the gas-fill pressure to be approximately neutral at the installation location—and there would be little if any deflection. This could be done by adjusting the temperature at which IG units are filled and sealed, though architect and window expert Nehemiah Stone, of the Heschong Mahone Group in Fair Oaks, California, suggests that this “may require more sophistication than the industry has demonstrated capacity for.” Also, preventing seal failure or glass breakage in transit could still be a problem.

8.Reduce the glass spacing. By reducing the spacing between the layers of glass, the volume of air (or gas) is proportionately reduced, so there will be less pressure and therefore less deflection or failure. This solution might appropriately be implemented with use of the higher-performance krypton and xenon gases, which have thinner optimal spacings.

The Bottom Line

The Hurd lawsuit may be one of those good news/bad news issues. On the one hand, it is forcing the window industry to take a closer look at the larger issues of long-term durability and quality control of insulating glass. If this forces manufacturers to improve seals and gas-fill retention, we will all reap the economic and environmental benefits (energy savings and enhanced durability). But on the other hand, this issue offers the very real threat that manufacturers will abandon low-conductivity gas fills altogether. Manufacturers, particularly those distributing nationally, may begin installing capillary tubes in all their windows—relying on desiccant in the spacers to absorb the additional moisture that will flow into the insulated-glass units. There have been very real energy savings (and commensurate environmental benefits) from improvements in window technology, including argon and krypton gas fill. Let’s not leave those benefits behind as a quick fix to the pressure equalization challenges faced by window manufacturers. If specifiers and buyers of windows continue to insist on high energy performance, manufacturers will find reliable ways to provide gas fills that work—even when windows have to be shipped to different elevations. Our recommendations? First, insist on the highest performance windows within your budget. In most cases, these will be low-e, gas-filled windows. Second, avoid breather tubes or capillary tubes. If you are building or designing in areas where elevation is a concern for sealed insulated-glass windows, choose a window company that deals responsibly with this issue and—if at all possible—uses a solution other than tubes. Bear in mind, however, that there are lots of other performance and durability differences among windows—the presence or absence of capillary tubes is only one of the considerations you should weigh. And third, if you must select windows with capillary tubes, do not attempt to crimp the tubes shut during installation unless such action is specifically called for by the manufacturer. The tubes are generally hidden in the sash and inaccessible. Even if you could get to the tubes, they are not designed for site modification and doing so would risk damaging the tubes or the seals or both (and almost certainly void the warranty). If the windows have capillary tubes, the length of the warranty against seal failure (fogging) is particularly important. Look for at least a 15-year warranty. As a final note, the issues of seal and glass failure with elevation and of long-term durability for all windows are not going to go away. EBN will continue to follow and report on these issues as well as industry response. It is very conceivable that entirely different solutions will emerge in the coming years.

For more information:

National Fenestration Rating Council

1300 Spring Street, Suite 500

Silver Spring, MD 20910


301/588-6342 (fax)

American Architectural Manufacturers Association

1827 Walden Office Square, Suite 104

Schaumburg, IL 60173


847/303-5774 (fax)

Randi Ernst, President

FDR Design, Inc.

303 12th Avenue S.

Buffalo, MN 55313

763/682-6096, 763/682-6197 (fax)

Andersen Window Care Solutions

888/888-7020 (toll-free)

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April 1, 2000