Sealed insulated glass units fabricated near sea level and shipped to Colorado’s Front Range or mountain corridors face a physics problem that no amount of careful handling can override: atmospheric pressure decreases with elevation, and the pressure inside those units does not.
High Altitude Windows: The Problem and the Quick Answers
Most insulated glass units are manufactured at low elevation factories, then trucked to installation sites thousands of feet higher. The glass panes arrive holding internal pressure that matches the factory altitude. Once installed at higher elevations, the lower external atmospheric pressure creates a pressure differential that pushes the glass outward. That sustained force stresses spacers, sealants, and the glass itself, which is why buildings in mountainous regions need insulated glass planned for altitude rather than standard units.
Here is what facility managers across the Denver Metro, I-70 corridor, and towns like Vail and Aspen see when altitude is ignored:
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Glass panes bowing or “pillowing” outward, visible as distorted reflections
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Fogging or clouding between panes from compromised edge seals
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Premature seal failure and loss of gas filling from the IGU cavity
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Glass breakage during transport over mountain passes or at installation
Fogging is often the first symptom building owners notice, and our article on why insulated glass fogs in commercial windows explains how moisture gets past a failed seal. ONE Glass Service handles commercial high altitude windows, reglazing, and replacement across the Denver Metro and Colorado mountain markets. This guide covers the mechanisms behind altitude-related glass problems and what to do about them.
How Insulated Glass Units Work at Low and High Elevation
An insulated glass unit consists of two or three glass panes separated by a spacer, with the perimeter sealed and the cavity filled with air or an inert gas like argon. This sealed unit acts as a captive air system: the internal pressure is locked at whatever atmospheric pressure and temperature existed at the factory.
Most commercial insulated glass units are fabricated at relatively low elevation. When they are shipped to the Denver market or mountain towns, the pressure inside remains higher than the surrounding atmosphere.
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Glass units may bow out if manufactured at low elevation and installed higher. The lower external atmospheric pressure cannot counteract the trapped higher pressure, so the panes flex outward.
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The reverse also occurs. An IGU manufactured at high altitude and transported to low elevation will experience inward “dishing” as the now-higher outside pressure pushes the panes inward.
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Both conditions load the spacer and sealant unevenly, degrading window performance and shortening service life.
Pressure, Bowing, and Structural Integrity at High Altitudes
Pressure differentials act as a constant, unrelenting load on every lite and seal in the unit. Spread across a large pane, even a modest pressure difference adds up to significant force, and over the elevation changes common in Colorado that force can permanently deform seals and fatigue glass.
Pressure differentials show up as glass distortion in two forms:
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Pillowing (convex bowing): Stand inside a commercial building and look at the window from an angle. If the center of the glass pane bulges outward, the unit is pillowed. A straightedge held against the glass will show a gap at center.
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Dishing (concave bowing): The pane center appears to sink inward. This is less common in Colorado installations (since most units come from lower elevations) but relevant for units shipped downhill.
Long-term pressure stress causes micro-movement at the spacer, perimeter sealant fatigue, and eventually gas loss and moisture intrusion. Temperature swings in the Rockies compound the problem. A storefront in a mountain town can see sun-warmed glass during the day and freezing temperatures that night, and each cycle expands and contracts the trapped air or gas, adding stress to seals already under altitude-driven pressure load.
Large IG units with thin glass are more vulnerable to deflection. Curtain wall and storefront window systems with wide spans deserve extra attention because deflection grows with span. Distorted reflections, difficulty opening adjacent doors or operable windows, and recurring fogging are early warning signs of compromised structural integrity.
Capillary Tubes, Breather Tubes, and High Altitude Windows
Capillary tubes and breather tubes are small tubes inserted through or near the spacer of an insulated glass unit. Their purpose is straightforward: they allow the pressure inside the sealed cavity to equalize with the outside atmosphere. Pressure-equalization is a common manufacturer recommendation for high-altitude windows because it prevents the bowing and seal damage described above.
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Breather tubes allow pressure equalization during shipping. A glazing unit leaving a low-elevation factory with the tube open can equalize gradually as the truck climbs to installation altitude. Capillary tubes do the same job through a smaller-diameter opening.
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Once the unit arrives and acclimates at the installation elevation, the tube is crimped and sealed with butyl or similar material, locking the cavity at the correct altitude pressure.
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Tubes cannot selectively retain argon while blocking moisture. An open tube allows both gas escape and humidity transfer, and that tradeoff affects which strategy a glazier chooses.
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For modular or relocatable buildings that move between large elevation changes, a permanently open tube or a different pressure-management strategy may be needed instead of a one-time crimp-and-seal approach.
On existing buildings, tube problems often surface only when units start to fail. ONE Glass Service evaluates high altitude windows with breather tubes as part of our commercial reglazing services, verifying whether tubes were properly sealed and whether the original pressure-equalization strategy is still performing.
Gas Fills, Low-E Glass, and High Performance at Altitude
High performance windows in commercial applications rely on argon-filled insulated glass and low-e coatings to control heat transfer. Argon improves thermal performance compared to air-filled cavities, which is why it is the standard gas filling for energy-conscious projects. But altitude complicates argon retention.
When a capillary or breather tube is left open, argon diffuses out and ambient air replaces it, so pressure-equalized units can lose some thermal performance. Some high altitude glass strategies accept that tradeoff, using air-filled units with open tubes to prioritize seal integrity over peak thermal performance. Others use sealed, gas-filled units equalized and crimped at the final installation altitude, maintaining energy efficiency while also protecting against pressure stress.
Mountain regions also bring intense UV exposure and severe temperature swings. Low-e coatings play a direct role here by reducing solar heat gain, improving U-factor, and managing glare. Understanding how low-e glass works in commercial facades helps facility managers select the right coating surface and emittance for their climate. At altitude, stronger solar radiation and wider daily temperature ranges make correct low-e selection just as important as managing internal pressure in the glazing unit.
Facility managers should confirm with their glazier whether published window performance data (U-factor, solar heat gain coefficient) assumes argon or air, and whether any capillary tubes will remain open in service. That distinction determines whether the installed unit will match the spec sheet.
Designing and Ordering Insulated Glass Units for High Altitude Projects
Most altitude-related window damage is preventable when design teams, contractors, and glaziers plan for elevation before the first unit is ordered.
Give your glazing contractor or window manufacturer this information up front:
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Project location and installation altitude
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Fabrication altitude, if constrained to a specific plant
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Shipping route, especially if crossing passes along the I-70 corridor
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Whether any units will later move to a different elevation (phased campus construction, for example)
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IGU size, glass thickness, spacer type, and whether gas filling is required
Coordinate glass thickness and spacer width with both structural and energy efficiency goals. Thicker glass and narrower cavities resist deflection better under pressure differences. Decide on the capillary or breather tube strategy before ordering: no tubes, tubes sealed on site, or permanently open tubes for air-filled applications. Document the plan in submittals and shop drawings so every party, from architect to installer, knows the approach.
Properties in mountain towns need extra attention. Commercial glass projects in Vail, Colorado and commercial properties in Aspen face UV exposure, freeze-thaw cycles, and snow loads that compound altitude-driven seal stress. Local code requirements in these jurisdictions may also affect glazing performance specifications. ONE Glass Service can assist with takeoffs and replacement strategies for existing buildings whose original glass units were not designed with high altitude in mind.
What to Ask Your Glazier About High Altitude Windows
Before ordering replacement insulated glass units or specifying new architectural glass for a high altitude project, use this checklist. Tubes and equalization only work if the glazier’s process accounts for them correctly.
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Fabrication vs. installation altitude: What is the elevation difference? How does the supplier adjust IG units for that gap?
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Shipping route: Will units cross passes with large elevation changes? Are tubes open during transport?
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Tube strategy: Will units include capillary or breather tubes? Are they for transport only, or will they remain open? Who crimps and seals them, and at what point in installation?
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Thermal performance assumptions: Do U-factor and SHGC values assume argon or air? Will those values hold once tubes are sealed at elevation?
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Structural limits at altitude: What glass thickness, aspect ratio, or pane size limits does the glazier recommend for curtain wall and storefront systems to protect long term performance?
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Entrance doors and operable units: How will pressure changes affect operation and seal integrity of door and window units over time?
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Inspection and maintenance: How should the facility team monitor for bowing, fogging, or edge seal distress? What triggers a call for professional reglazing or replacement?
Partnering with ONE Glass Service for High Altitude Insulated Glass
ONE Glass Service helps commercial clients across the Denver Metro, Colorado mountain towns, and Wyoming manage the challenges of high altitude insulated glass. Services include diagnosing fogged or bowed glass, reglazing failed IG units, replacing commercial windows and doors, and restoring sealant and surrounding framing to protect long term window performance.
If you are planning a high altitude window project or dealing with existing problem glass, contact ONE Glass Service to schedule an evaluation. Call (303) 505-3194 to speak with a glazing professional who understands how pressure, altitude, and insulation interact in Colorado’s commercial buildings.