Fabrication processes

Capillary & breather tubes in IGUs

An insulating glass unit is a sealed package of gas. Move that sealed package to a meaningfully different altitude and the trapped gas expands or contracts, bowing the glass and straining the edge seals. Capillary and breather tubes solve that by letting the cavity breathe, but they come with a real trade-off you have to design around.

6 min read Intermediate IGU fabrication

Why altitude bends a sealed unit

An insulated glass unit (IGU) traps a fixed amount of air or gas between its panes, sealed at the pressure, altitude, and temperature of the factory. That trapped gas now behaves like a spring. If the unit is taken to a higher altitude, the outside air pressure drops, so the trapped gas pushes outward and both panes bow out, a "pillowed" or convex look. Taken to a lower altitude, the higher outside pressure squeezes the panes inward (concave). Temperature does the same thing: a hot day expands the cavity gas, a cold day contracts it.

A little flex is normal and harmless. The problem is a large, sustained difference between where the unit was made and where it ends up, or a steep mountain pass on the delivery route. Sustained bowing means constant tension on the edge seals (fatiguing them and shortening unit life), visible distortion in reflections, and in extreme cases the centers of the two panes can touch or the glass can break.

Sealed unit at high altitude panes bow out (pillowing) constant stress on the edge seals With a capillary tube slow air exchange pressure equalizes, panes stay flat
Left: a sealed unit carried to high altitude, the trapped gas expands and pillows the panes outward, loading the seals. Right: a thin capillary tube through the spacer lets the cavity pressure equalize with the outside air, so the unit ships and sits flat.

What a capillary tube is

A capillary tube is a very thin metal tube inserted through the spacer and edge seal so it connects the sealed cavity to the outside air. Its bore is deliberately tiny: small enough that air and water vapor move through it only very slowly (by capillary action and diffusion), but open enough that the cavity pressure can equalize with ambient over time. That is exactly what you want for shipping a unit over a mountain pass or installing it at a high-elevation site: the unit travels and settles flat instead of ballooning.

Capillary tube vs. breather tube

The terms are often used loosely, but there is a practical distinction:

  • Capillary tube: very fine bore. Equalizes pressure slowly while limiting how much humid air can get in. It is commonly crimped and sealed shut after the unit has reached and equalized at its final elevation, returning the unit to a sealed (but not gas-filled) state.
  • Breather tube: a larger-bore tube, sometimes left open, used when the pressure swing is large (for example during transport across a high pass). It allows faster, freer air exchange and is typically sealed once the unit is at its destination.
The trade-off: you give up the gas fill
A tube that lets the cavity breathe cannot also hold a low-conductivity argon or krypton fill, the gas would simply diffuse out. So a capillary or breather unit is air-filled, which slightly reduces thermal performance versus a sealed argon unit. The open path also puts more load on the desiccant, since some humid air can still migrate in; the capillary's tiny bore limits this, which is why many fabricators crimp the tube shut after equalization to restore a sealed unit. If the tube is left permanently open, expect no argon and a more conservative service-life expectation.

When you need them

The trigger is the altitude difference between where the unit is fabricated and where it is shipped or installed, not the absolute elevation. As a common rule of thumb, fabricators start considering capillary or breather tubes when that difference is on the order of a few thousand feet (often cited around 2,500 to 5,000 ft / roughly 750 to 1,500 m), but the exact threshold is set by each manufacturer and depends on:

  • Cavity width: wider airspaces bow more for a given pressure change.
  • Glass thickness and size: thin glass in large units flexes most; thick glass resists bowing (and transfers more load to the seal instead).
  • The delivery route: a unit made and installed at sea level can still be pillowed badly in transit if the truck crosses a high mountain pass.

Always defer to the IGU manufacturer's written altitude policy for the specific unit makeup. Our IGU thickness calculator can help you sanity-check the overall build-up while you are specifying.

Alternatives to a tube

ApproachHow it helpsTrade-off
Capillary / breather tubeEqualizes pressure; ships and sits flatNo argon fill; added desiccant load unless crimped
Fabricate near the install altitudeLittle or no pressure difference to begin withNeeds a fabricator at the right elevation; not always possible
Thicker glass / narrower cavityResists visible bowingHeavier, costlier; shifts more stress to the seal
Altitude-adjusted fillFabricator slightly offsets the cavity pressure for the destinationRequires knowing the exact destination elevation in advance

In short: capillary and breather tubes are the standard fix for big elevation changes, and they trade a small amount of thermal performance for flat glass and protected seals. The right answer for any given project depends on the elevation difference, the unit makeup, and the manufacturer's policy.

Disclaimer: Altitude thresholds, tube practices, and gas-fill policies vary by manufacturer and unit makeup. The figures here are general rules of thumb for orientation, not specification limits. Always follow the IGU manufacturer's written altitude and capillary-tube policy for your specific product and project elevation.