Glass Glossary

Capillary Tubes

Last Updated: November 23, 2023

Capillary tubes are extremely narrow diameter tubes used in insulating glass (IG) units to equalize pressure differences between the inside of the unit and the external environment. They are particularly important when IG units are installed at higher altitudes than where they were manufactured. In such cases, the pressure inside the IG unit can differ significantly from the atmospheric pressure at the installation site, causing the glass panes to bow outward. This bowing creates stress in the glass and the IG unit’s edge seal, potentially leading to glass breakage and unit failure.

To mitigate these issues, capillary tubes can be installed at the time of manufacturing. They allow the IG unit to pressure-equalize with the local atmosphere, relieving the altitude pressure differential created by the difference in manufacturing and installation altitudes.

Units constructed with capillary tubes are not filled with argon. As a result, the U-factor for IG units with capillary tubes should be based on air-filled IG unit construction, rather than argon-filled. The use of capillary tubes helps to prevent the distortion and bowing of the glass panes, ensuring the IG unit maintains its aesthetic qualities even when installed in high-altitude environments.