What You Need to Know: Focus on IGU Cavity Width Manufacturing Tolerances

By Margaret Webb

Insulating glass (IG) units are dy-namic assemblies that change dimensionally based on environ-mental factors (temperature and baro-metric pressure), dynamic load factors (windload) and location (elevation). Since the cavity of a standard IG con-tains air, a fill gas or a mixture, the pressure and volume within the cav-ity are subject to these factors. As IG temperature increases, or elevation [at installation] is higher than at the point of manufacture, dynamic forces occur (i.e., negative windload), or there are lower barometric pressures, the volume of gas inside the IG increases relative to its conditions. This can cause outward deflection of the glass lites. Similarly, if the temperature decreases, the eleva-tion is lower than at the point of man-ufacture, or there are dynamic forces such as positive windload, or higher barometric pressures, the volume of gas inside the IG decreases relative to the conditions external to the IG, caus-ing inward deflection of the glass lites.

Guideline Development

The cavity width of an insulating glass unit (IGU) after fabrication varies based on changes in temperature, and elevation above sea level. A new Insu-lating Glass Manufacturers Alliance (IGMA) guideline currently in devel-opment will present voluntary man-ufacturing tolerances for IGU cavity width reduction at the point and time of IGU manufacturing. This guideline is not intended to address the dynamic aspect of cavity width variability once an IGU leaves the point of manufacture where environmental changes occur. Nor does the guideline address natu-rally occurring solar reflectance, which is a complex issue that’s affected by many variables such as environmental and climate factors with circumstances that may be unique to each occurrence.

These guidelines are intended to assist in determining tolerances in insulating glass cavity width reduction for double- and triple-glazed units at the time and location of manufacturing and are not intended for evaluation of IGU cavity width beyond the point of manufacturing as these fall outside the responsibility and control of the manufacturer.

Online Resources

IGMA is also developing a web-based information portal for consumers, designers, specifiers and building/home owners on the topic of cavity width variability in the installed environment. Topics that will be covered are altitude effect and temperature effect on IGUs, solar reflectance, design and remediation options used to reduce the impact of naturally occurring solar reflectance as well as a library of existing information materials.

Cavity width compensation technology exists such that some cases (such as when an IGU is manufactured at a lower elevation and shipped to a higher elevation) the cavity width will be intention-ally reduced at the point of fabrication so that equilibrium will occur at the point of installation. In cases where cavity width compensation technologies are being made, the recommendations contained in this guideline don’t apply. IGMA is developing another document that specifically addresses cavity width compensation techniques such as capillary tubes, bladders, valves, desiccant adsorption/desorption, pre-in-flating/pre-deflating and temperature manipulation techniques.

Increased cavity widths have little or no impact on critical performance attributes of U-factor and solar reflectance, though an excessive increase in cavity width may result in objectionable visual distortion. These should be managed in accordance with the manufacturer’s quality management system.

Cavity width reduction tolerance at the point of manufacturing varies based on intended cavity width and is summarized in the new guidelines. Conforming to the guidelines helps minimize the impact on U-factor as a result of reduced cavity width. Publication of the new guidelines is expected in Spring 2018.

To view the laid-in version of this article in our digital edition, CLICK HERE.