Glass Canada • June 2025 • Fenestration and Glazing Industry Alliance

The demand for highly energy-efficient fenestration products continues to rise. One can address this need by increasing the number of cavities in the insulating glass unit. This method, while effective, can create new challenges. IGMA TM-1300, Design Considerations for Multiple-Cavity Insulating Glass Units, the definitive technical manual on this topic, is set to be re-published with updated guidelines for designers, specifiers, fabricators and installers of multiplecavity IGUs. The document focuses on requirements for multiple-cavity IGU that differ from those for single-cavity units. As the world moves towards triple-glazed IGUs, it’s helpful to review some of the basic design considerations for these products.

One of the primary challenges in designing multiple-cavity IGUs is managing the air pressure changes within the cavities. TM-1300 gives guidance on how to deal with variations in temperature, barometric pressure, wind load, or altitude, helping to prevent seal stress and/or glass deflection. the use of capillary tubes is among the measures addressed.

Fabrication of multiple-cavity IGUs presents unique challenges. The inability to visually inspect the sealant bond line for centre lite(s) is a significant issue, as the next spacer layer visually blocks skips or voids. It is also possible to over-press one layer and under-press another when the unit sandwich is built. Another challenge is the link between handling and cleanliness. In multiple-cavity IGUs, the centre lite(s) has two “clean” sides unlike double-glazed IGU. Therefore, it is recommended to refer to TM-3100, Visual Quality Guidelines, when fabricating multiple-cavity IGUs. The glass can be processed vertically or horizontally just like a single-cavity IGU. However, sealant thickness can be inconsistent when processing horizontally. Narrower cavities (for instance, six-millimeter), which are often used with multiple-cavity IGU, may require vertical processing, especially for large sizes, to ensure glass centres don’t touch. When glazing, it is important to adequately support all glass lites with setting blocks. If not, the glass lite can create edge seal shear on the improperly supported lite, potentially causing premature IGU failure. Multiple-cavity IGUs are usually 50 percent heavier than dual-lite IGU as they contain additional glass and other materials. When selecting hinges, cranks and other hardware, the IGU’s weight must be taken into consideration. For hollow framing, stiffening channels and/ or inserts may be needed to ensure adequate rigidity.

Many glazing cavities are designed for a dual-lite IGU, and multiple-cavity IGU will be constructed, often as an afterthought, to fit an existing glazing cavity. It is important to review the water path for proper drainage to ensure all lites are fully supported and do not block water drainage channels. The basic structural considerations for the use of multiple-cavity IGUs will follow established guidelines for the structural framing portions of the units. The horizontal support member deflection due to the IGU’s weight must not exceed three millimeters or 25 percent of the recommended edge clearance, whichever is less. The twist of the member is to be limited to less than one degree. When multiple-cavity IGUs are being considered for structural glazing, the appropriate bond line width for the secondary silicone seal must be carefully reviewed to ensure negative design-wind-load conditions are met appropriately. Additionally, thermal stress must be evaluated to determine if glass lites require heat treatment to prevent glass breakage. To determine glass strength to ensure proper resistance to uniformly distributed loads, such as from wind and snow, one must consult ASTM E1300-24, Standard Practice for Determining Load Resistance of Glass in Buildings.

Adhering to guidelines and best practices, such as illustrated above, will be crucial to maximize the potential of multiple-cavity IGUs. By understanding the complexities, designers, specifiers, fabricators and installers can effectively leverage the benefits of triple-glazed IGUs to meet the growing demand for high-performance building solutions.