Knowing the Components of an IGU

The specification of insulating glazing technologies may require multiple fabricator sources, such as those requiring highly specialized processes for unique shapes, sizes, configurations and applications. In most instances, however, a single-source fabricator can be identified, and thanks to their integrated fabrication processes and reduced transport and coordination they are likely to provide a highly dependable and cost-effective product. The single-source supplier may also be the most comprehensive resource for IGU information and product support.

This is due in no small part to the demands placed on today's architectural project teams to meet high-performance and green design criteria while also juggling the many specialized manufacturing techniques that contribute to the ultimate glazing solutions. But the challenges are also inherent to the complex interactions of IGU components.

The building team must be as familiar as possible with the composition of glazing units in order to design and specify the building enclosure properly and also to make an informed choice between single-source fabricators and an alternative requiring two or more fabricators. Among the components requiring the most careful scrutiny are:

Insulating capabilities. With glass offering little in the way of intrinsic insulative property, architects strive to achieve an optimal enclosure U-value or ideal window-to-wall ratios to balance benefits like natural daylight with challenges like heat loss (and solar heat gain). Glazing becomes better insulating only by adding:

(a) components in the float—the results of which are quite limited and often negatively impact light quality and visibility—or

(b) air or gas space.

When at least one air/gas space is included—with no coating—the results are immediate and striking. Consider an uncoated monolithic design: the single 1/4” lite provides a mere 1.02 U-value and a SHGC of 0.82. If the design calls for a second uncoated lite and a 1/2-inch air gap between them—creating a basic kind of IGU—the U-value is cut by more than half to 0.47 while the SHGC also improves significantly to 0.70.⁸

Such gains are vital to optimizing building performance, but are highly dependent on the proper construction of the IGU. In the case offered, a U-value of 0.47 leaves a great deal of room for improvement, so fabricators and building teams look to coatings and other technologies, each of which adds complexity to the manufacturing process.

High-performance coatings. Pyrolytic and sputter coatings were introduced into the float process in order to manage heat-flow emissivity, explaining why the term low-E (for low emissivity) became almost inextricably paired with the word “coating.” The most common drawback of this first generation of coatings was a reduced visible light transmittance (VLT). But currently available technology can provide additional benefits of solar management, filtering radiation like infrared or short-wave more selectively and thereby offering a higher VLT.

The benefits of coating are clear and compelling: A basic low-E coating on the inside of the outboard lite (Surface #2) in the IGU configuration described above would achieve a 0.29 U-value and a 0.38 SHGC, a reduction of 38% and 46%, respectively.⁹

Laminated glass options. Sandwiching the PVB interlayer between two lites and bonding the layers with heat and pressure—laminated glass—offers a range of benefits depending on the specific configuration and choices of coatings, printing and more. The result can be safety glass, acoustically insulating glass or UV-reducing glazing, or a mix of these types.

Panels of laminated glazing can be used in IGUs, and insulating-laminated configurations typically have the laminated lites on the inboard side, an approach recommended when specifying for acoustic and solar performance. For safety performance, such as in hurricane zones, insulating laminated is typically preferred, placing the laminate inboard. Solar management coatings will most often be placed on the inside of the outboard surface touching the gas/air space. (The #2 surface for insulating laminated, the #4 surface for laminated insulating.)

Triple-insulating units. Adding a third lite and a second air/gas space certainly delivers next-level thermal performance to the base example given above: a 0.30 U-value (versus 0.47) and a 0.60 SHGC (versus 0.70), without coatings or interlayers.¹⁰

 

Triple-glazed IGUs can be costly, however, and for this reason they are still a rarity. Energy-minded owners and architects have been using them in Northern climates, where they achieve the highest energy savings. Single-source fabricators are seen by many as best positioned to understand how to achieve cost-effective triple-glazing performance and rapid return-on-investment, as well as to advise project teams on lower cost alternatives with comparable performance.

Silk-screen and digital printing. To combine performance and truly unique aesthetic expression, fabricators can apply frits: diffuse patterns of ceramic or glass batch (dot and line patterns are standard), fired and baked onto the glass substrate during production. Using a silk-screen in this process can achieve bold, highly visible and even customized patterns.

Frit is typically used to reduce solar heat gain. Single-source fabricators often suggest combining a silk-screen frit with a low-E coating on the #2 surface to achieve desired performance. Applying a 20% silk-screen coverage  to surface #2 may reduce the solar heat gain coefficient by approximately 13%.¹¹

 

Spacers and sealants. Just as brick is only as good as the mortar, the latest and best technologies in glass fabrication depend upon correct treatment of the edges. Spacers, the components that provide space between the lites, must be specified and detailed exactly in order to achieve optimal performance. Certain materials are not thermally broken; in other words, they conduct heat and cold more readily than others, reducing the IGU's insulating properties.

Durability is also crucial: certain materials may become damaged with age, pressure, and in the case of high-rise construction, variable wind loads that cause enclosure assemblies to shift frequently. Spacers that cannot endure such conditions may lead to reduced glazing system performance over time and more frequent replacement of glazing than should be necessary.

Edgework, cutouts, and holes. IGU fabrication requires a high level of precision, especially at the joints and edges. Single-source fabricators frequently offer reliable results based on their use of powerful, precise cutting and drilling tools. Just as properly honed edgework is crucial to the successful seating and sealing of IGUs into spacers and frames, so is accuracy and experience in cutting notches and cutouts and drilling holes for specialized applications.

Glazing units with holes for point-supported structural glazing systems can perform just as well as uninterrupted glazing units—if all component interactions are managed well and all cuts and edges accurately made and properly finished. In fact, they may have improved thermal performance as a result of the reduction of heat transfer because there is less metal around the edges of the glass units—a distinct advantage of point-supported glass designs.