To further improve thermal properties, IG units are sometimes filled with inert gas. Regular air in the space between the lites tends to circulate, down to the cold surface and up to the warm, creating heat loss by convection. Certain gases reduce this convection and conduct less heat than plain air. Argon is suitable for units with a space of 9.5 mm (0.38 in.) and more. Argon is used most often; it is inexpensive and occurs freely in nature. Krypton, though significantly more expensive and used less often, achieves good results in thinner units.
Given its molecular properties, silicone is unable to retain gas, which is why gas filling of glazing for structural applications has generally not been attempted. Though the butyl material of the primary seal should be able to contain argon or krypton, the application of the primary seal in real life is often not as reliable as would be required to assure adequate gas retention.
According to Insulating Glass Manufacturers Alliance (IGMA) guidelines, the gas fill level of an IG unit, if it is declared ‘gas-filled,’ must be at least 90 percent. (European standards [DIN 1286, Part 2] specify a maximum permitted gas loss of one percent or less per year. There are no comparable North American standards.)
Current IG systems
Most IG units installed over the past 20 to 30 years are of the conventional desiccant-filled aluminum box cross-section design. A large number of manufacturers offer aluminum spacers in a variety of sizes. For commercial applications, the aluminum spacer receives a bead of butyl on each glass-contact side, which forms the primary seal when the IG unit is assembled and pressed. The secondary seal of structural silicone is applied after pressing (Figure 2).
One of the first warm-edge spacers was Swiggle®, a soft, butyl-based material with an integrated corrugated thin metal strip that maintained the distance between the lites. Despite having a metal strip, its configuration conducted less heat than a conventional spacer (Figure 3).
Another successful spacer is SuperSpacer®, a silicone foam spacer with adhesives on both glass contact sides and a plastic multi-layer vapor barrier shield on the back. SuperSpacer requires a secondary seal, usually hotmelt butyl or one of the new warm-applied DSE (dual-seal equivalent) materials (Figure 4).
Though totally different in concept, Swiggle and SuperSpacer share common characteristics. Both have desiccant integrated in their respective materials, come in coils or rolls in sealed containers, and are suitable for manual application with relatively simple hand tools.
Also found in the warm-edge spacers group is a variant of the conventional spacer, which is similar to aluminum box-spacers but formed of stainless steel with extremely thin walls. The stainless steel walls conduct less heat than the thicker aluminum walls. Another variant offered by one spacer manufacturer uses two narrow, desiccant-filled metallic spacers that are separated and held together by polyurethane, creating a metallic spacer with a thermal break.
The most successful of the warm-edge spacers is Intercept®, a steel spacer with a U- profile rather than a box cross-section. The U, open to the inside of the IG unit, eliminates one of the heat bridges, and the remaining bridge—the bottom of the U—is usually buried in the window frame and not directly exposed to outside temperatures (Figure 5).
Unfortunately, the warm-edge IG systems are, by-and-large, limited to residential applications. For commercial applications, at least until recently, only conventional IG units could be used, given their larger size, heavier glass, and the requirement for a structural silicone seal.
TPS, the thermoplastic spacer system, is the latest warm-edge entrant. Though it has been in production in Europe since 1996, TPS only appeared on the U.S. scene in 1999. TPS offers a number of interesting attributes that satisfy many unresolved issues.
The TPS material is supplied in 208-L (55-gal) drums and formed into a spacer directly on the glass by a computer-controlled extrusion machine. The secondary seal may be polysulfide, polyurethane, or silicone. The fully automatic production method is the simplest available; there is no need for a separate spacer preparation activity because it is applied in the IG manufacturing system. The fully automatic process ensures consistent quality and the utmost flexibility.
Shaped and rectangular units as small as 191 x 349-mm (7.5 x 13.75-in.) and as large as 2692 x 4064-mm (106 x 160-in.) and even larger, with space widths between 1.6 mm to 19 mm (0.06 in. to 0.75 in.), with or without gas fill, can be produced in any sequence without interruption. Everything is now reduced to entering data in a computer. The picture illustrates the simplicity of the manufacturing process compared to the conventional method.
In addition to regular dual-pane IG units, TPS allows the processing of triple units with perfectly congruent spacers, as well as units with off-set, or ‘stepped,’ edges on one, two, three, or all four sides.
The end product is an IG unit with unmatched thermal characteristics. There is no heat conducting metal or material present, and laboratory tests certify a life expectancy of well over 80 years (see image below). The first TPS IG manufacturer in the U.S. has not reported a single unit failure since its introduction, over three years ago. The butyl-based TPS material retains gas, which makes it possible to offer gas-filled IG units with a structural secondary silicone seal for commercial applications, while ensuring excellent gas retention (losses of less than the industry criterion of 1 percent per year).
According to calculations and the product listing of Enermodal, TPS features the highest insulating value of any listed product. Accredited test and research facilities in Europe, such as Rosenheim in Germany, attest to an eight percent or greater improvement in the overall window U-value when installing a TPS IG unit compared with a high-quality European unit (measured according to DIN 52619-3).
Today TPS IG units are being produced in Europe, United States, and Asia, with approved TPS material being offered by several chemical companies. TPS glazing has been applied to many different projects of banks, department stores, office buildings, corporate headquarters, hospitals, universities, and government buildings (some of the most unique applications include monasteries and old hotels).
Several parallel efforts are under way to tighten the performance criteria for insulating glass, along with methods to mandate and enforce those standards. This momentum, plus rising energy costs, will force designers to project windows with better thermal and durability attributes. TPS is, and will be, the one glazing concept that easily meets today’s requirements and those of the future.
Article courtesy of:
Marcel A. Bally
Bridgehampton, NY 11932