Energy Efficiency Breakthroughs for Commercial Glazing
Glass attracts immediate attention to almost any building, and often defines a structure’s character. However, the energy loss associated with it is a major concern for designers and owners. Heat loss in winter and overheating in summer represent a major portion of the heating and air-conditioning (AC) load, contributing significantly to the cost of operating a building.
To mitigate this inefficiency, dual- and triple-glazed vision areas have been developed, aptly named ‘insulating glass’ (IG), which comprise approximately 86 percent of all vision areas in new commercial buildings. A recent development in IG—the non-metallic thermoplastic unit spacer (TPS)—offers a significantly higher overall insulation value and longer life for commercial fenestration. The architect can now apply glass of almost any size and shape without being limited by energy concerns.
There is also an aesthetic advantage, as the spacer is practically invisible. The owner will realize significant efficiencies through reduced heating and cooling expenses, and will not have to budget for glazing replacement until well beyond the warranty’s expiration. Finally, occupants will enjoy enhanced comfort, as the uniform warm surface temperature dramatically reduces occurrences of cold drafts and condensation on the inside of the window. Commercial insulating glass has come a long way, indeed, in product improvements.
History and evolution of IG concepts
Insulating glass consists of two, sometimes three sheets of glass, or ‘lites,’ held apart 6.4-mm to 19-mm (0.25 in. to 0.75-in.) by a spacer (a frame along the perimeter of the lites). The unit is permanently sealed around the edge to prevent contaminants from entering the space between the lites and gas from escaping gas-filled units (Figure 1).
Early examples of insulated windows stretch back to the 1930s, where they were used primarily in residential applications (people would mount a second removable window over the permanent one). Proper insulating glass—that is, a unit with two or three lites—was eventually introduced by some glass manufacturers, or ‘primaries.’ These early prototypes included concepts such as forming a dual-pane glass unit from a glass bubble filled with dry air, or two glass panes with a copper-coated edge zone soldered to a lead or a steel spacer.
While these attempts improved the window’s insulating properties, those early units, or ‘thermopanes,’ were often short-lived, as they generally did not tolerate thermal stress and pumping wind loads. In short, they would either break or separate from the spacer.
The 1960s saw the introduction of aluminum spacers, the concept of which is quite similar to the way things are done today. The aluminum box-spacer is filled with desiccant beads to absorb any residual humidity in the air space as well as any moisture possibly penetrating the edge seal. The IG unit could be assembled as a single-seal unit with hotmelt butyl applied around the perimeter, or as a higher quality dual-seal unit with butyl as the primary and either polysulfide, polyurethane, or silicone as the secondary seal.
Durability was gradually improved with the introduction of better sealant materials as well as dual-seal units. The performance of these spacers generally satisfied designers’ requirements and the technology stagnated for several years. However, with the introduction of sophisticated low-E (emissivity) glass, as well as gas filling, the metal spacer was found to act as a heat bridge—or rather, a ‘cold bridge.’ This became the focus for further thermal improvement.
Coated glass and gas improve the insulating properties, or ‘U-value,’ of an IG unit. Traditionally U-values are measured at the center of the IG unit, and top performing, high-quality units offer U-values approaching that of exterior walls. However, these impressive numbers are frequently qualified with ‘center of glass,’ but the center of the glass does not tell the whole story. Conventional IG units with a built-in thermal bridge at the edge account for significant heat loss in winter around the perimeter. The glass surface temperature falls off dramatically in the zone of about 64 mm (2.5 in.) around the edge during cold weather, creating condensation and cold air drafts.
The logical next step was to get away from the conventional, heat energy-conducting aluminum box spacer. Different spacer alternatives have been brought to market over the years, but they were designed primarily for residential applications. Since all of these newer spacer concepts attempted to alleviate the cold zone around the perimeter of the IG unit, the industry term ‘warm-edge’ was created.
The primaries, as well as larger manufacturers, have made good progress in developing glass that addresses both energy and comfort concerns. Improvements in thermal and optical performances have been achieved with surface coatings designed to shield against heat (i.e., infrared [IR] and ultraviolet [UV] radiation) while transmitting as much visible light as required. The industry distinguishes two basic types of this low-E glass:
1. Pyrolytic coatings, or ‘hard coats,’ are applied on the float line while the glass ribbon is being formed, and become an integral part of the finished glass. Pyrolytic-coated glass does not require special handling.
2. Sputter, or ‘soft’ coats, are microscopic metal surface coatings applied to the finished glass by vapor deposition. Sputter-coated glass has a delicate surface that is easily damaged by mishandling, but boasts better optical properties than pyrolytic glass.
The performance of coated glass is expressed using a number of different criteria, including:
- visible light and UV transmittance
- solar heat gain coefficient (SHGC)
- shading coefficient (SC)
The visible light transmittance of clear, coated glass is generally between 40 and 75 percent, depending on the application. SHGC, which measures the amount of heat allowed to pass through the glass, is higher than 60 percent in most cases for pyrolytic coated glass, but lower than 50 percent for sputter-coated glass—often between 20 and 40 percent.
Coatings have also been developed for specific regions (i.e. greater emphasis on heat protection in southern climates and the reverse for northern climates), while other coatings can accommodate some of the most extreme environmental conditions.
Given the fragile nature of the sputter-coated surface, the coating is applied on the inside surface. Microscopically thin, this coating represents a possible path for the permeation of humidity, so it must be removed from the edge zone to allow proper adhesion and sealing of the edge seal system.
A very recent development by the primaries is self-cleaning glass. Here, the exterior surface has either a hydrophilic or hydrophobic coating. The sun’s UV radiation causes the coating to react chemically with the dirt and break down its adhesion to the glass. The dirt falls off or is washed away naturally by rain. While this glass is promoted as ‘self-cleaning,’ it may be more appropriate—as one manufacturer states—to call it ‘low-maintenance,’ as some cleaning may still be required from time to time.
Another important glass issue is safety. Local codes in areas with high winds (and high vandalism rates), prescribe laminated or tempered glass, or a combination of both for the construction of IG units.
Ever since the primaries attempted to improve energy efficiency by installing two panes of glass instead of just one, the space between them became an issue. The edge seal performs a host of functions and defines IG unit types. The major component of the edge seal is the spacer, which defines and maintains the space between the lites. Besides, keeping out contamination or keeping gas in the spacer must also include a drying agent, or desiccant, to absorb any humidity present in the space at the time of manufacturing and beyond.
After all, there really is only one way an IG unit can ‘fail’ (besides smashing it in), and that is when the inside fogs up. Condensation forms on the inside surface of the glass if there is a leak in the edge seal. The better the edge seal, the longer the expected life of the IG unit.
The conventional edge seal as shown in Figure 1 consists of an aluminum spacer with a cross-section of a box filled with desiccant beads. For high-quality dual-seal units, the spacer is coated with polyisobutylene (generally referred to as ‘butyl’ or ‘PIB’ on the shop floor) on both glass-contact sides, forming the primary seal. The cavity between the glass edges on the outside of the spacer is filled with polysulfide, polyurethane, or silicone, thereby forming the secondary seal. Lower cost single-seal units receive only a hotmelt butyl seal on the outside of the spacer. A secondary seal of silicone must be used in IG units for structural glazing, which is often found in commercial applications.
Article courtesy of:
Marcel A. Bally
Bridgehampton, NY 11932