WINDOW TECHNOLOGIES: Assembly
The lites of glass in an insulating unit must be held apart at the appropriate distance by spacers. In addition to keeping the glass lites separated, the spacer system must serve a number of functions:
The standard solution for insulating glass units (IGUs) is the use of metal spacers and sealants. These spacers, typically aluminum, also contain a desiccant that adsorbs residual moisture. The spacer is sealed to the glass lites with organic sealants that provide structural support and act as a moisture barrier. There are two generic systems for such IGUs: a single-seal spacer and a dual-seal system (see figure to the right).
In the single-seal system, an organic sealant, typically a butyl-based material, is applied behind the spacer and serves to hold the unit together and prevent moisture intrusion. These seals are normally not adequate to contain special low-conductance gases.
In a dual-seal system, a primary sealant, typically PIB (Polyisobutylene), seals the spacer to the glass to prevent moisture migration and gas loss, and a secondary backing sealant, often silicone, provides structural strength. When sputtered low-E coatings are used with dual-seal systems, the coating is typically removed from the edge first (edge deletion) to eliminate the potential for coating corrosion that may lead to premature IG failure.
Since aluminum is an excellent heat conductor, the aluminum spacer used in most standard edge systems represents a significant thermal "short circuit" at the IGU edge, reducing the benefits of improved glazings. Component manufacturers have developed a series of innovative edge systems to address this problem, including solutions that depend on material substitutions as well as radical new designs and are collectively known as "warm-edge" spacers. One approach to reducing heat loss replaces the aluminum spacer with a less conductive metal, e.g., stainless steel, and changing the cross-sectional shape of the spacer.
Another approach is to replace the metal with a design that uses better insulating materials. An example is an insulating silicone foam spacer that incorporates a desiccant and has a high-strength adhesive at its edges to bond to glass. The foam is backed with a secondary sealant. Both extruded vinyl and pultruded fiberglass spacers have also been used in place of metal designs.
Warm edge spacers have become increasingly important as manufacturers switch from conventional double glazing to higher-performance glazing. For purposes of determining the overall window U-factor, the edge spacer has a thermal effect that extends beyond the physical size of the spacer to a band about 2-½ inches wide. The contribution of this 2-½-inch-wide "edge of glass" to the total window U-factor depends on the size of the window. For instance, edge of glass effects are more important for smaller windows, which have a proportionately larger glass edge area.
A more significant benefit may be the rise in interior surface temperature at the bottom edge of the window, which is most subject to condensation. With an outside temperature of 0° Fahrenheit, a "warm-edge" spacer could result in temperature increases of up to 18° Fahrenheit at the window sightline or 9° Fahrenheit at a point one inch in from the sightline as compared to an aluminum spacer. As new highly insulating multiple layer windows are developed, the improved edge spacer becomes an even more important element.