As window glazings have dramatically improved thermal performance, manufacturers have turned their attention to the edges, where today proportionately more heat can be lost than through the glass. New approaches to edge design include using spacers made with material that is less heat conductive than conventional metal. One problem with this is that these so-called organic spaces may outgas, coating the inside of the glass with a polymer film that impedes performance. The incentive to retain metal spacers is also strong because manufacturers have so much money invested in equipment to bend the metal. To save energy and still retain metal, manufacturers substitute lower-conductivity metals such as steel, reduce metal thickness, and use different shapes – such as a “C” cross-section instead of a “D.” PPG, Cardinal Glass, and Allmetal are among the manufacturers employing this approach. Here is how window-spacer designs have evolved:

conventional spacer: aluminum

In conventional edge designs for insulated glass windows, tubular metal spacers are filled with desiccant, a water-absorbing material.  Aluminum has been the preferred spacer material because it is easily bent during window fabrication, but it's also an excellent heat conductor, which reduces overall energy efficiency.  The heat loss through this type of aluminum window-edge spacer can offset a lot of the efficiency gains that are provided by high-performance glazings and insulative gas fill.

Swiggle: less metal

Tremco corp. of Beachwood, Ohio, was one of the first manufacturers to address concerns about heat loss through window edges when it introduced the Swiggle Strip spacer in 1979.  Composed of low-conductivity butyl rubber with built-in desiccant and just a strip of corrugated metal for structural rigidity, Swiggle Strip has gradually gained acceptance among manufacturers, says Tremco's Jim Plavescky.  Swiggle Strip's conductivity is one quater to one-half that of a standard aluminum spacer, which improves overall window energy performance by several percentage points. 

Super Spacer: Foam without Metal

In 1989, Edgetech of Ottawa, Ontario, upped the energy-savings ante with a spacer that uses no support metal.  Super Spacer employs silicone foam rubber with a built-in desiccant and a foil backing to prevent diffusion of insulative gas.  The silicone foam rubber is one-quarter to one-half as conductive as Swiggle Strip, and 6 to 11 percent as conductive as conventional aluminum spacers.  Super Spacers are used by Pella in its Architect Series windows and by several smaller makers.  Edgetech has also developed a hybrid spacer, which has yet to be adopted by a major manufacturer.  In the new design, a conventional aluminum edge spacer is placed next to a nonmetallic Super Spacer.

Warm Edge: A Hybrid

Southwall Technologies of Palo Alto, Calif., has yet another design approach for window edge spacers.  It will be used for its Heat Mirror glazing system that is to be introduced this year.  In the Southwall warm-edge system, two steel spacers are seperated by a 1/8-inch thick layer of rigid polyurethane foam.  The same polyurethane thermal break is used in Southwall's top-efficiency Superglass with two suspended Heat Mirror films, a product that is used in Hurd's insol-8 windows ("Windows of Opportunity," Dec, '89).  The conductivity of polyurethane foam is lower than that of silicone foam, boosting overall energy performance of a typical residential window by 20 percent, says Todd Sitrin of Southwall.  He adds that there is also no outgassing with the warm-edge design.