Volume 35, Number 3, March 2000

Warm-Edge Spacers

Energy Savings and Reduction
in Heat Loss are Just Some of the
Benefits of These Spacers that are Quickly Gaining Popularity

by Ingrid Quel and Chris Ranallo

Warm-edge systems have been the focus of considerable attention recently. While traditional metallic spacers act as a thermal bridge between two panes of insulating glass, a warm-edge spacer reduces the heat loss through this “bridge.” Additionally, warm-edge spacers must be considered as part of an up-to-date insulating window system. For example, it would be of no value to use a warm-edge spacer in a window made of aluminum or steel which in and of itself allows for major heat transfer between the inside and outside of a building.

A European Perspective

One reason for the recent focus on warm-edge systems, which is one of the largest concerns in Europe, particularly in Germany, is the saving of heating energy to reduce the emission of greenhouse gases like CO2 and of other pollutants. On average, existing residences in Germany need 220 kWh/(m²a) of heating energy per year. New regulations for energy saving in heated buildings in Europe require low-energy buildings as standard. Passive houses will be built in the future. Compared with the average for existing buildings in Germany, these passive houses offer potential energy savings of more than 90 percent.

How Does “Warm-Edge” Work?

Heat transmission through the center of an insulating glass unit happens in three ways:

• Radiation: This transport mechanism can be reduced with low-E coatings.

• Thermal Conduction: This can be diminished by reducing the thermal conductivity of the gas fill, by the use of noble gases like argon, krypton or xenon.

• Convection: This is reduced to a minimum by optimization of the cavity.


The design of a spacer influences the size and extent of the thermal bridge. Although a material might have a small thermal conductivity value, if the bridge is wide it can have the same effect as a higher conducting material with a smaller cross-section.

Therefore, to reduce a thermal bridge, drastic reduction in the thermal conductivity is necessary, as well as reduction of the wall thickness of the bridge in the direction of the heat flow.

 Which Systems are
Available in Europe?

The certified testing institute in Rosenheim, Germany, has conducted a comparison of these systems, which are available in Germany and other European countries. All systems, except one, are hollow profiles with a diffusion barrier made of metal. Systems without metallic diffusion barriers around the cavity for the desiccant are not able to pass the German insulating glass tests due to structural and long-term moisture resistance requirements.

To be sold in Europe, the systems must pass severe insulating glass tests that rigidly test the long-term structural and moisture resistance integrity of the system. Before a new spacer system can be used for insulating glass in Germany, it has to pass severe insulating glass tests. Glazing, which cannot provide certification according to the German standard (DIN 1286), is not allowed in buildings and can incur legal penalties for the manufacturer. Drafts of new European standards for insulating glass tighten the requirements even more. The insulating glass manufacturer has to run daily audits and be certified annually by an independent organization. This expenditure of time and money is necessary to guarantee an expected service life for windows of more than 20 years.

The calculated linear thermal transmission coefficients, the y-values, do not show a clear separation between warm and cold edge, but a constant improvement of thermal performance.

Thermix® spacers are made of a high quality engineering plastic, containing an extremely thin stainless steel foil which acts as a diffusion barrier.

The plastic material, a modified polycarbonate, will not outgas, has excellent rigidity over a wide range of temperatures and enhanced UV stability. Due to the stainless steel foil, the coefficient of linear thermal expansion does not exceed that of an aluminum spacer. Thermix spacers can be processed on most existing insulating glass lines that use traditional metallic spacers. Specific equipment is only necessary for bending corners.

Modified polycarbonate has a thermal conductivity of 0.19 W/(mK), which is 1,052-times less than the thermal conductivity of aluminum. Stainless steel contributes 15 W/(m²K), which is still 13.3-times less than aluminum. Because the stainless steel foil in the Thermix spacers is very thin, U-shaped, and completely insulated by the plastic material, the profiles have an excellent thermal insulating performance.


European (and in particular, German) standards for insulating glass units demand a warm-edge spacer designed to meet long-term structural and performance standards. The ideal spacer to meet these requirements is non-metallic, hollow, and contains a thin metallic (preferably stainless steel) diffusion barrier.  

Ingrid Quel is a product manager for Thermix GmbH, an Ensinger Company, located in Ravensburg, Germany. Chris Ranallo is vice president/general manager for Ensinger Inc.’s OEM Division, headquartered in Washington, PA. This article was adapted from a presentation given in November at iGm/FW ’99, held in Atlanta


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