AGRR - Winter 2005 - Structured For Performance

CONTENTS

Volume 19, Issue 2 Winter 2005

Structured for Performance

Warm-Edge Insulating Glass Technology for Structural Glazing
by Marcel A. Bally

There are many options available for improving the thermal performance of insulating glass (IG). Low-E coated glass products, gas filling the space between the lites of an IG unit and “warm edge” spacers are some of the possibilities.

The Winter 2004 issue of the Architects’ Guide to Glass & Metal featured an excellent article by Jim Plavecsky of Windotech Sales Inc., titled “Understanding the Elements of Warm-Edge Technology.” (See the Winter 2004 issue of the Architect’s Guide to Glass & Metal, page 6.) That article primarily addressed IG concepts as used in residential windows. While commercial projects were mentioned and the cited data and technologies also applied to that field, structural glazing was not covered specifically. This article builds on the earlier discussion, enlarging the scope to include these projects as well.

Warm Edge Advantages

First, let’s recapitulate the major advantages of warm edge compared with conventional aluminum box spacers:

• Warm edge spacers, especially the ones that do not contain metal, are insulators rather than heat conductors like aluminum.
• Warm edge spacers permit less energy loss through the edge zone of the window compared to conventional spacers, resulting in warmer edge zones in cold weather compared to aluminum spacers, hence the term “warm edge.”

A higher glass surface temperature in the edge zone avoids, or at least reduces, the likelihood of condensation. Little or no condensation also reduces the potential for mold growth.

Finally, since indoor glass surface temperatures are warm, the creation of cold drafts (thermal air movements) are also avoided. This can enhance the occupant’s comfort.

Warm edge spacers, however, also change the stress conditions within the IG units. Metal spacers, with their heat conductance in the edge zone and the resulting colder edge temperatures, create thermal stress between the warm center and the cold edge of the inboard lite, and do the reverse for the outboard lite. Ironically, warm edge spacers create a stress on the spacer itself. At cold outside temperatures the outboard lite remains cold over the entire surface and the inboard light shows a relatively evenly distributed warmer temperature over the entire glass surface. The different rate of expansion of the two lites creates a stress on the edge seal. Flexible warm edge spacers tolerate this condition well, whereas rigid spacers are at a disadvantage.

Structural Glazing

Turning now to the issue of interest: structural glazing. Traditionally, the most common edge seal or spacer option used in structural glazing was an aluminum box spacer filled with desiccant, PIB primary seal and a silicone secondary seal.

Since silicone’s molecular structure does not permit it to retain gas, IG units for structural glazing are generally not gas-filled. If a specification does call for gas filling, there is a chance that the gas won’t stay inside the unit very long. The PIB primary seal may, generally, not be relied upon to hold the gas at the expected loss-rate of less than 1 percent per year. Or, as one cautious and honest IG manufacturer says: Gas filling of structural IG units is not practical.

Warm edge spacers are suitable for structural glazing and, at the same time, serve as an effective gas barrier. They improve energy conservation on two fronts: the spacer itself conducts less heat and the gas-filled IG cavity has an improved insulating property.

According to a study by Ducker Worldwide that was presented by Nick Limb at the Insulating Glass Manufacturers Alliance annual meeting in February 2005, there is tremendous potential for warm edge applications in commercial projects. Currently warm edge represents only 18 percent of all commercial IG units, compared to 86 percent of all residential IG units.

The study also showed that energy efficiency was the number one industry trend reported by architects. Energy efficiency, combined with low-E coatings, was found to be more than twice as popular than increased natural light, safety/security, aesthetics or any other trend. Clearly the potential is there, awaiting only the appropriate product.

Today there are warm edge options available for structural glazing that architects can specify.

TPS

In 1995 the Thermo Plastic Spacer (TPS) was introduced. TPS consists of a butyl-based thermoplastic material formulation that is UV-stable with integrated desiccant applied directly onto the glass by a computer-controlled extrusion nozzle. It is non-metallic and designed to provide enhanced heat insulation. It is also an effective gas and moisture barrier.

Since its introduction TPS has been used successfully in applications all over the world, primarily in Europe. Commercial TPS IG units were manufactured and became available in the United States in the late 1990s.

Advantages

An IG unit will last as long as the edge seal remains intact. Aside from glass breakage, an IG unit fails if and when the edge seal is compromised and permits humidity to enter the space between the lites.

Generally, edge seal systems are rated by their moisture vapor transmission rate (MVTR), the rate at which moisture is permitted to penetrate the edge seal. One advantage of TPS is that there is no moisture vapor transmission. Desiccant is required with both conventional spacer systems and most other warm-edge systems. With these systems, humidity is expected to pass through the seal and enter the space between the lites where it is absorbed by the desiccant. The humidity must enter the IG space before it can be removed by the drying agent in the spacer.

This is not the case with TPS. The TPS material itself contains desiccant. The small amount of humidity present in the IG space after manufacturing is absorbed by the inside of the TPS spacer. But once the space between the lites is dry, there will be no further humidity penetration (at least not for 80 or more years). The TPS material absorbs any humidity that penetrates the secondary seal from the outside. Once the outermost layer of the TPS spacer is fully saturated, humidity has to pass through that layer and start to saturate the second layer. However, it will become increasingly difficult for humidity to penetrate, as the gas laws now come into play and “new” moisture (most likely air with less than 100 percent relative humidity) encounters 100 percent humidity in the outermost TPS layer, and is therefore not drawn into the material. The entry-barrier for moisture becomes increasingly difficult to pass. Even though the moisture will encounter a tough barrier early on, the TPS spacer contains desiccant all the way through.

Super Spacer TriSeal

Super Spacer TriSeal, which is very new to the market, from Edgetech IG, is another option for high performance structural glazing. This flexible spacer is based on the silicone-foam spacer discussed in the previously mentioned article by Jim Plavecsky. It is designed specifically for higher performance and for structural glazing. It features three seals and has also passed the rigorous European certification standard 1279, retaining gas also with a structural silicone secondary seal.

Most attributes described for TPS apply also for Super Spacer TriSeal.

Warm edge, and all its advantages, are now available to the architect to specify for structural glazing.

Marcel A. Bally is the sales and marketing director for Bystronic Inc. in Hauppauge, N.Y.