Volume 26, Issue 6 - November/December 2012

Metal Matters

Comfortable Connections
Thermal Barriers Add to Aluminum Performance
by Ken Brenden

Any fenestration framing material must provide a comfortable interior environment while minimizing building energy use in both hot and cold climates. In addition to their structural integrity, design flexibility and sustainability, aluminum fenestration products offer optimal thermal performance by the means of thermal barriers.

Over the years, aluminum’s naturally high thermal conductivity has been overcome largely by advanced thermal barrier technology. This becomes particularly important in the winter months, as heating consumes a large proportion of a building’s energy use and stricter government regulations require the reduction of power usage in buildings to combat environmental concerns.

Thermal Barrier Technology
Specifically, thermal barriers are made from resins that create a thermal break between the inner and outer surfaces of aluminum fenestration, which allows for the reductions in heat loss or heat gain. Seventy percent of aluminum fenestration systems produced in North American contain a thermal break, with principal types including pour and debridge (P&D) and polyamide thermal barrier strips.

For P&D thermal barriers, a high-performance polymer (e.g., polyurethane) is poured into a specially designed cavity formed by a thin metal bridge temporarily connecting the interior and exterior sections of the frame. Once the material has cured, the metal bridge is removed (debridged) to form the thermal barrier. Most often in commercial fenestration products, a dual cavity design is used to improve thermal performance in storefront and curtainwall systems.

Polyamide thermal barrier strips are pre-extruded profiles made from polyamide with 25 percent of glass fiber. The polyamide profiles are locked in place in aluminum extruded “pockets” into two separate (inner and outer) aluminum extrusions. In order to create a structurally-secure finished assembly, the pockets must be “knurled” and “rolled” prior to insertion of the polyamide profiles. The profiles also can be supplied with sealing wire which gives a reliable seal and enhanced strength when activated by heat.

Other Thermal Performance Contributors
In addition to thermal barrier technology, today’s highly engineered profiles must take maximum advantage of an assemblage of components that optimize all aspects of thermal performance:

Insulating glass unit (IGUs): Double glazing is well-recognized as a key means to minimize U-factor. Today’s most advanced double pane IGUs typically reduce U-value by 34 percent, while an advanced triple-pane window can be 60 to 70 percent lower.

Gas fill: Replacing the “dead air” within IGUs with an inert gas such as argon or krypton further reduces heat flow.

Warm-edge spacers: This technology upgrades the more traditional metal spacers to material that reduces heat flow around the edges of IGUs and also reduces the tendency for condensation to form around the perimeter of the glass. Moving from the least efficient metal spacer to the most efficient spacer in a double glazed product can yield a 0.04 U-value improvement.

Low-E coatings: Advanced “triple-silver” spectrally-selective low-E coatings combine the best qualities of low-E, tinted and reflective glass.

Sealing and weatherstripping: Effective sealing and weatherstripping minimizes air infiltration around frame and sash members. Also, effective sealing is essential in preventing excessive water penetration.

Thermal performance of a completed commercial fenestration unit is quantified by testing according to industry consensus standards, such as AAMA 1503-09, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections.

Additionally, the recently published QAG-2-12, Voluntary Quality Assurance Processing Guide for Polyamide Thermal Barriers establishes assembly guidelines for architects, manufacturers, designers and/or owner(s) utilizing polyamide thermal barriers.

Ken Brenden is the technical services manager for the American Architectural Manufacturers Association in Schaumburg, Ill.

Architects' Guide to Glass & Metal
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