Discussing Thermal Barriers
Aluminum remains the framing material of choice for architectural fenestration products used in non-residential applications. Aluminum has many excellent qualities from recyclability to engineering versatility to corrosion resistance. It also is an excellent conductor of temperature.
Adding a barrier or “break” to insulate between the inside and outside aluminum surfaces, reduces the transfer of hot and cold temperatures. This minimizes the potential for interior condensation and frost build-up in cold weather and helps save energy.
The most common types of thermal breaks used in aluminum-framed fenestration products include the following:
• Slot thermal breaks remove 1-inch-wide by 6-inch-long sections of aluminum extrusion with only one inch between these slots. This typically is reserved for vertical framing members. Slotted horizontals are typically not strong enough to handle the dead load of glass.
• Clip thermal breaks use a small, plastic “clothespin” clip to separate the interior and exterior aluminum surfaces.
• Strut thermal barrier systems insert thin fiberglass reinforced “struts” into the aluminum extrusion.
• Pour and debridge thermal barriers involve pouring a two-part, chemically cured resin into a cavity in the extrusion. After it hardens some of the aluminum is cut away creating the break. The pour and debridge area can also be “lanced” to provide an additional bond between the metal and thermal break.
“Double” pour and debridge are emerging in response to new energy code requirements. This barrier system utilizes two pour and debridge cavities, thus reducing the amount of conductivity between inside and outside. Most manufacturers have developed these systems in order to meet new, more stringent, energy codes.
When thermally broken aluminum framing is matched with high-performance glass, performance is further improved in all categories, including lower U-factors and higher Condensation Resistance Factors (CRF). CRF is a dimensionless ratio of surface temperature to ambient temperature difference that is determined through surface temperature measurement in guarded hot box testing. A non-thermal, single-glazed unit may have a CRF 29; a standard, uncoated insulating glass unit may have a CRF 52 and the highest-performing curtainwalls may reach a CRF 80.
CRF is especially important in cold-climate, high-humidity applications such as multi-family and mixed-use buildings, hotels, kitchens, computer rooms and hospitals. Hospitals may require a CRF as high as 72 or more. For instance, with an outdoor temperature of 0°F, indoor temperature of 70° and relative humidity of 50 percent, condensation will occur at about 50°, requiring a CRF of 72. Below 32°F surface temperature, condensation forms as frost, which can persist even when temperatures moderate. Additional CRF information and calculations are available through the American Architectural Manufacturers Association website.
The benefits presented by fenestration systems using thermal barriers is attractive from an energy and cost savings perspective, and also creates a comfortable thermal environment that supports the productivity and well-being of building occupants. These characteristics also align with the U.S. Green Building Council’s LEED® Rating System’s criteria for Indoor Environmental Quality.
Along with the improved comfort and performance, the aluminum frames’ thermally broken construction utilizing thermal struts or snap-on face covers also allows some manufacturers to offer dual finishes, where different colors and techniques may be specified for each side of the aluminum’s visible surface. As examples: a curtainwall system could be ordered to match different exterior and interior color schemes, or an entrance system could be finished with a durable anodize on the exterior and a brightly painted surface on the interior.
Innovative manufacturers continue to develop new products and technologies to enhance both the appearance and the performance of their products, meeting the needs for best-in-class building envelope performance in any climate zone.