Most anyone attending a seminar, webinar, box lunch, or other architectural presentation has a chance of hearing about the now ubiquitous topic of the ‘building envelope’ with possibilities including roofs, air barriers, exterior building panels, rainscreen assemblies, and energy efficient openings. The last category relates to doors, windows, storefronts, and curtainwall assemblies. Windows being non-structural, offer the greatest possibility of material selection using wood, wood clad, vinyl, fiberglass, and aluminum, while storefronts and curtainwalls are almost always aluminum. This posting relates to current technologies in fabricating energy efficient aluminum framing, which are generally comprised of two components: aluminum framing and glazing assemblies.
ALUMINUM FRAMING: The ‘older’ process is to extrude the aluminum through a die, somewhat channel shaped in the center to divide the frame into interior and exterior sections. A polyurethane based product is poured into the channel, eventually becoming the thermal break and preventing the exterior aluminum temperature from conducting to the interior aluminum. Once hardened, the back side of the channel is removed, debridged, such that the two aluminum sections are separated by the thermal break. This process creates a poured and debridged frame. If the designer and Owner are interested in creating aluminum framing with an exterior color separate from the interior, the poured and debridged process is costly.
Another process extrudes the exterior and interior aluminum sections independently, allowing for a cost effective way of finishing the frames in a two color scheme, and then brings them together via several glass-fiber-reinforced nylon bars. Sometimes called insulbars, this process introduced in the late 1970s typically results with a slightly higher resistance (R) value, meaning the aluminum framing is a bit more energy efficient. Some claim that the insulbar concept is: the strongest structural thermal break available, better than a poured and debridged or PVC; is 50% better thermally than polyurethane or PVC; and easily allows for aluminum framing of varying widths.
This process of fabricating an aluminum frame generally includes 3 processes: knurling the aluminum cavity to receive the insulbar, inserting the insulbars into the frame, and rolling the aluminum frame and nylon insulbar composite into a single frame.
The adjacent graphics indicating aluminum frame sections for both systems are reproduced from the Ensinger, Inc. Website, www.insulbarusa.com. While some window manufacturers have more recently converted to the insulbar fabrication process, others adopted it decades ago.
GLAZING ASSEMBLIES: In climates with large temperature ranges, the use of single pane glazing does not meet energy efficient design; two pieces of glass separated by an air space improves the R value. This can be accomplished using a single insulated unit or via non-hermetically sealed dual glazing, where the interior lite can be removed and allows for the installation of a venetian blind between the glass.
The insulated glazing unit generally consists of four components that will be briefly mentioned in this blog: the type of spacer, the type of glass, the use of low-E glass coatings, and the space between the glass.
SPACER: While the ‘older’ process used a desiccant filled aluminum spacer to separate the glass, a more energy efficient design was introduced by PPG in the early 1990s with their Intercept Warm-Edge Spacer. This technology allows the spacers to flex during expansion and contraction, whereas the conventional aluminum spacer requires the sealant to flex, potentially causing sealant failure and loss of the system’s ability to insulate. Given the same materials, the edge U-value with the Intercept Steel Plated Spacer is 0.34 Btu/sq. ft. x h x deg F, while a conventional spacer is 0.45 (the lower number is more energy efficient). Other types of energy efficient spacers are now design possibilities.
GLASS: Current glazing options now allow glass with lower solar heat gain coefficients (SHGC), which is the amount of solar heat entering an opening and where a lower SHGC number is better, while allowing more visible light.
COATINGS: Glass coating technology also allows various a low-E coatings, normally applied to the second surface (inner side of the exterior piece of glazing), but also to the fourth surface for even greater energy efficiency. AIR SPACE: Thermal improvement can be improved by filling the inner air space with argon gas or better yet, krypton gas. All of these energy improvements create a glazing assembly with a higher R value, where a higher number is better.
Selecting these energy efficient assemblies for windows, storefronts, and curtainwall will also have a positive effect on the building’s mechanical system. A higher R value, the same as a lower U value, of these building envelope components, results in smaller heating and cooling loads. When totaling the number of energy efficient windows, storefronts, and curtainwall areas in the building facade with lower U values, there is the potential of selecting smaller mechanical units; life cycle costing these smaller mechanical units means an annually savings to the Owner. If not already doing so, one should consider utilizing these newer energy efficient technologies for aluminum framing and glazing assemblies in exterior building openings.