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We Have the Technology: Steps to Take that Can Improve Thermal Performance
By Helen Sanders
It is important to specify and provide aluminum fenestration solutions that can meet the requirements of the new, more stringent energy codes. Here are several strategies that can be used individually or in combination to create fenestration with high thermal performance.
Improving the thermal performance of the perimeter of a fenestration system (frame and glass edge) is recommended first. This has the most impact on the system U-factor and causes subsequent center of glass improvement strategies to be more effective. Doing so also improves condensation resistance.
Frame Thermal Performance
The key to improving frame thermal performance is reducing thermal bridging between the outside and inside. Using wider and more complex thermal barriers (see figure) is a typical strategy to lower fenestration U-factor by reducing conduction and convection. If redesigning extrusions is not possible or system depth is limited, lower conductivity polyamide (PA) thermal barriers can often be swapped easily for regular PA.
In curtainwall systems, swapping aluminum for non-metal pressure plates (e.g. PA) is a straightforward way of improving thermal performance. And, structurally glazed systems are more thermally efficient than captured pressure plate systems, everything else being equal. This is because thermal bridging between the outside and inside is reduced.
Edge of Glass Thermal Performance
Reducing thermal bridging at the edge of insulating glass units (IGU) is also important. Aluminum box spacer is highly conductive and degrades system thermal performance. Substituting with stainless-steel box spacer improves performance somewhat (by ~0.1 BTU/°F.hr.ft²). In contrast, using a high-performance warm-edge spacer, such as plastic hybrid stainless-steel (PHSS), typically reduces system U-factors (NFRC sizes) by 0.02-0.03
BTU/°F.hr.ft² in captured systems. In structurally glazed curtainwall, where the weakest thermal link is the edge of glass, it can reduce U-factors by up to 0.05 BTU/°F.hr.ft².
Center of Glass Performance
Once the thermal bridging is addressed at the edge, improvements in the center of glass can have more impact. Typical strategies for a basic 1-inch air-filled IGU with low-E coating on surface two include:
• Replacing air with argon gas;
• Adding a room-side low-E coating. This reduces the temperature of the room-side surface, so increased condensation risk must be considered;
• Replacing double-silver low-E with a triple-silver low-E coating on surface two; and
• Changing from a double- to a triple-pane configuration which, in combination with better frames and warm-edge spacer, typically is needed to achieve system U-factors below 0.30 BTU/°F.hr.ft².
Helen Sanders is in strategic business development for Technoform North America Inc. based in Twinsburg, Ohio. Read her blog each month at www.usglassmag.com/insights
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