Curtain wall assemblies, large expanses of aluminum framing, are not traditionally known for their thermal efficiency. While insulated spandrels within the curtain wall framing have been used in an attempt to improve the thermal performance of the curtain wall system, research and testing has shown that typical insulated backpan systems fall short of expectations for the thermal resistance of opaque walls in many codes and standards.
This low performance of spandrels is due to several limitations in typical spandrel designs and materials. New technologies have been developed which try to address some of these issues. One strategy is to use Architectural Insulation Modules (AIM) which comprise of highly insulating vacuum insulated panels (VIP) sandwiched within an insulated glazing unit (IGU). When AIM units are coupled with large thermal breaks in framing, the thermal performance of the spandrels can greatly improve.
Unfortunately, there is no single commonly validated and accepted approach in evaluating the thermal performance of curtain wall spandrel sections as there is with the curtain wall vision sections (NFRC-100). For this study, three approaches were examined:
- Guarded hotbox testing of physical specimens according to ASTM C1363,
- Two-dimensional thermal modeling following variations of NFRC-100, and
- Three dimensional thermal modeling following ASHRAE-1365 RP.
For each of these methods, a square section of high performance curtain wall spandrel was analyzed for thermal transmittance for five spandrel configurations, including typical IGU and backpan systems, as well as AIM units.
This paper outlines a high level comparison between these three approaches in analyzing the thermal performance of spandrels. The results, in turn, help identify what the realistic thermal performance values are that can be expected from curtain wall spandrels and how they can be improved using highly insulating materials.