Spandrel Thermal Simulation Techniques

Glazed wall systems, such as curtain walls and window walls, are one of the most commonly used façade systems in modern buildings in North America. These systems consist of transparent and opaque areas, which allow natural light, views, and solar gains through the transparent sections, as well as thermal resistance and concealing interior building components covered by the opaque sections. While the thermal and optical performance of the transparent areas is very well understood and evaluated through industry standards such as NFRC-100 and 200, the thermal performance of the opaque sections of glazed wall systems, also known as spandrel assemblies, is not.

The process of calculating heat loss through spandrel assemblies is complicated due to thermal bridging caused by highly conductive mullions and framing members. This makes it challenging to evaluate using traditional 1D or 2D calculations. While thermal bridging is also present in transparent sections, the additional heat loss near the center of the spandrel assembly is different. Using similar evaluation techniques can lead to misleading results. Studies reveal significant differences in the performance of current simulation methods for spandrel assemblies, which can diverge up to 20% compared to laboratory testing. This divergence has caused many designers to believe their systems perform better than they do, resulting in increased heating and cooling demand and condensation problems. This uncertainty also poses a risk for compliance with more stringent energy code requirements. As building codes in North America are reducing energy demand requirements to lower operational greenhouse gas emissions through lower facade U-factors, there is a need for more accurate methods to evaluate facade systems to reflect their actual thermal performance.

This paper compares various thermal simulation techniques for evaluating the thermal performance of spandrel assemblies in typical curtain wall systems with variations for glass types, insulation levels, and cladding, among others. These include 2D and 3D simulation methods:

• NFRC-100 standard for fenestration that is commonly adopted by the industry.
• Recently published NFRC 100 new guidelines for spandrel assemblies
• FEN-BC/FGIA Reference guideline
• 3D thermal simulations following CSA Z5010

This paper aims to highlight the differences in thermal simulation results for spandrel assemblies in terms of overall heat loss (U-factors) and surface temperatures to evaluate condensation risks. Results aid in identifying shortfalls of procedures to determine the right tools for the correct application.