Living on the edge: Improving curtainwall thermal performance
Credit: © Nic Lehoux
Meeting thermal performance requirements for curtainwall is increasingly challenging as codes are becoming more stringent. And it is becoming more difficult to trade off envelope performance with more efficient lighting and HVAC systems in performance compliance paths.
Silicone structurally glazed curtainwall systems typically have better thermal performance than their captured counterparts – all other things being equal. This is because thermal bridging associated with metal pressure plates and associated fasteners is eliminated. In the absence of an external frame, the major conduction path from inside to outside is directly through the edge of the glass. This means that the choice of spacer in structurally glazed systems has an outsized impact on the assembly U-factor.
The table shows the thermal performance (U-factor) of a captured curtainwall, a two-sided structurally glazed curtainwall and a four-sided structurally glazed system. Assembly U-factor data in the table is based on two different insulating glass edge spacer systems: A highly conductive aluminum box spacer and a high-performance plastic hybrid stainless steel (PHSS) warm-edge spacer, which limits thermal conduction through the glass edge. In both cases, the center of glass comprises a low-e coated argon-filled dual-pane insulating glass unit (IGU).
The four-sided structurally glazed system has a U-factor 10%-18% lower (better) than the fully captured system that uses aluminum pressure plates, depending on the insulating glass edge spacer used.
The data also demonstrates the relative impact of using a high-performance warm-edge spacer in the IGU instead of a highly conductive aluminum spacer. In a typical captured curtainwall system, a high-performance warm-edge spacer reduces the assembly U-factor by 0.02 to 0.03 BTU/oF.hr.ft2 or ~5-10%, but the impact of insulating glass edge spacer is even higher in a structurally glazed system. In this example, a high-performance warm-edge spacer reduces the U-factor of the structurally glazed system by 14% or 0.05 BTU/oF.hr.ft2.
This is a significant amount, given the performance enhancements needed to meet more stringent code requirements with high glazing areas. In the Space Needle renovation, the edge of glass performance was critical because of a reverse structurally glazed curtainwall creating a heat sink on the observation deck and a system with two unsupported edges on the restaurant level. A high-performance warm-edge PHSS box spacer was needed to meet both thermal and structural performance.
If moving to structural glazing is not an option, exchanging the aluminum pressure plate with a non-metal version could be considered. Polyamide pressure plates can deliver a U-factor improvement of 20% in a thermally improved curtainwall system. Combining with a high-performance warm-edge spacer and wider thermal breaks can improve the performance further.