Thermal Bridging Research: Curtain Walls

One of our many in-house research efforts is a large, collaborative project. We investigated into the Thermal Performance of Façades through the AIA Upjohn Research Initiative.

Thermal bridging in building construction occurs when thermally conductive materials penetrate through the insulation creating areas of significantly reduced resistance to heat transfer. These thermal bridges are most often caused by structural elements that are used to transfer loads from the building envelope back to the building superstructure. Though design professionals generally understand that thermal bridging is a concern, few can quantify the extent of its impact on building performance.

Small changes in designs can still lead to dramatic improvements in performance. With careful detailing and attention to the issues of thermal bridging, the design and construction industry can improve the performance of our building envelopes.

Today we’re sharing our findings regarding curtain walls.

The mullions of curtain walls have long been understood to act as thermal bridges within vision glazing systems. Building codes and other energy standards provide maximum allowable U-values for the whole assembly, accounting for the frame, the edge of the glass that has been de-rated by the frame and the center of glass performance. In most curtain wall buildings, however, this is only part of the installation. Areas between floors and sometimes across the façade are blanked off to create spandrel panels and these are insulated in a variety of ways. However, because the mullions are simply part of the system, few of us really consider the thermal impact the mullions can have circumventing the insulation. Our thermal images demonstrated that these areas are often substantial sources of heat transfer and the magnitude of the problem is amplified by the density of the mullions and the conductivity of the pieces.

Because curtain wall frames are made of highly conductive aluminum, which is about four times more conductive than steel, and typically go from the exterior of the building through to the interior, they are significant thermal bridges. To combat this, a thermal break in the assembly, which is typically ¼ of an inch to one inch thick and made of a less conductive polyester reinforced nylon, has become a typical component in modern curtain walls. The thermal break is located between the face plate and the structural part of the mullion, the rail, in line with the glazing pocket. This creates a “cold” side for the portion of the frame in front of the glass, and a “warm” side with the structure on the backside. When insulation is added in a spandrel panel, it is most often added along the backside of the panel, between the innermost surfaces of the rails, and is often supported with a metal back pan. The insulation creates a “warm” side and “cool” side of mullion rail and completely disconnects the thermal barrier of the insulation from the thermal break in the frame. In our installations that used this detail, we observed a 70% decrease in thermal performance.

As the industry has progressed over the past few years, we have become savvier. We did have examples of projects where attempts were made to explore alternative approaches to thermal bridges at spandrel panels. The first option that we examined included spray foam inserted into the rail in an attempt to create a more insulated structural part of the mullion and continuity between the insulation and the thermal break. As might be expected, this showed little improvement over the same rail filled with air, because the heat is conducted by the aluminum, which is unaffected by the insulation inside the frame. The resulting assembly reflected a 60% decrease in the thermal performance.

The second alternative added a two-inch thick by six-inch tall band of insulation along the back side of the curtain wall rails. This created the promise of a continuous thermal barrier on the backside of the assembly. However, because the rigid insulation is flammable and subject to damage, it included a metal backpan and that was wrapped around the sides and attached to mullion frame. Much like the case of the insulated mullion, the metal pan created a continuous path from warm to cold and though it was very thin, provided an efficient path for heat loss. This too showed a 60% decrease in thermal performance. In our modeling we determined, however, that if the metal pan was removed from the assembly and the insulation could be held in place by a non-conductive material, the thermal performance decrease could be reduced to only 17%.

Though it involved a less conventional and more expensive curtain wall, the last detail studied was a structurally glazed steel frame curtain wall with triple glazed insulating glass units. Because this system inherently keeps the mullions in-board of the glass, it restricted thermal bridges even for spandrel conditions. The spandrels saw an approximate 30% reduction in the R-value over theoretical calculations and the spandrels achieved an R-15 for the assembly.