Managing convection: designing high-performance aluminum fenestration

Twinsburg, Ohio. by Helen Sanders, Ph.D
test window BEST

The next step in window design

With the adoption of more stringent energy codes, there is an increased focus on designing commercial fenestration with higher thermal performance.  A fenestration assembly’s thermal performance, or U-factor, is determined by the weighted average of the U-factors of the center of glass, the edge of glass and the frame. Improving the U-factors of the frame and edge of glass, as well as the center of glass, is essential to achieving the best performing fenestration system. Otherwise, heat will escape through the edges of the assembly, negating any improvements in the center of glass. Also, since condensation resistance performance is driven by thermal bridging at the frame and edge of glass, improving the thermal performance of the fenestration perimeter results in a more balanced design.

There are three mechanisms for heat transfer at the fenestration assembly edge – conduction, convection , and radiation – accounting for approximately 50%, 35% and 15% of the heat flow respectively. Typically, in fenestration designs, the conduction mechanism is addressed first, using simple thermal barriers such as polyamide strips or polyurethane pour-and-de-bridge systems in the frame, and warm-edge spacer at the edge of glass. 

For the frames, the wider the barrier, the lower the conduction, the better performing the system. According to the National Fenestration Rating Council’s definition, a “thermally broken” fenestration system has a low conductivity barrier of at least 0.210 in (5.3 mm) between interior and exterior frame elements. This should not be confused with “thermally improved,” which requires only a minimal 0.062 in (1.60 mm) thermal isolator. Today’s polyamide thermal barrier systems are as wide as 3.9 in (100 mm).

Once conduction is addressed, convection and radiation mechanisms must be reduced to impact performance further. There are several strategies for reducing convection in the large cavities inside the aluminum extrusions. Polyamide barriers can be made in more complex shapes with “legs” that extend perpendicularly into these cavities to break up the convection currents. Further performance enhancements can be achieved by combining reflective foils to reduce the radiative thermal transfer component with these more complex polyamide barriers. Alternatively, similar high-performance can be achieved using dimensionally stable shaped foam to fill the cavity, which is attached to a simpler thermal barrier.
The use of foam in combination with a polyamide barrier has both cost and logistical advantages compared to more complex shaped barriers with foil. Since the barrier itself is a simpler shape, it is generally less expensive. Because the foam can be applied using adhesive tape to the barrier as needed, the same polyamide barrier can be used for two or more window performance variants and customization can occur by applying foam to the barrier at the point of fabrication.

While fabricators may have different design requirements or manufacturing constraints, they can be confident that the range of thermal barrier and IGU solutions currently available today allow fabricators to reduce conduction, convection, and radiation, which reduce condensation and improve system U- values.  Applying these solutions will ensure fabricators design high-performance fenestration systems which that exceed today’s stringent energy codes.

The image below shows 3 barrier systems with increasingly high-performance.  Assembly U-factors assume a high-performance plastic hybrid stainless steel warm-edge spacer and a center of glass U-factor of 0.26 btu/

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