Envelope Performance: It’s the Edges that Matter When It Comes to U-Factor

By Helen Sanders

The importance of the edges of fenestration—frame and edge of glass—to the overall U-factor and condensation resistance of the building envelope’s glazed area is becoming more appreciated. These “edges” drive condensation resistance and U-factor performance and are the reason why, in general, the U-factor of a window is higher than its center of
glass.

But edges matter elsewhere on the envelope, too. In fact, they matter so much that in some cases, envelopes with floor-to-ceiling fenestration can be more energy efficient than those with lower window areas. How can that be when opaque walls have a much better R-value than windows?

It’s because of the thermal bridging that occurs around the fenestration perimeter at the interface to the wall. For this reason, the greater the window perimeter, the more the envelope performance is degraded. As a result, an envelope with many small punched opening windows—and a relatively low window-to-wall ratio—may have worse energy performance than one with floor-to-ceiling windows.

MINIMIZING HEAT LOSS

Rick Ziegler at Morrison and Hershfield has been studying the impact of these interfaces. He reports that 13% of the heat loss through a typical steel stud wall with punched opening windows (~40% window area) is due to the window-to-wall transition and has found it to be even higher with poorer edge details. Reconfiguring the glazed area to reduce the perimeter length or further reducing the thermal bridging at the interface can improve performance. This upsets the conventional wisdom that a lower window-to-wall ratio is always better in terms of energy performance.

The performance of opaque wall elements also can be degraded by how they are attached to the building. Conventionally, long metal Z-girts are used to attach panels to the building frame, which can cause significant thermal bridging, degrading the thermal performance of the opaque panel assembly, and causing a high potential for condensation (and mold growth) inside the wall and building. Thermally-broken cladding clips are recent innovations that reduce this performance degradation substantially.

OTHER CONSIDERATIONS

Non-thermally broken slab edges and parapet connections are also significant causes of thermal bridging. Even in the absence of a balcony, according to Ziegler, an un-isolated slab edge can contribute as much as 40% to the heat loss of a concrete wall.

The moral of the story is that envelope performance is not just driven by that of the window, opaque wall and their area ratio. The details of wall component edges and interfaces have a significant impact on the envelope’s overall performance and can negate the often-cited energy benefit of reducing window area.

This means that envelope performance calculations must account for the degradation caused “at the edges” of windows, opaque panels, slabs and parapets; solutions must be implemented to reduce these sources of thermal bridging; and energy codes must become more sophisticated, so that reducing window area is not counter-productive.

Helen Sanders is in strategic business development for Technoform North America Inc. in Twinsburg, Ohio. Read her blog each month at usglassmag.com/insights.

To view the laid-in version of this article in our digital edition, CLICK HERE.