Volume 28, Issue 4 - July/August 2014

What's the Difference?
Several Factors Come into Play in Energy Performance of Curtainwall vs. Window Wall
by Nick St. Denis

Increased stringency in energy codes has prompted architects and builders to be much more careful when specifying glazing systems for their projects. Curtainwall and window wall systems are a customary part of that conversation. Both systems can be adapted and improved in many ways to allow them to meet varying degrees of performance requirements, though there are still significant factors that separate the two—starting with the glass-to-aluminum ratio.

“The less aluminum you have and the more glass you have, the better the thermal performance,” says Tubelite architectural specification manager Tom Minnon. “Right now, aluminum takes away from the thermal performance. It’s getting better ... but with curtainwall, you’re using less aluminum and more glass.”

He adds, “Looking at the total system’s U-factor, the window wall might not perform better because of the percentage of glass to aluminum.”

According to the North American Fenestration Standard, a curtainwall is defined as “a non-load bearing exterior wall cladding that is hung to the exterior of the building, usually spanning from floor to floor.” A window wall, meanwhile, is “a non-load-bearing fenestration system provided in combination assemblies and composite units, including transparent vision panels and/or opaque glass or metal panels, which span from the top of a floor slab to the underside of the next highest floor slab.”

The way each system relates to the slab is another key difference in their thermal performance potential.

“With a curtainwall, you have the spandrel area where the curtainwall bypasses the slab,” says Minnon. “You can put a lot of insulation in there, between curtainwall and the slab edge. With a window wall system, the challenge is, ‘How do you insulate slab exposed to outdoors?’ You’re probably not getting as much insulation.”

Though the slab can make a difference regarding options with insulation, a window wall system can be made to model the outside appearance of a curtainwall.

“Window wall systems can be designed to look as much, or as little, like curtainwall as is aesthetically desirable, through creative detailing of slab edge covers,” says Wausau Window and Wall Systems’ vice president of technical services Steve Fronek. “Units can be installed in any sequence, and can meet the same high-performance requirements as curtainwall.

“Window wall systems can be located such that the glass plane and edge of slab can be very tight to lot line setbacks. Window wall systems often are provided with integral operable vents, for natural ventilation, egress, or ease of cleaning. Vents in the closed position can be designed to be almost indistinguishable from adjacent fixed lites.”

Adds Minnon regarding the vents, “We’re seeing more and more of that with the sustainability movement.”

Minnon says another advantage a curtainwall has compared to a window wall in terms of thermal performance is that a curtainwall can be triple-glazed, while most window wall units can only handle up to an inch of glass.

However, he says that “you can come close with double-glazing,” adding that most manufactures put a low-E coating on the interior side of the glass, as well, to further improve its U-factor.

Given the ability to bolster a window wall unit by those measures, Fronek concludes, “The differences between curtainwall and window wall are more evident in sequencing and anchorage provisions than in performance.” AGG

Nick St. Denis is an assistant editor for the Architects’ Guide to Glass & Metal magazine. He can be reached at

Architects' Guide to Glass & Metal
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