Installation Differences Between Curtainwall and Window Wall Systems

By Jordan Scott

Window wall and curtainwall have structural differences that affect the applications for which they are suited. With these differences come several installation and design considerations that are important for glazing contractors to understand, especially upfront in the design-assist phase when they can steer system decisions.

Structural Definition

Different companies have different definitions of curtainwall and window wall (also known as ribbon windows). Spencer Culhane, senior building envelope specialist at Schüco USA of Newington, Conn., offers his company’s definitions: A curtainwall is any façade assembly that is suspended in front of the superstructure of the building and a window wall is any assembly that is load-bearing or deadloaded to each level of the structure.

“For curtainwall, think of a continuous wrapping applied to the skeleton of a building’s structure. It’s unbroken and continues along the rise and width of the whole building, uninterrupted by the structure … It hangs like a curtain,” he says. “… Window wall systems are very common in concrete slab construction. On the edge of the floor, you’re resting and erecting continuous windows horizontally, but they only span floor-to-floor with the slab dividing the system.”

Culhane points out that the system includes the framing, glass and operables. Façades with window wall systems may include slab edge covers, façade paneling or spandrel glass, but the structural part of the system is what matters.

Tom Mifflin, director of marketing for Tubelite of Walker, Mich., says the optimal and most economical system largely depends on where the project is located and who is available to install it.

Marty Trainor, senior vice president at Chicago-based Ventana, says his company uses four-sided structural glazing for both product categories, which makes their performances similar.

“For some, window wall [when compared to curtainwall] is inferior and less of a high performance product,” he explains. “That’s not the case [for us] and a few other companies on the market.”

Trainor adds that many people believe window wall is also a cheaper product, but that depends on the project. Curtainwall is good for projects with steel structures or high spans between floors while window wall is better for concrete structures and small spans, according to Trainor. This is why window wall often is used for hotel and residential projects while curtainwall is used in large office projects where an HVAC system needs to be hidden.

He points out that trying to use a window wall when there’s 14 feet between floors could result in a cost higher or equal to using curtainwall, which is already designed to accommodate that distance.

Earl Patrick, executive designer for Enclos of Eagan, Minn., says that if the two system types are designed correctly, there should be no difference in performance. He says it can be difficult to achieve the same air infiltration values with window wall as with curtainwall.

“Though you could make the argument that window wall doesn’t perform as well seismically,” he says, adding that curtainwall isn’t pinned between two slabs like window wall.

Mifflin offers a further explanation.

“Another important performance consideration is the inter-story movement, which is determined by loads from the building structure, occupants, walls, equipment, column foreshortening, creep, and thermal expansion and contraction. When inter-story movement requirements exceed +/- ¾ inch, window wall systems should be carefully reviewed, or curtainwall systems should be considered,” he says.

Installation Differences

Installation also plays an important role in the system selection process. Trainor says that at Ventana, there’s one major difference between curtainwall and window wall: curtainwall is installed outside of the slab and window wall is installed inside the slab. Culhane echoes Trainor’s statement, saying those are the predominate installation methods for each system.

“Curtainwall can have panels that start inside but are then swung into the exterior, craned into position and then hung,” says Culhane. “Curtainwall installation requires a hoist mechanism on the outside of the building to hang the panels on the exterior. Window wall systems don’t have the same requirement and are installed on the inside from each floor level, though a glazier may need to go to the exterior for slab edge covers.”

He explains that logistics and the structural condition of a building often drive the product decision. If cladding a tower or high-rise where there’s rigging access on the exterior, it can be faster to clad using a curtainwall system. If cladding a long, low-rise building with a long, horizontal orientation where a crane would have to be brought onsite for the purpose of setting a façade, it might be better to go with a window wall system.

Trainor points out that installing a window wall system on a steel structure can be much more labor intensive since glaziers can’t drill into concrete when installing the system between slabs.

“On a steel structure there are structural pieces to support the slab called outriggers. It’s easy to install window wall in a concrete structure because you literally tip it into place from the inside. If there are steel beams and outriggers you can’t do that anymore because they’re in the way,” says Trainor. “Structure type and span really influence the decision more than anything else. Generally, you can get the same water and air performance, U-value performance and heat gain based on the glass you use. The performance is pretty much equal.”

Design Considerations

“Thermal, air, water, acoustic, structural performance; horizontal and vertical movements, windloads, deadloads, fire stop, installation methods, aesthetics, available labor, site constraints, schedule and budget—all must be analyzed to determine the appropriate system for every project,” says Mifflin.

Mifflin explains that window wall is used most commonly in low- to mid-rise buildings with dimensional façade features such as inside/outside corners with walk-out terraces and balconies.

“A curtainwall system height is almost unlimited and its mullions are much deeper. This allows curtainwall to create a sleek, monolithic, exterior appearance on high-rise buildings. Curtainwall also has a more versatile glazing pocket to accept thicker glass or other infill. This makes it the default system for such challenging requirements as blast mitigation, hurricane resistance and high acoustic performance,” he says.

Patrick believes window wall only should be used when the application drives it, such as when the projected slab is needed structurally.

“There’s a misconception in the marketplace that window wall is cheaper, but that’s rare. If there’s a situation where window wall is not demanded structurally it almost never pencils out to use window wall over curtainwall,” he says. “There’s a lot more field labor involved with window wall and it increases the schedule duration as well.”

According to Culhane, slab edge cover details are critical in window wall façades. They can become a source of heat loss if not insulated correctly, but they can also become a moment of expression, he explains, adding that there’s an opportunity for improvement.

When using a mixed palette of materials, such as terracotta cladding, shading structures or fins, Culhane says curtainwall may be easier because each panel can be designed to hide joints more elegantly, whereas window wall requires the system to be captured between each slab.

“With window wall you don’t have that robust, unitized frame to hold the materials more easily,” he says. “It can be done but it depends on how much customization you want to put in it.”

Trending Façades

Higher thermal performance remains a steady trend as building owners seek to meet model energy codes and to improve energy-efficient performance in existing properties, says Mifflin.

While there are aesthetic differences between curtainwall and window wall, Trainor says that hybrid window wall systems are closing that gap.

“The aesthetic differences were small to begin with and they’re getting smaller as the industry evolves with the use of hybrid window wall,” he says.

Patrick has seen an increase in demand for organic materials.

“There’s definitely a move to more opaque and organic materials for infills rather than all glass boxes,” he says. “Then, of course, architects are continuing to push the boundaries of form as we get more capabilities with parametric modeling.”

Culhane says curtainwall systems continue to be a reliable approach for custom, high-rise buildings or bespoke façades while window wall is more standardized and useful for low-rise construction.

“We’re not seeing one outweigh the other,” he says.

Jordan Scott is an assistant editor for USGlass magazine. She can be reached at jscott@glass.com

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