Here&#;s what you need to know about steel curtain wall systems to get started on your next project.
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What is curtain wall?
Glazing Contractor: Binswanger Glass Company
Glazing Contractor: Binswanger Glass Company
Throughout much of history, buildings were constructed with their outer walls serving as part of the structural support of a building. But when structural steel and reinforced concrete entered the picture, a building could be supported by relatively thin columns, independent of the exterior walls. This, in turn, freed walls from the burden of having to carry structural loads. With fewer restrictions, architects could re-envision the outer façade of a building. The exterior wall was nothing more than a curtain, hence the term &#;curtain wall&#; was born.
Today, the term curtain wall describes non-structural, non-load bearing walls on or in the exterior skin of a building. These walls serve as a &#;curtain,&#; separating the exterior weather from mechanically conditioned interior spaces. Curtain walls are also suitable for interior applications like multi-story atriums or stairwells where the separation of space is necessary.
Curtain walls are typically composed of aluminum or steel frames that contain infills of glass, thin stone or metal panels. The frames are connected to support and anchor the assembly back to the building structure. This enables design teams to set a building&#;s skin apart from its structure, achieving a specific design intent while providing thermal and moisture separation between exterior and interior spaces.
What is the difference between a storefront and a glazed curtain wall?
A storefront is similar to a glazed curtain wall in that it is non-load bearing. However, one key differentiator is that a storefront is generally restricted to a single-story application. It is usually employed at ground floor applications. Storefront systems also generally have lower performance standards for air- and water-resistance testing than those employed for glazed curtain wall systems.
Given its specific use, storefront profiles tend to be smaller and their heights more limited when compared to curtain wall systems. Storefront systems also frequently incorporate doors, usually from the same manufacturer. In these instances, the combination of the glazing and door system as a whole is considered the &#;storefront.&#;
By comparison, a glazed curtain wall is meant for taller, multi-story applications (anything from low- to hi-rise buildings). In many instances, it is designed strictly as a glazed system rather than one with doors. Glazed curtain walls also offer more resistance to air and water penetration than a standard storefront. Additionally, they tend to perform better structurally in higher wind load applications. These conditions are normally found higher on a building&#;s façade or at corner conditions, rather than the street level entrance type situations where storefronts are employed.
Is there a difference between unitized and stick-built glazed curtain walls?
Yes. In a unitized (or modular) system, the framing materials, component parts and glass lites are pre-assembled and glazed in the factory. They are then shipped to the site as complete units ready for installation. This approach requires significant engineering and assembly resources at project inception. Involved parties must detail, fabricate and then fully assemble, glaze and seal the unitized wall panels under controlled conditions in the factory. All of this must occur before the panels are shipped to the site. The panels are generally one vertical module wide by one story tall. Today, these systems have been developed to the point where very little unit-to-unit sealing operations in the field are required. Perimeter conditions are generally treated in a similar fashion to stick systems.
With a stick-built system, the various framing materials and component parts are erected in place in the field. To make this process possible, the parts are first fully fabricated prior to shipment to the site. They are then assembled, sealed and glazed on site. This approach helps simplify transportation. To help standardize and expedite the installation process, some steel curtain wall system suppliers have developed all component parts to the point where a complete system is available. This includes connection detailing and hardware; gasketing; exterior pressure plates and cover caps; complementary door and entry systems, as well as detailing.
What are the different types of framing materials for glazed curtain walls?
Building and design professionals have two primary options when it comes to selecting framing materials for curtain wall systems: aluminum and steel. Wood is also emerging as an increasingly viable option.
Many design professionals are already familiar with aluminum as a fenestration material. It is lightweight, readily available, corrosion resistant and adaptable to many different designs.
New to many design professionals is the fact that manufacturing innovations have allowed steel framing to reemerge as a favorable material for glazed curtain walls. By employing advanced material forming methodologies pioneered in Europe, it can now be used as the primary framing material for tall, uninterrupted expanses of glass. These processes capitalize on steel&#;s strength over more conventional aluminum framing. This allows steel systems to support larger wind loads, larger lites of glass and meet more demanding project criteria. Learn more about steel vs. aluminum curtain walls.
While wood is a relative newcomer as a framing material for curtain wall, the use of timber or glue-laminated members make it attractive aesthetically for some applications. Most &#;conventional&#; curtain wall manufacturers don&#;t engineer or fabricate the wood members. Traditional sourcing of the wood framing is coordinated with a glazing subcontractor, who can apply a glass receptor system to the face of the wood members to accept the glazing infill selected by the design team.
What is the difference between a glass curtain wall, a butt-glazed glass wall and a channel glass wall?
Glass curtain walls, butt-glazed wall assemblies and channel glass systems help architects achieve aesthetically pleasing, high-performing building designs that are naturally lit, energy-conscious and code-compliant. However, each system differs from the others.
Glass curtain walls are non-structural, non-load bearing walls on or in the building envelope that typically use aluminum or steel frames with glass infills to separate spaces. Fully or partially captured, these systems help designers push the building envelope to total transparency with expansive free spans or by achieving a &#;frameless&#; glass aesthetic. When made with steel framing, they can facilitate free spans up to 40-feet, as well as form challenging curves, angles and slopes.
Able to foster longer stretching dimensions and enhance energy savings, today&#;s glazed curtain walls are frequently used in monumental entrances, contemporary facades, visually appealing workspace interiors and more. Appropriately designed and tested systems can also achieve a fire rating, if they pass the requisite fire tests.
Like curtain walls, butt-glazed glass walls are non-load bearing and can be fire rated. However, a key differentiator between the two systems is how the frames retain the glass. With butt-glazed wall systems, a full perimeter frame supports panels of glass from the head and sill only while a sealant joins the unsupported edges vertically. This creates a seamless glass appearance, with minimal metal visibility. While aesthetically desirable, it can have stringent limitations on size and load criteria. As such, designers typically employ butt-glazed systems to maximize uninterrupted views in places likely to receive relatively low-load, including storefronts, interior office partitions, lobbies and atriums.
Channel glass systems differ from curtain walls and butt-glazed assemblies in both composition and makeup. Consisting of slender, self-supporting &#;U&#;-shaped cast-glass panels and an aluminum perimeter frame, they can span up to 23-feet vertically, depending on wind load and project requirements. Compared to conventional glass blocks, the channel glass creates an inviting ambience by evenly diffusing natural light without casting unwanted glares or shadows and protecting privacy. Available in a range of colors, translucencies and textures, channel glass offers premium sound and thermal insulation, resulting in high-performing building envelopes.
Back
Since reinforced concrete and structural steel entered the picture, engineers have learned that thinner columns could still support a building. Even more interesting, they could exist independent of the exterior, meaning outer walls no longer had to shoulder structural loads.
Today these outer &#;curtain&#; walls offer amazing aesthetics, but they still also serve an important role in sustainability and protecting a building envelope.
Exactly what is a curtain wall in construction, and what part does it play in protecting your build for the long term? Let&#;s talk more about curtain wall construction, including an in-depth explanation of each type, their materials and products, and their overall benefits.
Definition: What Is a Curtain Wall?
A curtain wall system is a thin, framed external structure consisting of fill-ins and non-structural partitions. It&#;s grown into arguably the most popular glazing system &#; just look around you.
Consider the sweeping skyscrapers you see in the city. The sleek, floor-to-ceiling glass walls and framed windows make up a curtain wall system.
How do these walls hold up the entire building? They don&#;t. The framing is attached to the building structure and doesn&#;t carry any of its load.
So they&#;re just an aesthetic feature of the architecture, right? Nope again. In most uses, commercial curtain wall systems have a huge job &#; protecting the building envelope design&#;s integrity from:
Water
Cold & hot air
UV radiation
While these walls are thin, they can hold up against the elements. This is because they use corrosion-resistant materials and any forces they endure (i.e. wind) transfer to the connected building structure.
Curtain wall construction requires assembly materials for gasketing, glazing, structural adhesion, weatherproofing, and more.
What Isn&#;t a Curtain Wall?
Sometimes people confuse or compare curtain walls with similar elements of a building&#;s envelope.
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Curtain walls vs. window walls: Curtain walls are suspended over the building&#;s structural elements, providing no support, and extend beyond the length of one floor. Window walls set glass within a wall&#;s structural components, and each unit is only one floor long.
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Curtain wall vs. storefronts: Units of a curtain system are also taller than the typical storefront component, which is generally 10-12 ft.
Curtain Wall Materials
A curtain wall gets its unique look from two external cladding components: frames and infills.
Frames
When choosing a framing material for a curtain wall, steel vs. aluminum is almost always the debate.
Aluminum has become a familiar face in building design because of its impressive strength-weight ratio. It has other advantages too:
Steel framing is making a bit of a comeback for glazed curtain walls thanks to advances in material grades and forming technology. While steel is inherently heavy, the right system can support larger wind and glass loads.
Wood has grown into another viable option in the right situation. Timber and glue-laminated members can bring a unique aesthetic, but that also means sourcing it is a bit unconventional. The process often involves coordinating with a glazing subcontractor, who can apply a glass receptor system to the members to receive the glazing infill.
Infill
Infills (aka spandrel panels) take up the vast majority of space in a curtain wall, as they&#;re the large panels that go between each mullion. Infill options are a little broader than framing options:
Glass: A popular choice due to its near-infinite customizability in color and opacity. In commercial glass construction, 1&#;4 inch monolithic is common in spandrel areas, while 1 in. insulating glass is often the choice elsewhere on the building.
Metal panels: Aluminum, steel, and even copper wall cladding are available, providing a nice range of design possibilities. Since these metals can withstand harsh weather conditions, they&#;re excellent for projects where longevity and low maintenance are key.
Stone veneers: These infills elegantly replicate the appearance of natural stone, adding a timeless charm that&#;s hard to beat. However, weight and installation considerations should factor into your potential purchase.
Louvers: They provide excellent airflow and light management,
adding to a building's energy efficiency
. On the downside, they may require additional maintenance over time.
Windows & vents:
These provide must-have ventilation and emergency exits. Choosing the right type will depend on building codes, energy efficiency requirements, and aesthetic tastes.
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Types of Curtain Walls
Everyone in the construction world seems to develop their own language, and it&#;s no different with curtain walls. There are two ways builders categorize them:
By build type
By how they&#;re glazed
By Build Type
Builders may also refer to curtain walls by their method of assembly. Your project requirements will likely push you toward one system over the other.
Stick Systems
Stick-built curtain wall systems are the traditional method, assembled in the field with inside or outside glazing. They&#;re most common in small buildings with low required volumes, but complex conditions.
After fabrication off-site, the manufacturer ships the components to the job location. By sealing and glazing on-site, transportation is simpler.
Other stick system facts to consider:
Unitized/Modular Systems
This is often the system of choice for tall buildings.
The unitized curtain wall installation process starts right in the factory. The manufacturer handles fabricating, assembling, glazing, and sealing the parts. Only then does the curtain wall head to the job site (carefully, due to its size), where a separate field team works on installation.
Advantages and drawbacks of unitized glazing include:
By Glazing
You can also choose curtain wall types based on how the glass is held to the wall structure.
Fully Captured
As &#;conventional&#; as it gets in this relatively new field, fully captured curtain walls are aluminum-based systems. They use gaskets and pressure plates to surround the glazing on all sides.
Fully captured varieties are straightforward to install and are compatible with a wide variety of building types. On the downside, their components can conduct a lot of heat in or out of the facade.
2-Sided
This newer option introduces a different approach of bonding the glazing to the mullion.
In two-sided structural glazing, there are still mechanical supports on two sides of the unit. On the other two sides, wet-sealed or extruded silicone replaces the traditional gaskets and plates. The sealant must be approved for structural use.
Structural silicone glazed (SSG) systems create a clean and smooth visual, as fewer visual breaks and external accessories get in the way.
4-Sided
Four-sided systems take the idea a step further in the name of sustainability. Four-sided structural glazing includes silicone joint sealant on all edges of the glazing. Note that some sealants approved for use on two-sided systems aren&#;t usable in four-sided systems.
These systems offer similar aesthetic benefits as their two-sided sisters, while also improving energy efficiency. The downsides are added cost and vulnerability to poor field conditions during installation.
Curtain Wall Protection: Assembly Products
Something has to affix all that architectural glass and metal paneling safely to the structure. This often happens through adhesives, chemical bonding, or mechanical bonding.
Then there&#;s the matter of keeping it all in place. Curtain wall installers rely on a variety of weatherproofing products to stave off water, air, and UV rays:
Joint sealants
Weatherseals for window walls
Glazing & bonding structural tape
Expansion joints
Pre-molded silicone sheets/boots (inside & outside corners)
Paintable caulking
Two-part, fast-cure silicone for on-site reglaze repairs
These products protect your curtain wall in two ways: from sudden water intake, and from gradual wear and tear. If you&#;re new to curtain wall products, an experienced distributor can help you with material selection, in-field and shop adhesion testing, and warranty support.
Curtain Walls & Water Ingression
To fully grasp the importance of a curtain wall, think about what causes the ingression of water. Five major forces are to blame:
Gravity
Kinetic energy
Capillary action
Surface tension
Air pressure
To withstand these forces, the curtain wall must also be glazed and incorporate drainage details, interior sealants, perimeter flashings, and more. They must also account for thermal expansion in any of the included materials.
The best way to avoid leaks is to identify any forces the curtain wall will likely be under. From there, make sure it has expansion joints, seals, and gaskets to account for the inevitable thermal expansion.
Long-Term Maintenance
Maintenance means more than just keeping a glass curtain wall nice and shiny. Regular upkeep also boosts protective performance.
For extra safeguarding, apply sealant compounds every 10 years after installation. In general, prompt and proactive repair of chips, cracks, and other damage will promote a better-looking, longer-lasting glaze job.
Beware of Product Compatibility!
This one&#;s been a hot topic among the large glaziers, particularly in unitized/modular builds.
It&#;s a must that the field and in-shop teams operate in conjunction &#; not just literally, but also when selecting products. The entire curtain wall design process requires both sides to use materials suitable for the project &#; and communicate with each other about those materials.
Educating Yourself on Weatherproofing Products
A curtain wall is a vertical facade for many of the latest and greatest commercial construction projects. From office buildings to other uses, they bring aesthetic impact while still excelling at what&#;s most important &#; protecting the building and its occupants.
For more tips on structural glazing and related products, visit our full resource hub:
(Editor's note: This article was originally published in May and was recently updated with new information.)
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