Tag Archives: Curtain wall

Specifying Custom Curtain Walls

Hyatt Place - Banner Photo

All images courtesy Bellweather Design Technologies

by Brian Harrington, CSI
Custom curtain wall assemblies provide an excellent way to create a true architectural signature on a building. These systems are most often used to make a key statement at the entrance or podium level, and to convey the design language of the building.

The ‘investment’ made in them is as much about the firm’s design reputation, as it is about actual project budget. For this reason, it is important to design, budget, and specify custom systems in a way guaranteeing the best chances of survival beyond value engineering and budgeting rounds.

A typical value engineering process is designed to balance product function with cost—basically to make certain all product alternatives have been considered, while managing budgets and meeting architectural intent. The best protection against losing signature design elements to budget cuts is to engage in a full design-assistance stage upfront. This stage fully documents the options, tradeoffs, and development process undertaken to produce the final system design.

The most significant difference in specifying a custom curtain wall versus off-the-shelf systems is the time needed to:

  • understand the design elements requiring a custom system;
  • accurately research the options and qualified suppliers for custom systems; and
  • create construction documents that accurately convey the project-specific requirements.

To illustrate an efficient process, this article uses the creation of the ‘Crinkle Wall’ at the new Hyatt Place in Portland, Maine. (The project was completed in March 2014, with a ribbon-cutting in May.)

Partnering with design and manufacturing teams
It was clear from the start the Crinkle Wall would not be achieved with a standard, off-the-shelf curtain wall. The design included extreme mullion angles, both left and right, and in and out, with minimal sightlines at slab levels, which could not be achieved using typical shear block construction.

“The expression we were aiming for was that of pleats, or folds, like you’d see in fabric or clothing,” explained Tim Hart, one of the founding members of the architectural firm Canal 5 Studio.

An architectural sketch and rendering was completed to illustrate the intent, and a team was assembled to work through challenges posed by the wall’s unique requirements. These included design, coordination, installation, and budget. Situated on a tight city lot, even a project-specific installation process/schedule needed to be considered up-front.

To consider all perspectives, the initial team consisted of architect, construction manager, and custom curtain wall designer/fabricator. Participation and buy-in from the owner/developer was perhaps the most important component, as he shared in the vision for the curtain wall, to differentiate the property. Once the basic system design and assembly strategy was developed, installers were added to the team to share their viewpoints on how to manage logistics.

This type of partnership is the definition of the design assistance approach. Just as the geometric and aesthetic requirements are communicated up-front, it is advisable to share budget criteria at this stage to help quickly filter possible design approaches, and rule competing options in or out. While pricing consultants and services may be a good fit for off-the-shelf system comparison, custom design requires a collaborative, bottom-up approach between designers and product developers to get to true representative system costs.

It is also important to understand a design assistance stage requires its own schedule within the overall project schedule in order to anticipate product lead times and keep them on track.


The unique geometry of the Crinkle Wall, designed by Canal 5 Studio for Hyatt Place in Portland, Maine, required a system design that could accommodate variable angles in four directions.

Identifying key criteria to meet design intent
The best way to truly manage project budgets with custom curtain walls is for the architectural team to determine the project’s key design criteria—specifically, which items are negotiable, and which are not. In other words, there needs to be agreement on what is a ‘must-have’ and what is a ‘nice-to-have.’

The outside supplier members of the design assistance team can use this information to advise the architectural team about how each item impacts overall price and availability. The Crinkle Wall, for example, is a ‘true’ curtain wall by design. This means it hangs on the building outside of the slab, providing a façade that is not interrupted at slab levels by closure covers, or even spandrel glass.

Therefore, a non-negotiable related to system design was that since the joinery between horizontal and vertical mullions would be visible within guest rooms, shear block constructionisible fasteners would not be allowed. A ‘hybrid’ approach was developed to create a split horizontal mullion that could hide fasteners by keeping them all inside the system.

Since the assembly was designed with the entire construction and installation team efforts in mind, the split-mullion hybrid approach served another important function: It allowed installers to shop-assemble horizontal ‘ladder sections’ of curtain wall that were one-story tall, and could be hung as a complete unit. This approach allowed the installer to significantly shorten installation time on the tight construction site.


The Crinkle Wall is designed as a ‘hybrid’ system, with a split horizontal mullion that conceals fasteners and allowed for horizontal ‘ladder sections’ to be factory-assembled and set in place, one story at a time.

Determining pricing ‘levers’ and their impact on target project budgets
All architectural design comes with trade-offs. A strong understanding of material options and fabrication limitations can help greatly in keeping within budget targets. For instance, the more exotic the glass make-up becomes, the fewer suppliers there are from which to choose, and the pricing generally becomes higher.

With the Crinkle Wall, a good understanding of glass fabrication requirements and criteria was key to understanding and managing this cost element. First, the design dictated each insulated glass unit (IGU) was a unique trapezoidal shape, with offset edges on specific edges, to make silicone joints at consistent widths.

Next, all but two lites in the original design were within dimensions glass fabricators consider to be ‘standard’ block sizes. These two largest lites were only a few square feet into the ‘oversized’ category, so to avoid the extra charges (which can be as much as 1.5 times the cost of standard square foot charges), the lites were able to be easily resized without impacting the overall aesthetic.

Writing the custom curtain wall specification
In order to accurately capture the unique attributes of a one-of-a-kind curtain wall, the specifier should be part of the design assistance team from the start. It is clear through the design assistance process a standard specification cannot easily be used for a custom curtain wall application.

However, the design assistance process yields most of the raw materials needed to write the construction documentation. The custom system manufacturer should have a base specification that can be modified with this project-specific information, to create a specification that accurately reflects an up-to-the-minute understanding of how the wall should be built.

Two areas that should be heavily considered, and championed by the specifier in both the design assistance and specification writing phases, are the requirements for mockups and performance testing. Mockups are great to consider for proof-of-concept, but require significantly more scheduling time, since any extruded parts have their own die-and-extrusion schedules, not to mention small-run fabrication and assembly processes.

Likewise, performance testing ranges from in-place air and water tests to full laboratory tests. The size, location, and number of penetrations from windows and doors can help guide the level of testing the team deems appropriate. Mockup and testing budgets must also be considered as part of the entire project budget.

It is important to note the final custom curtain wall specification will be focused on a sole-supplier, or just a handful of capable suppliers—those that partnered on the development of the system. It goes without saying careful vetting and selection of capable suppliers must be done as a first step in the system’s development.

A key to cost-containment success on the Crinkle Wall was a collaborative process that included ongoing discussions and reporting of cost impacts, as well as numerous budget stages to confirm the direction the project was moving in along the way.


Slab plans at each floor were used by the steel fabricator to construct the precise angles needed for the curtain wall geometry. The 3D elevation wireframes included the surrounding storefront for coordination and allowed for the best visual representation of changes like reducing oversized lites to meet glass fabrication limitations.

Coordinating with surrounding trades
One of the final considerations with the development of the custom curtain wall specification is how it may impact the specifications of surrounding materials and trades, including the building engineer, along with suppliers of structural steel, concrete, claddings, HVAC/mechanical, lighting, electrical/automation systems. The development of custom curtain wall includes engineering assumptions and requirements that, in turn, place requirements on surrounding structures. The custom system specification must therefore be written first in order to inform the specified requirements of surrounding materials and structures. The custom curtain wall engineer and building engineer should also be part of the design team, whether as a fully present member in development meetings, or as part of the manufacturer/supplier’s team. (It is a good idea to begin this coordination during early design assistance meetings.)

Picture 024

Curtain wall installers anchored the ladder section to pre-located steel shelf angles at the edge of the slab.

Unsurprisingly, the slab edge on the Crinkle Wall was unique at each floor. Each level had a custom-built steel structure at the curb edge that mimicked the exact angles of the wall, allowing for correct placement of anchor shelf brackets. This structure was then filled with concrete to complete the unique slab at each floor. The architect and curtain wall designer collaborated on slab edge drawings for the contract documents, and for early release to the steel supplier to build from. Coordination meetings, communications, and 3D model-sharing between design team and all surrounding trades often begin during the design assistance stage, continuing through project execution.

As with structural engineering requirements, the material choices for interfaces between curtain wall and surrounding walls may define the price and quality level of those surrounding materials. It is critical for architectural designers and specifiers to build a working knowledge of the changing relationships between these systems. At times, it can be beneficial to bring members of surrounding trades into the design assistance process to establish a working relationship long before the building
is constructed.

The development of a custom curtain wall and its accompanying specification can only be successfully accomplished with an appropriate amount of time set aside for a comprehensive design assistance process. The selection of a design assistance team to drive this process should be based on its prior experience with custom systems, and its ability to see the development process through to completion.

Pricing and adherence to budget is as important to most projects as the adherence to the design intent. If the project is over-budget, it can be difficult to keep it from being value-engineered to a simpler and cheaper system that may not meet the original design goals. Losing a custom assembly to a typical system in a value engineering stage means more work for the specifier, since a new specification must be developed—often with much less time to do so.

The key to keeping the budget on track is the development of a solid understanding of pricing levers and material options that have the potential to push the project off-track. An integrated design, with thoughtful choices of materials (and even manufacturing and installation approaches), can not only manage the expense of the curtain wall, but sometimes also the overall project cost by shortening the required time onsite and other factors.

A custom curtain wall specification is a true team effort, with suppliers, installers, and even surrounding trades contributing to its success.

Brian Harrington, CSI, is a founding principal at Bellwether Design Technologies, a designer and supplier of custom glazing systems, including curtain wall, skylights, structural glass canopies, and vestibules. He manages the design assistance process for each project in collaboration with the architect, installer, and design team. Harrington has worked in the construction industry for a decade. He can be reached at b.harrington@bellwetherdesigntech.com.

Making the NAFS Short-form Specification Work


Images courtesy AAMA

by Dean Lewis
Fenestration products are becoming undeniably more complex as performance expectations diversify and tighten. The same is true of the standards guiding both designers and specifiers of these products.

The focus of these standards is the American Architectural Manufacturers Association/Window and Door Manufacturers Association/Canadian Standards Association (AAMA/WDMA/CSA) 101/I.S. 2/A440, North American Fenestration Standard/Specification for Windows, Doors, and Skylights (NAFS). The 2011 edition of NAFS serves as the basis for product certification as required by the International Building Code (IBC) and International Residential Code (IRC).

Specifying NAFS compliance ensures the specification is based on an objective evaluation because of two essential properties of the standard:

  • performance basis: rather than attempting to prescribe detailed physical attributes such as frame thickness, NAFS rates complete, fabricated products according to how well they perform under prescribed conditions; and
  • material neutrality: since NAFS is performance-based, it plays no favorites, allowing use of any qualifying framing material.

Basic specification using NAFS
For specifiers, using NAFS to choose a fenestration product involves three basic steps:

  1. Select the type of window, door, or unit skylight desired. This is also known as the operator type as determined by the way in which the product opens and closes, such as double-hung, casement, or awning. The 2011 NAFS addresses 36 different operator types, ranging from the venerable single-hung to the newer dual-action and parallel-opening windows, as well as sliding and side-hinged doors, plus unit skylights, roof windows, and tubular daylighting devices (TDDs). Each of the 36 types is assigned a unique letter code identifier.
  2. The environment or application in which the product is to be installed must be considered. In the standard, guidelines are provided to help determine which class of product is suited for a particular application.
  3. The performance level required for the specific project must be determined.

The starting point: design pressure
The key to understanding product performance requirements, testing, and basic fenestration product specification is the major structurally related aspects pertain directly to the Design Pressure (DP). This is the force, expressed in Pascals (Pa) or pounds-per-square-foot (psf), exerted by the wind velocity likely to be experienced at the building’s location. From this, the Performance Grade and Water Penetration Resistance Test Pressure can be determined. The DP is a starting point for defining the performance of a given fenestration product, but not a performance rating in itself.

The classic reference for determining design wind load is American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) 7, Minimum Design Loads for Buildings and Other Structures. This standard includes the well-known map depicting maximum wind velocity contours for the United States (although local codes may cite other requirements) so it is important to consult the authority having jurisdiction (AHJ).

ASCE/SEI 7 cites three additional variables used to establish the actual maximum likely wind load for building envelope components and cladding due to the expected wind speed:

  • mean roof height;
  • importance factor (i.e. criticality of the structure to life safety); and
  • exposure category based on surrounding terrain.

Performance Class

Performance Class roughly describes the likely target application for a window or door. Like DP, it is not in itself a specific performance rating. The four classes defined in NAFS-11 are:

  • R Class: one- and two-family dwellings;
  • LC Class: low-rise and mid-rise multi-family dwellings, and other buildings where larger sizes and higher loading requirements are expected;
  • CW Class: low-rise and mid-rise buildings where larger sizes, higher loading requirements, limits on frame member deflection, and heavy use are expected; and
  • AW Class: in high-rise and mid-rise buildings to meet extreme loading requirements and limits
    on deflection.

These designations specify incrementally more stringent basic performance requirements to meet increasingly demanding product applications. It should be noted these designations are merely guidelines; the specifier must determine the Performance Class needed for a given job and the specific applicable performance requirements.

To accomplish this, NAFS defines the following four mandatory basic performance requirements within each Performance Class for a completely fabricated product. They are:

  • minimum structural load a product must withstand due to wind at the established DP;
  • resistance to water penetration due to wind-driven rain, also tied to the DP;
  • ability to seal out air leaks that can decrease energy efficiency; and
  • security in terms of its ability to resist forced entry.

Performance Grade

The specific performance level of a fenestration product that falls within a given Performance Class is defined by the Performance Grade (PG).

Performance Grade is not the same as the Design Pressure; a product only achieves a specific PG rating if it complies with all NAFS requirements for a certain Design Pressure corresponding to a given maximum expected wind velocity. Key among these requirements are the following elements.

Structural performance
requires a window or door to withstand a pressure test load applied per ASTM E330, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights, and Curtain Walls by Uniform Static Air Pressure Difference, first at the Design Pressure, during which the deflection of structural members is measured and recorded. This is known as the Uniform Load Deflection Test. The specimen is further tested at a pressure 150 percent of the DP in the Uniform Load Structural Test. The test pressure is 200 percent of DP for unit skylights and TDDs.

Ph Atmos Living Overture

Folding patio doors have been a popular trend in high-end homes as well as some hospitality environments. The structural integrity of these products is important due to the wide expanse of the opening.

Water penetration resistance
The water penetration resistance test pressure
is per test method ASTM E547, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Cyclic Static Air Pressure Difference. It simulates the force behind wind-driven rain and is generally based on 15 percent of the DP for R, LC, and CW Performance Classes, subject to a minimum of 140 Pa (2.90 psf) and a recommended maximum of 580 Pa (12 psf). The exception is the AW class, which is tested for water penetration per both ASTM E547 and ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference, at a pressure of 20 percent of design pressure, but also is capped at 580 Pa (12 psf) in the United States. The Performance Grade earned by the product will be the Design Pressure as modified by the results of water leakage testing. The lower of the ratings determines the PG.

Air leakage resistance
The air leakage resistance is determined per test method ASTM E283, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen.

While not directly related to DP, the maximum permissible rate of air leakage (expressed in L/s∙m2 [cfm/sf]), is a key performance attribute relating to occupant comfort and energy efficiency. Unlike structural and water penetration resistance requirements, NAFS specifies applied test pressures and maximum allowed leakage rates based on different Performance Grades and operator types.

These three criteria—often referred to within the industry as air, water, and structural (AWS)—are the foundation of fenestration performance and the elements for basic third-party certification.

Figure 1

NAFS_Fig 1_edit


By definition, each Performance Class requires a specific PG be met as the result of testing; the minimum PG increases as one moves up from R to LC to CW and AW Class. This lowest level is known as the entry level or ‘gateway’ performance requirement. Gateway Performance Grades for the four performance classes are shown in the second column of Figure 1.

The level of Performance Grade called for by a specific project may not correspond to the general use categories suggested by the Performance Class. For example, some residential applications may need a CW or even AW Class, while LC might be appropriate for some commercial buildings. It all depends on the specific project’s needs.

A product only earns its way into a given Performance Class when it complies with all the minimum requirements for the designation under consideration. This means not only does the product have to comply with the structural loading performance requirements, but it also must meet:

  • air infiltration resistance;
  • water penetration resistance;
  • resistance to forced entry; and
  • various product-specific auxiliary tests that confirm hardware operation under load.

Often, a higher performance level than that of the minimum gateway level is required—in a hurricane zone, for example. If such a higher performance level is to be specified, the standard allows for products to be tested to meet any design load above the gateway minimum in increments of 240 Pa (5 psf). NAFS caps these optional higher grades at 4800 Pa (100 psf) for R, LC, and CW classes; there is no cap for AW products. Again, only after successful entry into the Performance Class at the Minimum Performance Grade tested and at the minimum test specimen size can additional optional Performance Grade testing be conducted.

Additional requirements under NAFS
Specifying NAFS-compliance encompasses a defined suite of performance criteria, as well as referenced standards for materials and components. In addition to those mentioned, there are other product type-specific performance factors falling under the NAFS umbrella that are unrelated to design wind pressure exposure. Among these are:

  • forced-entry resistance (if applicable) determined per ASTM F588, Standard Test Methods for Measuring the Forced Entry Resistance of Window Assemblies, Excluding Glazing Impact, or ASTM F842, Standard Test Methods for Measuring the Forced Entry Resistance of Sliding Door Assemblies, Excluding Glazing Impact;
  • operating force (if applicable) per ASTM E2068, Standard Test Method for Determination of Operating Force of Sliding Windows and Doors;
  • deglazing force determined per ASTM E987, Standard Test Methods for Deglazing Force of Fenestration Products;
  • lifecycle testing for AW Class products per AAMA 910, Voluntary Lifecycle Specifications and Test Methods for AW Class Architectural Windows and Doors;
  • manufacturing tolerances permitted;
  • materials (aluminum and wood specifications plus various polymeric standards governed by standards such as AAMA 303, Voluntary Specification for Rigid Polyvinyl Chloride (PVC) Exterior Profiles; AAMA 304, Voluntary Specification for Acrylonitrile-Butadiene-Styrene (ABS) Exterior Profiles Capped with ASA or ASA/PVC Blends; AAMA 305, Voluntary Specification for Fiber-reinforced Thermoset Profiles; and AAMA 308, Voluntary Specification for Cellular Polyvinyl Chloride (PVC) Exterior Profiles;
  • sealed insulating glass durability as per ASTM E2190, Standard Specification for Insulating Glass Unit Performance and Evaluation or CAN/CGSB 12.8, Insulating Glass Units; and
  • components (e.g. hardware to meet cited AAMA and/or Builders Hardware Manufacturers Association [BHMA] standards).

Specifying NAFS compliance means the embedded requirements for these factors also must be met and are implicitly included in the specification.

PPG_CSU San Marcos_highres_image1

Air, water and structural criteria are the foundation of fenestration performance. Air infiltration is particularly important to occupant comfort and energy efficiency, as shown in these windows and doors at California State University San Marcos.

Product Designation
Using the key elements of product type, Performance Class, and Performance Grade, a performance rating can be constructed to specify a window or door for a specific application. The rating, then, is a four-part product designation consisting of:

  • Performance Class;
  • Performance Grade;
  • maximum size tested (which qualifies all smaller products within the manufacturer’s product line of the same design); and
  • Product Type (abbreviated).

For example, a horizontal sliding (HS) window may be tested at a size of 1600 x 1118 mm (63 x 44 in.) in Performance Class R with a Performance Grade of 720 Pa (15 psf)—the gateway level for R-class approval. The designation would be:

SI designation: R – PG720 (SI) – 1600 X 1118 – Type HS

IP designation: R – PG15 – 63 X 44 – Type HS

Care must be taken, however, to ensure all requirements of a given Performance Grade are met, or the product must be rated at a lower Grade, which also may result in a lower Performance Class. The manufacturer cannot meet one condition without meeting the other.

For example, an AW-class window tested at 80 psf for structural performance and at 8 psf for water resistance is an AW-PG40 since it only meets the 8 psf water test (20 percent of 40 = 8 psf). Also, this window qualifies as a CW-PG50 since it only meets the 8 psf water test (15 percent of 50 = 7.5 psf).

The same product tested at 50 psf for structural performance and at 12 psf for water resistance is either an AW-PG50 or a CW-PG50 since it exceeds the water test pressure at both the 15 percent and 20 percent level (15 percent of 50 = 7.5 psf and
20 percent of 50 = 10 psf) and since it meets the maximum 12 psf water test ‘cap’ for both classes.

Finally, the same product tested at 75 psf for structural performance and at 12 psf for water resistance is either an AW-PG75 or a CW-PG75 (by virtue of having met the 12 psf cap).

The short-form shortcut
Once the product selection is complete, a product designation is determined, and appropriate compliance testing decided, the selection for the project at hand must be documented. NAFS provides assistance with this by presenting a recommended short-form, fill-in-the-blank specification. Simply add the specific designation (for the Performance Class and the Performance Grade needed, as previously discussed) and the manufacturer that is preferred.

The guide language of the short-form specification is as follows:

All (windows) (doors) (secondary storm products) (tubular daylighting devices) (roof windows) (unit skylights) shall conform to the (product designator) requirements of the voluntary specification(s) in AAMA/WDMA/CSA 101/I.S.2/A440-11, be labeled with the AAMA, CSA or WDMA label, have the sash arrangement(s), leaf arrangement(s) or sliding door panel arrangement(s) and be of the size(s) shown on the drawings, and be as manufactured by (preferred manufacturer) or approved equal.

An example of a completed short form specification for a horizontal sliding LC class window rated at a Performance Grade of 25 (in IP units) would read:

All windows shall conform to the LC-PG25-HS voluntary specification in AAMA/WDMA/CSA 101/I.S. 2/A440-11, be labeled with the AAMA, CSA, or WDMA label, have the sash arrangement and be of the sizes shown on the drawings and be as manufactured by (XYZ Windows) or approved equal.

The specifier is not constrained by the standard’s requirements, as more stringent exceptions still may be included. Additional requirements also may be specified beyond the basics for performance considerations relating to impact resistance, blast mitigation, acoustical characteristics, thermal values, and tornado mitigation.

The specification writer is burdened with the responsibility for a great many building elements; the product designation system, performance requirements and methodology included in the NAFS standard can significantly lighten that burden without sacrificing completeness or quality.

Dean Lewis currently serves as American Architectural Manufacturers Association’s (AAMA’s) educational and technical information manager, bringing his knowledge of technical training to advance the FenestrationMasters professional certification program. Lewis began his career in the fenestration industry at PPG Industries with positions in project engineering, product design, and sales and customer technical support, and he has served on committees of ANSI, ASTM, and ASHRAE. Further experience includes teaching in the industrial and military sectors, and 35 years of managing technical training, publishing, and certification. Lewis holds a bachelor of science in physics with graduate work in engineering management. He can be contacted by e-mail at dlewis@aamanet.org.

More great walls of fire: Exterior separations

by Jeff Razwick


A fire-rated curtain wall provides lot line protection in a dense city. All images courtesy TGP

As shown in this author’s previous article, fire-rated walls typically stand guard inside buildings, ready to compartmentalize fires from within at any moment. As urban density and demand for daylight and visibility in the building envelope increase, these assemblies are also proving valuable for a growing number of exterior applications.

Fire-rated curtain walls can prevent a fire from traveling to or from neighboring buildings without restricting visibility. Unlike gypsum, masonry, and other opaque fire-rated materials, this multi-functionality can bring fire and life safety goals in line with the aesthetic design intent where building codes deem the threat of fire is significant from adjacent construction.

For design professionals evaluating when to use the assembly in the building envelope, it can be helpful to look at situations where it can benefit exterior separations with fire safety requirements.

Property line protection
As it becomes more efficient to build upward and closer together in cities to accommodate growing populations, property line setbacks are narrowing. This is generating an increase in the number of buildings required to use fire-rated materials as exterior separations—a safeguard building codes typically only require for structures in close proximity to each other.

Generally, lot line protection is required when a building is close to its neighbor, regardless of whether that adjacent structure is on the same lot. To provide clarity on this requirement, building codes specify the horizontal separation distances requiring fire-rated materials. For example, see International Building Code (IBC) Sections 705.5 and 705.8. In Section 705.3, IBC uses an imaginary line to determine whether buildings on the same piece of property are in close proximity to each other.

Where codes deem it is necessary to protect against the spread of fire between buildings, fire-rated curtain walls make it possible to do so while maintaining visibility and light. For example, they can provide lot line protection without sacrificing light transfer. Well-designed fire-rated curtain walls can even extend the surface area through which light can transfer to help illuminate a building’s core and better support green building goals. Some fire-rated curtain walls are available with fire-rated insulated glass units (IGUs) incorporating tinted or low-emissivity (low-e) glass for more efficient solar energy management, while taking advantage of daylighting techniques.

Transparent fire protection
Opaque fire-rated materials like gypsum and masonry can satisfy property line requirements and provide compartmentalization for both exterior and interior spaces. The downside is they restrict light transfer and visibility. Fire-rated glass curtain walls can serve as a clear alternative given their heat blocking characteristics; specifically, their classification as fire-resistance-rated wall construction.

Fire-rated curtain walls are tested to ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, and Underwriters Laboratories (UL) 263, Fire-resistance Ratings. Receiving classification as non-directional fire-resistance-rated construction (meaning they can maintain the same fire-rating from both sides) rather than an “opening protective,” they can exceed 25 percent of the total wall area to provide transparency from the outside where fire and life safety is a concern.

Exterior cladding performance criteria
The air and water penetration resistance of fire-rated steel curtain wall systems (tested per ASTM E283, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure, at 30.47 kgf/m2 [6.24 psf] and per ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference, at 20 percent of design wind load, respectively) is typically better than comparable, non-rated aluminum systems. The steel profiles are protected from air and water penetration by a continuous, full-width silicone gasket mounted to the face of the profiles in the glazing pocket.

Regarding thermal performance, the increased thickness of the rated glass in fire-rated curtain walls can help reduce potential for heat flow. Where energy-efficient curtain wall design is critical to building goals, fire-rated IGU constructions allowing low-e glass to be incorporated in the ‘glass sandwich’ can further improve energy performance. As an added benefit, narrow steel frames paired with high-performance fire-rated glazing can help lower the potential for heat transfer and therefore increase condensation resistance. Simulations of the actual construction can be modeled, giving the designer the ability to know how the fire-rated curtain wall will affect the sizing of the building’s HVAC systems.

Fire-rated curtain walls with steel frames can also work in close conjunction with surrounding materials to help ensure a sound building envelope as the temperature changes. Steel’s coefficient of expansion is nearly half that of aluminum, and is similar to glass and concrete. This also reduces the size of perimeter sealant joints, especially at locations where expansion is being addressed.

Tested to ASTM E119 and UL 263, fire-rated curtain walls can provide fire protection from the outside in.

Tested to ASTM E119 and UL 263, fire-rated curtain walls can provide fire protection from the outside in.

Support for demanding applications
Industry standards for exterior curtain wall frames typically limit deflection due to wind load to L/175 or 19 mm (¾ in.)—whichever is less—for spans under 4 m (13 ½ ft), and L/240 for greater spans (where L equals the length of the span between anchor points). These standards were originally developed to prevent sealant failure of insulating glass units due to mullion deflection.

In fire-rated curtain walls, the rated glass may impose stricter limits on the framing, such as L/300. Since steel has a Modulus of Elasticity three times that of aluminum, it can more easily meet these deflection limits without increasing the system profile size. It can also reduce the need to reinforce the frame members. As a best practice, one should consider verifying deflection requirements with the glass manufacturer before accepting typical industry standards.

For all the ways fire-rated glass can enhance building design goals for interior fire separations, there is an almost equal amount of options to do the same for exterior fire-rated glazing applications. To ensure the safety of people and property while still providing a high-performance product required by specification for exterior applications, it is important aesthetic goals align with fire and life safety standards in local building codes. Where necessary, the design team can consult with the manufacturer or supplier.

Jeff Razwick Head ShotJeff Razwick is the president of Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, and other specialty architectural glazing. He writes frequently about the design and specification of glazing for institutional and commercial buildings. Razwick is a past-chair of the Glass Association of North America’s (GANA) Fire-Rated Glazing Council (FRGC). He can be contacted via e-mail at jeffr@fireglass.com.


Great walls of fire: Interior separations

by Jeff Razwick

Fire-rated curtain walls can satisfy life safety requirements without sacrificing transparency. All images courtesy TGP

Fire-rated curtain walls can satisfy life safety requirements without sacrificing transparency. All images courtesy TGP

Glazed curtain walls are best known for their ability to visually integrate two otherwise separate spaces. Less talked about—though, perhaps more important—are curtain walls with the capability to retain visibility and access to daylight while standing guard against fire.

Tested to ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, and Underwriters Laboratories (UL) 263, Fire-resistance Ratings, fire-rated curtain walls can satisfy life safety requirements without sacrificing transparency—for better safety and aesthetics. Their multi-functionality is critical to helping design teams meet a complex set of performance criteria with one product, eliminating redundant systems and streamlining construction.

Simply put—fire-rated curtain walls allow design teams to do more with less in areas where fire and life safety is a concern. For design professionals interested in using the tough-yet-transparent form of such curtain walls to tackle multiple project demands for interiors, certain questions may arise during the specification process.

1. What constitutes a fire-rated curtain wall?
Fire-rated curtain walls block the transfer of flames and smoke, as well as radiant and conductive heat, for the duration of their given fire rating. To achieve this level of defense, fire-rated curtain walls incorporate fire-resistive glass and framing.

Fire-resistive glass is typically a clear, multi-laminate product with an intumescent interlayer that turns opaque during a fire. This reaction allows the glass to carry fire ratings up to 120 minutes, pass the fire and hose stream tests, and remain relatively cool on the non-fire side of the glass for its designated fire rating.

Fire-resistive frames serve as the support structure in fire-rated curtain walls, and can block the transfer of radiant and conductive heat for up to 120 minutes. While many framing systems employ fire-resistive insulating materials to achieve the necessary defense, those using inherently heat-resistant framing materials like carbon steel do not typically require thermal barriers within their core to protect against heat transfer. Regardless of the material chosen, packing the perimeter of the framing system to the rough opening with firestop insulation or an appropriately rated intumescent sealant is critical to the system’s overall performance.

Some manufacturers offer comprehensive fire-rated curtain wall systems, complete with frames, glass, seals, and component parts. These integrated assemblies ensure all components are designed and tested in the same assembly and to the same standard. This is critical since the International Building Code (IBC) requires all elements within a fire-resistive glazing assembly to provide the same category of fire resistance and carry the minimum fire rating as stated in the code.

Fire-rated frames can be wet-painted or powder-coated to match virtually any color scheme.

Fire-rated frames can be wet-painted or powder-coated to match virtually any color scheme.

2. Where are fire-rated glass curtain walls suitable for use?
Fire-rated curtain walls are typically suitable wherever building codes require an assembly designated “fire resistant” to enclose a space. Examples include wall applications requiring a 60-minute or greater fire rating that must meet temperature-rise criteria, such as stairwells, walls in exit corridors, or other fire barriers dividing interior construction exceeding 25 percent of the total wall area.

Since the choice to incorporate fire-rated curtain walls is often at the design team’s discretion, it is important to evaluate whether the daylight and visibility provided is advantageous to occupant safety and well-being. For example, an expansive multi-story, fire-rated curtain wall may prove beneficial to people working in a hard-to-light office. Similarly, a single-story fire-rated curtain wall enclosing a stairwell, lobby, or gathering area can extend line of sight to boost safety levels or create a sense of collaboration.

3. Are fire-rated glass curtain walls suitable in areas where they are susceptible to impact?
Fire-rated curtain walls are available with glazing that provides up to Category II (Consumer Product Safety Commission [CPSC] 16 Code of Federal Regulations [CFR] 1201, Safety Standard for Architectural Glazing) impact-safety ratings. This is the highest rating, indicating the glass can safely withstand an impact similar to that of a fast-moving adult. As such, fire-rated curtain walls are ideal for use in high-traffic areas, including schools, gymnasiums, and hospitals.

4. How do fire-rated and non-fire-rated curtain walls compare?
Unlike the bulky, wraparound form of traditional hollow metal steel frames, modern fire-rated frames have a slender profile and sleek aesthetic. They can be much narrower, have well-defined edges (rather than rounded profiles), and have vertical-to-horizontal framing joints without visible weld beads or fasteners.

In areas where a frame-free exterior surface is desirable, it is now possible to specify fire-rated curtain walls with the smooth, monolithic appearance of a structural silicone glazed system. One available assembly is silicone-sealed and requires no pressure plates or caps. Its toggle retention system becomes completely hidden once installed, creating a seamless, uninterrupted surface appearance.

5. What finishes are available for fire-rated curtain wall systems?
Design professionals can achieve nearly any look when it comes to fire-rated frame appearance. Carbon steel frames can be wet-painted or powder-coated to match virtually any color scheme, from aluminum to bright greens and blues. Framing materials also include polished or brushed stainless steel.

Fire-rated frames are also available with finished stainless steel or aluminum custom cover-caps to provide design professionals with even greater aesthetic flexibility. The face caps are available in numerous shapes and sizes—from H- and I-shapes to custom configurations. Stainless caps are typically brushed finish while aluminum ones can be wet-painted, anodized, or powder-coated to match the framing.

Modern fire-rated frames have a slender profile and sleek aesthetic to improve sightlines and views between spaces.

Modern fire-rated frames have a slender profile and sleek aesthetic to improve sightlines and views between spaces.

6. Are there any limitations to be aware of?
Since mismatched fire-rated glass and framing ratings can jeopardize the safety of a fire-rated curtain wall, it is important to verify the entire assembly provides the same type of fire protection and has a fire rating equal to or greater than the code requires. This includes the glass, frames, hardware, and all component parts.

From a performance standpoint, use of fire-rated glass requires stiffer deflection limits due to imposed wind loads. Typical curtain walls will allow L/175 (where L = span of the framing member between anchor points) or 19 mm (3/4 in.), whichever is less. Due to the nature of the fire-rated glass, deflection is limited to L/300. This may not be critical for interior applications where the only wind load is from mechanical systems, but it becomes important when designing fire-rated curtain walls for exterior applications.

Regarding installation, it is helpful to keep in mind many frames in fire-rated glass curtain walls are shipped as knock-down (K-D) kits ready for onsite assembly. While frame components may be pre-assembled or welded in the factory, pre-assembly is often done on a case-by-case basis. If pre-assembly is critical to a job’s timeframe, one should verify the manufacturer has the resources to assist with this process.

While one of the primary advantages of selecting a fire-rated glass curtain wall system is the ability to do more with less, aesthetic goals should never come at the cost of safety. Manufacturers and suppliers are available to help problem solve or create a custom work-around to balance life safety with design goals.

Jeff Razwick Head ShotJeff Razwick is the president of Technical Glass Products (TGP), a supplier of fire-rated glass and framing systems, and other specialty architectural glazing. He writes frequently about the design and specification of glazing for institutional and commercial buildings. Razwick is a past-chair of the Glass Association of North America’s (GANA) Fire-Rated Glazing Council (FRGC). He can be contacted via e-mail at jeffr@fireglass.com.


Testing Glazing in the Field: Performance Classes

Up until the 2008 edition of American Architectural Manufacturers Association/Window and Door Manufacturers Association/Canadian Standards Association (AAMA/WDMA/CSA) 101/I.S.2/A440, North American Fenestration Standard/Specification for Windows, Doors, and Skylights (NAFS), there were five performances classes of windows with differing requirements for test pressures, allowed leakage rates, and other variables. The current four types, and their minimum performance grades are:

  • R ([15 psf]);
  • LC ([25 psf]);
  • CW ([30 psf]); and
  • AW ([40 psf]).


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penetration resistance test pressure (for laboratories) is 15 percent of the performance grade for R, LC, and CW classes; 20 percent for the AW Class.

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