Architectural Aluminum Curtain Wall Systems

The curtain wall is the element of a project on which, if you are not doing things right, everybody can get hurt... Walter Scarborough, HKS Inc.

December 2005
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Continuing Education

Use the following learning objectives to focus your study while reading this month’s Continuing Education article.

Learning Objectives - After reading this article, you will be able to:

  1. Understand the proper applications of architectural aluminum curtain wall systems to best meet code and design requirements.
  2. Have a greater understanding of new technologies and system advancements in architectural aluminum curtain wall systems.
  3. Recognize industry standards in product testing and classification of architectural aluminum framing systems.

One of the most catastrophic and widely reported curtain wall failures occurred in January, 1973, when, during construction of Boston's John Hancock Building, designed by Henry Cobb of then I.M. Pei & Partners (now Pei Cobb Freed & Partners), 75-mile wind gusts caused more than 65 500-lb glass panels to shatter and fall.

More panels broke and fell in the following months and, ultimately, all 10,344 panels on the sixty-story building were replaced, at a cost of $7 million. In the interim, exterior openings were covered with plywood, leading to the joke in Boston that the Hancock Tower should be renamed the U.S. Plywood Building.

Jokes aside, Nicholas Isyumov, research director for the Boundary Layer Wind Tunnel Laboratory at the University of Western Ontario, says engineers learned from the experience that they must pay attention to the effects of wind not only on the structure, but also on the nonstructural cladding, especially when that skin is of aluminum or reflective glass.

Despite the occasional calamity, the idea of an all-glass skin remained perhaps the most persistent curtain wall theme of the 20th century. Starting with small metal window systems containing relatively small glass panes, and moving towards larger glass sizes with smaller mullion profiles, the most technically advanced glass walls of the recent past managed to eliminate mullions entirely by using the glass itself as a structural material, relying on structural sealant joints, or pinning the glass to elegantly-detailed lightweight steel sub-structures.

In the 21st-century, the emphasis of curtain wall and slope wall manufacturers is on more highly weather resistant packages with heightened energy efficiency, using more durable and long-lasting materials and finishes, recyclable materials. Glass glazing, long thought to be a building's weak link with regard to energy performance, now rivals insulated walls in its ability to maintain a desired interior climate while reflecting unwanted effects of the sun. Today's walls are more sophisticated in every respect.


One PPG Place, Pittsburgh, PA
John Burgee Architects with Phillip Johnson

"Today," says Carl Wagus, technical director for the American Architectural Manufacturers Association, "we are working on test methods for evaluating the performance of thermally broken aluminum-we've already developed a series of standards for evaluating thermal barriers structurally."

The movement to "green" architecture has meant that structural systems are being asked to support more material, including sun shades and light shelves. "Our systems are being tested structurally right now," says the director of the curtain wall division of one manufacturer. "Architectural firms say 40 percent of what they are designing right now has some type of sun shade built into the curtain wall system," says the source.


Radio Shack Corporate Headquarters
Forth Worth, Texas

Today, various curtain wall infill products can be used instead of glass. Aluminum panels, for example, were used on the Delphi Building.

"We take for granted in our industry that architects know what simple architectural framing systems are and what applications fit them best, but we get questions all the time that indicate otherwise," says the director of the curtain wall division of one manufacturer.


Gaylord Texan Resort & Convention Center
Grapevine, Texas
Hnedak Bobo Group

Strom Thurmond Wellness & Fitness Center
University of South Carolina
The Budreaux Group Architects

An operable vent within a curtain wall system.

Many manufacturers offer extensive lines of custom and standard skylight systems, translucent systems and architectural-grade window and vent systems; some offer one-stop shopping-engineering, extrusion, fabrication anodizing, painting and testing-all from one location. Some manufacturers also can direct the entire material flow for a project. The results, say manufacturers: lower project costs, ease-of-installation and fewer jobsite headaches.

Most manufacturers test their products through standardized test methods established by the American National Standards Institute (ANSI) and the American Architectural Manufacturers Association (AAMA). Once having defined performance characteristics for the project, the designer can specify products through common guide specifications available from a number of sources. AIA, and The Construction Specifications Institute specification guides reference ANSI/AAMA guidelines.

Storefronts and curtain walls do not have a single performance standard, but are grouped via a series of standardized performance test methods. The three primary weather performance standards are ASTM International E 283, Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls and Doors Under Specified Pressure Differences Across the Specimen; ASTM E 331, Test for Water Penetration of Exterior Windows, Curtain Walls and Doors by Uniform Static Air Pressure Difference; and AAMA 501.1, Standard Test Method for Exterior Windows, Curtain Walls and Doors for Water Penetration Using Dynamic Pressure.


Shakespeare Theater, Chicago, IL
VOA Associates

Structural performance is measured by ASTM E 330, Test for Structural Performance of Exterior Windows, Curtain Walls and Doors by Uniform Static Air Pressure Difference. Thermal performance, U-values, and condensation resistance factors are measured under AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows Doors and Glazed Wall Sections and are standardized through AAMA 1504, Voluntary Specification for Thermal Performance of Windows, Doors and Glazed Wall Sections.

It is important to note, however, says David W. Altenhofen, CSI, CCS, AIA, chief of architectural technology at Philadelphia-based Kling Lindquist, that a project's performance values cannot be determined solely through standardized test procedures and specifications alone.

"The architect must interpret and interpolate test results" for manufacturer's standard assemblies, says Altenhofen. "The discrepancies between the system required for the project and the tested one may be substantial."

"Selecting window, storefront and curtain wall systems is a far cry from just reviewing standard products in catalogues and selecting the desired profile," Altenhofen says in a recent article in The Construction Specifier.

"A lot of architects-most architects-are very good at what they do. However, most of them can't keep up with the nuances of curtain walls," says Jerry Johnson, senior design consultant for Dallas-based Curtain Wall Design and Consulting Inc. "There is so much going on, and the complexity of these systems has increased dramatically."

Contemporary architectural aluminum framing systems are carefully isolated from the building frame so that they support only their weight and the force of wind. They are insulated and "thermally broken" to obtain higher energy efficiency and reduce moisture condensation; utilize glazing and spandrel materials that offer precise control of thermal performance and emissivity and are carefully drained and gasketed to prevent water leaks.

Part of the confusion about specifying framing systems may stem from what now have become industry-wide definitions, themselves.

There exists what Walter Scarborough, vice president and director of specifications for Dallas-based HKS Inc., calls "a terminology issue" within the curtain wall industry.

"Our documents, for instance, now refer to ‘glazed aluminum wall systems'," says Scarborough. "The terms that the industry has come to use have different meanings to different people. The term ‘storefront,' for example has come to represent, what, by comparison, is a low-performance system. The contractor will, too often, acquire a low-performance system, regardless of your needs, and it is an uphill battle from there.

"Until they have tried to understand window systems for what they are, a lot of people don't understand the complexities of those systems or see the importance of one system over another system," he says. "If you have a building 20 stories tall on the coast of Florida and you install what is commonly referred to as a ‘storefront' system, your building will perpetually suffer leakage, and owners don't want to spend millions of dollars for a (cladding) system and have it leak all the time."

Essentially, says Fred Grunewald, research and development manager for a Texas-based manufacturer, "architects specifying (cladding) systems need to know the local code requirements regarding items such as wind loads, seismic considerations, and life safety issues to allow them to determine the specific performance requirements for a project."

Before designing a curtain wall system, there are five items to consider. They include: design criteria (wind loads, codes, etc.), structural criteria (live load and deflection), thermal considerations (CRF and U values), anchorage considerations and secondary water control.

Anchorages were a critical consideration in developing plans for Chicago's new waterfront Shakespeare Theater. The design of the 75,000-square foot, seven-story glass encased Shakespeare Theater, completed in 1999 as a final phase of an overall 1.1 million square-foot urban redevelopment of Chicago's Navy Pier, was to be reminiscent of a century-earlier Chicago side street. Chicago-based VOA Associates, looking to contain costs while sprinting from schematics to opening night in 15 months, chose an off-the shelf system, which despite being pre-fabricated, "was able to deliver many of the things we were looking to do," including a prism-like bay that permitted panoramic views of the waterfront, served to diffuse the impact of exterior sound, and reduced the visual impact of seven-inch-deep mullions, says Jim Spacek, VOA vice president and project manager.

Although not a contractual design-build project, the CM early on worked with a curtain wall manufacturer and an installer. "As a result," says Spacek, "we were able to consider proprietary issues earlier than normally would have been the case." Among the issues: at which floors the attachments would take place. "We ended up with a plan in which alternate floors carry wind and gravity loads and were able to plan for that in a way that minimized erection issues down the road," Spacek says.

"In this business, the water infiltration value is the thing that separates the men from the boys," says Scarborough. "The higher the resistance, the higher-performing the system, the more expensive the system is. On the vast majority of projects we do, we start at the upper end of the (resistance) range," he says. "We generally start a project looking at 12 pounds (per square inch), in some cases 15 pounds." Low-ratio systems offer water control at 1.77 and 6.24 pounds per square inch. At the upper end of the range are highly customized, high- performance curtain walls.

"At HKS," Scarborough says, "there are a half dozen of us who have spent our careers writing specifications. There are nine of us in the specifications group. We meet for a half-hour every morning. At lunch, we frequently have manufacturers come in and make presentations. We devoted September to discussions of waterproofing issues. We spent the month approaching the subject from an academic standpoint. Not every architect has that luxury, but you can't do this stuff casually."

"Architects, in general," he says, "tend to think of themselves knowing every-thing, as the master builders-those are the guys who get themselves in trouble."

"You have got to read-a lot," says HKS vice president Joe Sprague. "You have got to get together with the manufacturers. The thing I stress the most is that if the architect doesn't understand something-anything-find the manufacturer. Those guys will bend over backwards to accommodate us. Every product in our industry is becoming more sophisticated, more technical," says Sprague. "The weakness in the design industry in the U.S. is that there is not a formal mechanism to help the architect understand industries like curtain wall manufacture. This stuff is not taught in school. There are good architects out there, who have gone to good schools, but when it comes to the technical aspects of putting a project together, they fall down. The manufacturers know that, and they know that they possess the knowledge that will enable architects to do their jobs successfully. I spend time in plants. I call manufacturers.

They will fly in here to teach us how their systems perform. I say ‘I don't understand. Teach it to me.' They will."

"Ten years ago, it seems to me, manufacturers were more a part of the design process," says Dan Rogers, director of the curtain wall division for a Texas manufacturer. "Due to many fast-track projects, we have lost some of that interaction with architects, and we need to get it back."

"We've gone to great lengths to design and manufacture high-performance systems," says Rogers. "A lot has changed since Boston's John Hancock fiasco. There are a lot more products available and they are available now with zero sight lines, beveled, sloped or radiused exteriors, any color you want, any depth and profile." Architects are best served, however, Rogers says, by using those systems for the applications for which they were intended.

"You wouldn't put a residential window in a school," says Rogers. "If you did, it might last five years. An architectural window in the same environment might last 50 years." Systems are tested in the laboratory by subjecting them to 2,500 wind cycles.

Nearly all of what designers need to know to specify aluminum architectural framing systems is available at manufacturers' websites.

"You can go to the sites and pull up details. There are specification and performance criteria," Grunewald says. "It is important that you read the performance requirements. What you end up with is a cut-and-paste Word document, a reproducible file that can be transported directly to design documents. The same is true with product details. Most manufacturers offer a wide variety of products with various levels of performance and aesthetic attributes. When the designer has determined the structural requirements and elevation configurations, the manufacturer can assist with specifying a product that meets both the performance and aesthetic needs of the project.

Historically, architectural aluminum framing systems generally were limited in color to gray, black or bronze (anodized). Today, specialty coatings mean that systems are available in nearly any color. As much as half the product lines of some manufacturers today are coated.


These two CAD drawings illustrate storefront (top) and curtain wall systems (bottom).

"The systems are also becoming less complicated to install and more flexible in their applications," Grunewald says. Tomorrow's systems will be even more installer friendly, he says, and nearly all manufacturers are developing systems that can be unitized, pre-fabricated and assembled off-site. Because off-site labor costs can be better controlled and the product can be assembled and sealed in a protected environment, unitized and pre-assembled units are likely to make architectural framing systems increasingly cost competitive.

"High-performing, unitized systems, which some aluminum framing manufacturers now offer and others are turning to, still are a little bit more expensive than stick-built, maybe in the range of 20-25 percent more," says CDC's Johnson. "But these systems offer secondary water control.

That issue is our biggest hang-up in this industry, always has been, always will be. Secondary control covers your backside. Even if the system leaks, you don't hear about it. A lot of owners don't want to spend the extra money, but they are the first to squeal when water run down the inside of the glass. Secondary systems should be mandatory."

Tomorrow's architectural framing systems will be developed with heightened consideration of sustainability, will see even higher thermal performance, will accommodate specialty glazing-photovoltaics, for example-"and the bar keeps being raised in terms of water and air infiltration," says one manufacturer.

Storefront or Curtain Wall:
Which Product is Right for my Application?

Left: The Millennium Tower in Addison, Texas, is a good example of a curtain wall application. HKS Architects. Right: A storefront system was used on the H&R Block Headquarters building in Kansas City, MO. BNIM Architects.

It used to be simple to choose between a curtain wall and storefront glazing system. Curtain walls were for large, towering buildings. Storefront systems were for small, one-or-two-story buildings, often retail stores.

The name said it all.

Today, things are a bit more confusing. There are more storefront systems and more curtain wall systems. Many two-story buildings combine uses, such as office/retail. As you look through brochures and CDs, you see center-set applications, front-set applications, structurally silicone products, tubular and I-beam curtain walls-and you wonder which of these many products will fit your project application.

To make matters more difficult, those towering buildings are not as common now. Most have been replaced by buildings no more than 10-stories tall. So the natural question is, "Can't I use a storefront system on this six-story office building?"

CDC consultant Jerry Johnson says the answer is not a simple one, but the following rule of thumb can make it a pretty basic decision: "Storefront systems were designed primarily for use in one-to-four-story applications-usually small retail buildings or strip centers."

Other factors to keep in mind are the following differences between the two systems:

Storefront Curtain Wall
Finish One Finish Dual Finish
Gasket Vinyl EPDM, Silicone
Performance Air 6.24 =.06 CFM/FT² 6.24 = .06 CFM/FT²
Water Test 10 psf per ASTM E 331 15 psf per ASTM 331

The key factor in selecting a curtain wall or storefront system is the enclosure's ability to handle and control water. Water control is the ability of the glazing system to collect and drain to the exterior of the building. As you can see by the storefront and curtain wall detail illustrations, a storefront system has a very limited water head compared to a curtain wall system, but a storefront system will perform very well in the proper application.

Water control is a much bigger problem for specification writers than concerns about structural requirements, says Johnson. "It is usually obvious when a storefront glazing system is not capable of meeting structural and wind load requirements. When a system fails, water control is usually the problem."

The following checklist of questions about the project requirements and system capabilities may help make the selection easier:

  • What do the specifications require • CRF and "U" value ratings?
  • Air, water and structural requirements? • Specified gaskets?
  • Finish requirements? • Live-load slab deflections and seismic?

Cutting corners is not recommended when it comes to overall building construction, and the glazing system is no different. A building owner might get by using a storefront system when a curtain wall system is needed. However, that decision might prove costly in the long run. That is why it is important to determine the exact requirements of a glazing system before making your selection.

"We are going through a new cycle," says Grunewald. "The glass box became popular in the 1960s, then the energy crunch did away with the glass box. But with advancements in technology, daylighting and high-visibility buildings are undergoing a revival. The goal is to allow as much light as possible to enter a structure and still maintain energy efficiency. As technology improves, it is becoming easier to do that.

Ohio, Grunewald says, has begun a statewide school construction program, with an eye toward efficiency, in which all windows will be triple-glazed and include venetian blinds, an extremely expensive option, but a highly efficient one. "In 15 years we have gone from single pane windows to triple-glazed systems," he says.

"These are reliable, soundly performing systems that work around standard assemblies but can be manipulated to achieve a variety of aesthetics," says Blanchfield. "Many of these manufacturers have a lot of engineering horsepower in-house, and they have a lot of production capacity. That makes them attractive for jobs with tight schedules and not too many peculiarities.

"What I see consistently, however, among architects who have not done a lot of curtain walls is an insufficient knowledge of tolerances---in the manufacture, the construction, and in the ability of builders to hold dimensions true," he says. "There is a false expectation that contractors can build as straight and as true as the architect can draw, and that is not always the case. These kinds of things aren't discovered until the actual work is underway and they can then become a source of problems. Sometimes contractors are errant in their work, but often problems stem from a lack of judgment and experience on the part of architects. If they anticipate real circumstances, they can avoid many of the hardships."

"Curtain wall systems are a commodity item," says Blanchfield, "and like most commodities, the price of curtain wall systems is market-driven. Although both materials and energy costs have risen, the price of most curtain wall systems has gone down. It is a buyer's market right now."

"We now live in a world of specialization," says Sprague. "Whether it's a wall system or a power system, we owe it to ourselves to seek the best information we can find. Our clients don't want excuses."

"Fenestration Beyond Standards
Successfully integrating windows, storefronts, and curtain walls
by David W. Altenhofen, CSI, CCS, AIA
Reprinted with permission of The Construction Specifications Institute
99 Canal Center Plaza, Suite 300, Alexandria, VA 22314, from The Construction Specifier

Standard performance specs, test methods, sizes, profiles, details... it might be reasonable to assume window and curtain wall systems can easily be selected, detailed, and specified through standards. On the contrary, specialized skill and knowledge are required to integrate windows into a building, and fenestration design certainly should not be limited by standards. Installing windows is rarely standard, as most designs require some degree of customization for anchorage, flashing, trim, perimeter seals, and continuity with the thermal envelope. Windows and curtain walls have a substantial impact on HVAC and electrical systems, and should be customized to suit the project. Furthering the need for customization, many of design issues are interrelated and sometimes mutually exclusive. The application of standards is simply inadequate.

Performance standards

Windows vary considerably in performance, from barely adequate under normal conditions to extremely tight during hurricanes. Most manufacturers test their products with the standardized method American National Standards Institute/American Architectural Manufacturers Association/National Wood Window and Door Association (ANSI/AAMA/NWWDA) 101/IS 2-97, Voluntary Specifications for Aluminum, Vinyl (PVC) and Wood Windows and Glass Doors. Performance values vary from air penetration of 2.06 m3/h at a test pressure of 75 P (0.37 cfm at 1.57 psf) to 0.56 m3/h at 299 P (0.10 cfm at 6.24 psf).

Water penetration values also span a similar range. This standardized measurement allows a relatively simply comparison between different windows. Acceptable performance for a window in a particular project cannot, however, be completely predicted by simply complying with one of the performance classes in ANSI/AAMA/NWWDA 101/IS 2-97. Of particular concern is manufacturer-advertised performance might be based on a window much smaller than the one required for the project, and may not be representative of actual project performance. Other variables-such as glass type, anchorage, and stiffness of the wall in which the window is mounted-can also result in a difference in performance between tested samples versus and the installed window.

Storefronts and curtain walls do not have a single performance standard, but rather a series of standardized test methods for establishing performance. The three primary weather performance standards are ASTM International E 283, Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls and Doors Under Specified Pressure Differences Across the Specimen; ASTM E 331, Test for Water Penetration of Exterior Windows, Curtain Walls and Doors by Uniform Static Air Pressure Difference; and AAMA 501.1, Standard Test Method for Exterior Windows, Curtain Walls and Doors for Water Penetration Using Dynamic Pressure.

Structural performance is measured by ASTM E 330, Test for Structural Performance of Exterior Windows, Curtain Walls and Doors by Uniform Static Air Pressure Difference. Thermal performance, U-values, and condensation resistance factors (CRFs) are measured under AAMA 1503-98, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, and standardized through AAMA 1504-97; Voluntary Specification for Thermal Performance of Windows, Doors and Glazed Wall Sections.

"Fenestration Beyond Standards
Successfully integrating windows, storefronts, and curtain walls
by David W. Altenhofen, CSI, CCS, AIA
Reprinted with permission of The Construction Specifications Institute
99 Canal Center Plaza, Suite 300, Alexandria, VA 22314, from The Construction Specifier

Note these standardized test procedures and specifications do not establish the performance values required for a project, which must be determined by the architect. He must carefully specify the performance levels without over-specifying and driving up project costs. It is unreasonable on most projects to perform custom laboratory testing on mock-ups of the project design. As such, the architect must interpret test results provided by the manufacturer from standard assemblies or from similar, previously tested assemblies, requiring a further level of interpretation and interpolation. The discrepancies between the system required for the project and the tested one may be substantial.

Standards for glass also tend to set a standardized test method or minimal performance, but do not necessarily establish fully adequate performance. ASTM E 1300, Standard Practice for Determining Load Resistance of Glass in Buildings, sets the method for determining the structural performance of glass, but the architect or engineer must set the actual performance criteria. Some of the variables they have to juggle include substantial snow/drift loads on skylights, high wind loads at corners, simultaneous positive/negative wind loads on canopies, or limited allowable deflection to minimize visual distortion.


Figure 1
Project-Specific Window Jamb Detail

ASTM E 774, Standard Specification for the Classification of the Durability of Sealed Insulating Glass Units, establishes standards for insulated glass (IG) units, but a variety of options are not included in the standard. Several different types of primary and secondary seals are allowed, which may not be compatible with adjacent glazing sealants. Likewise, fine details, like the color of the spacer or special spacers to improve thermal performance, are not included.

National Fenestration Rating Council standard NFRC 100, Procedure for Determining Fenestration Products U-Factors, establishes the method measuring U-values of IG units while NFRC 200, Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence, establishes how solar heat gain coefficients are measured, but does not give any performance values. The architect must select the appropriate values to meet energy codes and match the assumptions used by the mechanical engineer in the design of the HVAC system. Problems will arise when only the standard is referenced without providing performance values, particularly when the contractor submits an inferior product.

"Fenestration Beyond Standards
Successfully integrating windows, storefronts, and curtain walls
by David W. Altenhofen, CSI, CCS, AIA
Reprinted with permission of The Construction Specifications Institute
99 Canal Center Plaza, Suite 300, Alexandria, VA 22314, from The Construction Specifier

System selection

Selecting window, storefront, and curtain wall systems is a far cry from just reviewing standard products in catalogs and selecting the desired profile. The first step involves determining all the criteria affecting the project.

The most basic of criteria is the wind speed and resulting structural loading. Multiple standards exist and more than one may be necessary on a project. Building codes typically establish loading. International Building Code (IBC) requires compliance with American Society of Civil Engineers (ASCE) 7-98, Minimum Design Loads for Buildings and Other Structures, but allows for compliance with internal requirements, as well as use of Southern Building Code Congress International (SBCCI) SSTD 10, For Standard Hurricane Resistant Construction. FMG Property Loss Prevention Data Sheet 1-28, Design Wind Loads, applies when the project is covered by Factory Mutual (it is generally more stringent than building code).

AAMA also publishes standards for wind loads. Technical Information Report (TIR) A10, Wind Loads on Components and Cladding for Buildings Less than 90 Feet Tall, is based on ASCE 7, Minimum Design Loads for Buildings and Other Structures. For tall, large, or complicated buildings, it may be prudent to use wind tunnel testing to determine design loads as described in AAMA CW-11, Design Wind Loads for Buildings and Boundary Layer Wind Tunnel Testing. Wind tunnel testing will likely reveal some areas of higher and lower wind loads than those indicated by code. When it comes to windows, it is important to remember smaller tributary areas require a higher structural load, while corner zones are higher than in the field. Finally, seismic loads, building movement caused by loading, temperature and wind, and deflection of adjacent building structures and materials all affect the window, storefront, or curtain wall system.

After determining the design wind pressure, the next step is to determine the test pressure for air and water penetration. AAMA recommends 20 percent of the wind load for "most parts of the country under the normally prevailing weather conditions." However, AAMA goes on to warn higher loads may be necessary in areas of sustained high winds and rain. Again, performance criteria need to be adjusted to suit the particular project situation.

Thermal performance criteria must also be established. U-value and glass shading coefficient are typically set by an energy code and by their impact on the HVAC load. Aesthetic issues may influence reflectivity and the color of glass, both of which affect U-value and shading coefficient. A minimum condensation resistance factor (CRF) needs to be established based on local weather, interior temperature, and relative humidity (RH). A proper CRF is crucial for eliminating condensation on the interior surface of window frames and glass.

Other criteria that could be important to the project are acoustic performance, fire ratings, hurricane resistance, security, bullet or blast resistance, and sustainable/green issues. Each has associated standards and guidelines, but it is not enough to instruct builders to just follow the standards. They must be interpreted by the architect and reduced to clear and measurable performance criteria. All of these issues need to have clear criteria established to select window, storefront, and curtain wall systems that will be able to adequately perform.

"Fenestration Beyond Standards
Successfully integrating windows, storefronts, and curtain walls
by David W. Altenhofen, CSI, CCS, AIA
Reprinted with permission of The Construction Specifications Institute
99 Canal Center Plaza, Suite 300, Alexandria, VA 22314, from The Construction Specifier

Documentation

The architect must prepare a set of construction documents indicating every requirement to allow for accurate bidding and construction. Documentation is crucial at the areas of most customization. For example, drawing detailed inside profiles of manufacturer's standard aluminum extrusions is a waste of time-the required exterior profile will suffice. On the other hand, half- or full-scale drawings indicating every material at the interface of window systems and adjacent construction are warranted. The construction documents must also include a specification carefully tailored to meet the unique requirements of the project. Using unedited, standard specifications for special project requirements is unacceptable.

Detailing

Standardized details prepared by the window or curtain wall manufacturer are almost never suitable for direct use on a project. In fact, many of them carry disclaimers stating the manufacturer is not responsible for the adequacy of the details provided for the joint between the window and wall system. The details a manufacturer presents generally meet only the lowest level of performance. (No manufacturer will risk losing business over more conservative installation requirements that make their product more expensive than the competition.)

As such, many manufacturer standard details show a simple, single line of sealant as the only method for keeping the joint between the window and the adjacent wall system watertight. Even resources like Architectural Graphic Standards only give generic information about the detailing of window or curtain wall systems. It is up to the architect to develop appropriate systems and details resulting in systems meeting performance criteria. (Refer to Figures 1 and 2 for an example of the difference between a manufacturer's standard jamb detail and the same condition customized to suit the project.)


Figure 2
Project-Specific Window Jamb Detail

The architect must consider several issues when detailing window, storefront, and curtain wall systems. Perhaps the most important is understanding whether the wall and window systems serve as rainscreen or barrier systems. By understanding the type of wall system and the functions of its various layers, it is possible to trace the continuity of thermal envelope, and moisture, air, and vapor barriers from the wall system, across the joint and onto the window, storefront, or curtain wall. It is also important to detail for realistic (i.e. imperfect) levels of performance within both the window systems and the interface with the adjacent construction.

It is doubtful the sealant applied in the field between dissimilar materials, and installed by different subcontractors, will perform perfectly for the life of the building. There will probably be gaps in the sealant at substantial completion, let alone in five, 10, or 20 years. Likewise, most storefront systems are designed to be watertight at the interior side of the glazing pocket, but in actual installed conditions, it is not unusual to see water penetrating farther towards the interior of the building than the glazing pocket. A reasonable solution is to detail sub-sill flashing with end dams with provisions for weeping any water harmlessly to the exterior.

"Fenestration Beyond Standards
Successfully integrating windows, storefronts, and curtain walls
by David W. Altenhofen, CSI, CCS, AIA
Reprinted with permission of The Construction Specifications Institute
99 Canal Center Plaza, Suite 300, Alexandria, VA 22314, from The Construction Specifier

Specifications

Similar to detailing, standards are only the starting point for successful window and curtain wall specifications. As discussed above, unedited manufacturer standard specifications will confound a detail Likewise, standardized master specification systems, such as American Institute of Architects (AIA) Masterspecâ„¢ and the Construction Sciences Research Foundation (CSRF's) Inc.'s Spectext®, are only starting points for project-specific specifications. Minimum performance values suggested in off-the-shelf master specs are frequently inadequate for project conditions beyond low-rise residential or light commercial work. The criteria developed during selection of the system must be carefully incorporated into the specifications to ensure the completed building performs as desired.

It is especially important to cover minor details that could seriously affect performance. Requirements should include manufacturer tests be based on windows substantially similar to project conditions. Two excellent examples of this are multiple units, ganged together, or units installed with perimeter trim or receiver channels. AAMA 450, Voluntary Performance Rating Method for Mulled Fenestration Assemblies, is the standard method determining the performance of mulled assemblies. However, details many manufacturers provide for mulling units together or for adding trim have never been performance tested. (It is of little use to specify a pressure-equalized rainscreen window rated to an architectural window performance class of 60 (AW60) and set it into untested, barrier-type perimeter receiver channels with a mullion joint of a single line of sealant.)

The specifications must also include requirements for quality control and testing during the construction period. Although there are standards for many individual laboratory and field tests, there are no standards explaining how and when to apply them. The architect must rely on his own knowledge or retain a qualified consultant to determine testing protocol. Opinions vary widely on testing order, modifying standardized procedures, and determining passes and fails. Lab testing can easily require half a dozen different tests, performed in specific order, with air and water penetration tests repeated periodically for a total of a dozen or more individual tests.

Finally, the specifications for all of the materials and systems adjacent to the windows, storefront, and curtain wall must be coordinated. For example, a vapor retarder generally should seal to the window frame to maintain a continuous envelope. By coordinating provisions between the window, insulation, vapor retarder, and sealant specifications, the seal cannot be missed.

"Fenestration Beyond Standards
Successfully integrating windows, storefronts, and curtain walls
by David W. Altenhofen, CSI, CCS, AIA
Reprinted with permission of The Construction Specifications Institute
99 Canal Center Plaza, Suite 300, Alexandria, VA 22314, from The Construction Specifier

Construction phase

With the exception of chain stores and some residential work, virtually every project becomes a prototype with new and unique solutions. As such, , testing is a crucial part of the successful installation and long-term performance of windows, storefronts, and curtain walls. Large and complex projects require the construction of mock-ups at off-site laboratories. These mockups are rigorously tested per the specifications for conformance to the indicated performance criteria. On more typical projects, on-site testing to look for leaks and verify installation quality is important.

Even standard systems with performance proven through laboratory testing need field testing. Many systems depend on careful installation, and testing will determine whether the proper methods were used. Something as simple as a omitting dab of sealant at the corners of glazing gaskets, or not using calibrated torque wrenches for fasteners on a pressure plate can cause systemic failure. More importantly, the interface between the window and the adjacent wall needs to be tested. This joint is far from standard and is probably one of the most customized aspects of the building envelope.

Conclusions

All standards-details, test methods, specifications, and so forth-are useful tools, but on their own are incapable of delivering well-performing windows, storefront, and curtain wall systems. In fact, standards can become a crutch for the ill-informed or unqualified. Architects, specifiers, engineers, and other professionals must remember the purpose of the standards is usually to provide a scientific comparison method of performance criteria. Standard details are just the starting point for the proper integration of windows, storefront, and curtain wall systems.

Additional Information

Author
David W. Altenhofen, CSI, CCS, AIA, has over 20 years of architectural experience, and is a past chair of the national level, American Institute of Architects (AIA) Professional Interest Area (PIA) Committee on Specifications and Building Technology. He chairs a similar committee at the local Philadelphia, Pennsylvania, AIA chapter, and has presented at AIA's national convention. Altenhofen is a guest lecturer and critic at Drexel and Philadelphia universities, and has already published two articles in The Construction Specifier. He can be reached via e-mail atdaltenhofen@kling.us.

MasterFormat No.
08050-Basic Door and Window Materials and Methods
General Data-Specifications
General Data-Standards

UniFormat No.
B2020-Exterior Windows

Key words
Division 8
American Architectural Manufacturers Association
American Institute of Architects
American National Standards Institute
American Society of Civil Engineers
ASTM International
Construction Sciences Research Foundation
International Building Code
National Fenestration Rating Council
National Wood Window and Door Association
Southern Building Code Congress International

Abstract
All standards-details, test methods, specifications, and so forth-are useful tools, but on their own are incapable of delivering well-performing windows, storefront, and curtain wall systems. Architects, specifiers, engineers, and other professionals must remember the purpose of the standards is usually to provide a scientific comparison method of performance criteria. Specialized skill and knowledge are required to integrate windows into a building, and fenestration design should not be limited by standards.

 

Originally published in the December 2005 issue of Architectural Record.