Tag Archives: curtain walls

Additional thoughts on the NAFS short-form specification

In the December 2014 issue of The Construction Specifier, Dean Lewis wrote about the North American Fenestration Standard/Specification for Windows, Doors, and Skylights’ (NAFS’) short-form specification. American Architectural Manufacturers Association/Window and Door Manufacturers Association/Canadian Standards Association (AAMA/WDMA/CSA) 101/I.S. 2/A440 serves as the basis for product certification as required by the International Building Code (IBC). Due to space constraints, two short ‘mini-articles’ were excluded from the final magazine’s layout. That information is now provided below.

ASTM and AAMA Standards
These are the ASTM standards and test methods cited in the 2011 edition of North American Fenestration Standard/Specification for Windows, Doors, and Skylights (NAFS). (All are assumed to be most current revision level unless otherwise cited.):

  • 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;
  • ASTM E330, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights, and Curtain Walls by Uniform Static Air Pressure Difference;
  • ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference;
  • ASTM E547, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Cyclic Static Air Pressure Difference;
  • ASTM E987, Standard Test Methods for Deglazing Force of Fenestration Products;
  • ASTM E2068, Standard Test Method for Determination of Operating Force of Sliding Windows and Doors;
  • ASTM F588, Standard Test Methods for Measuring the Forced Entry Resistance of Window Assemblies, Excluding Glazing Impact; and
  • ASTM F842, Standard Test Methods for Measuring the Forced Entry Resistance of Sliding Door Assemblies, Excluding Glazing Impact.

American Architectural Manufacturers Association (AAMA) polymeric profile standards include:

  • 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;
  • AAMA 308, Voluntary Specification for Cellular Polyvinyl Chloride (PVC) Exterior Profiles;
  • AAMA 309, Standard Specification for Classification of Rigid Thermoplastic/ Cellulosic Composite Materials;
  • AAMA 310, Voluntary Specification for Reinforced Thermoplastic Fenestration Exterior Profile Extrusions;
  • AAMA 311, Voluntary Specification for Rigid Thermoplastic Cellulosic Composite Fenestration Exterior Profiles; and
  • AAMA 313, Voluntary Specification for Molded Aliphatic Polyurethane Elastomer Frame Materials.

Design Pressure vs. Performance Grade
In the past, the terms ‘Design Pressure’ (DP) and ‘Performance Grade’ (PG) have been loosely used by some in the field. The specific definitions of these terms have been carefully confirmed with the publication 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).

Design pressure
Design pressure is a rating identifying the load induced by wind and/or static snow a product is rated to withstand in its end-use application—this is not to be confused with performance grade or structural test pressure (STP). Loads induced by static snow are applicable only to unit skylights, roof windows, and tubular daylighting devices (TDDs).

Performance grade
Performance grade is a numeric designator that defines the performance of a product in accordance with this standard/specification—this is not to be confused with DP or STP. PG is achieved only on successful completion of all applicable tests specified in Clause 5.

Structural test pressure
Structural test pressure is the pressure differential applied to a window, door system, TDD, or unit skylight. In this standard/specification, the STP is 150 percent of DP for windows and doors and 200 percent of DP for TDDs and unit skylights. This is not to be confused with DP or PG.

In other words, DP and STP are strictly structural qualifications, irrespective of the results of any air leakage resistance testing or water penetration resistance testing. On the other hand, the PG of a product is limited by the lowest/least performance of its structural, air leakage resistance, or water penetration resistance test results; operating force and/or forced-entry resistance requirements may also apply.

Port Canaveral’s Exploration Tower: Specifying unitized curtain walls and wind tunnel tests

FIGURE 1 Port Canaveral Exploration’s Tower ALT

Photo courtesy Slade Henson

by Patrick Condon, PhD, and Slade Henson
In the April 2014 issue of The Construction Specifier, the cover story centered around properly specifying color-changing mica coatings on Port Canaveral’s Exploration Tower. While the article acutely explored this Florida landmark’s iconic use of this new type of color-changing coating on aluminum, it also referenced an important wind tunnel study that bears closer inspection.

The facility was designed in accordance with American Society of Civil Engineers (ASCE) 7-10, Minimum Design Loads for Buildings and Other Structures, for a Risk Category III building with 254-km/h (158-mph) wind speeds and a 1700-year peak gust at 10 m (33 ft) in open country. Wind tunnel studies are used for prototype airplanes, helicopters, and cars, but they can also be employed for buildings that bend the wind load assumptions inherent in ASCE 7-10. In the built environment, wind tunnel study candidates include the world’s tallest buildings, architectural screens and solar panels, rooftop equipment, smoke stacks, and odd-shaped projects.

Due to the distinctive sail shape, a wind tunnel study was ordered by Baltimore, Maryland-based GWWO Inc./Architects that could predict loads on the structure as well as cladding. This test was based not only on the aforementioned ASCE 7-10, but also the 2010 Florida Building Code (FBC) and the 1999 ASCE Manual of Practice (67, “Wind Tunnel Testing”).

The architect chose Cermak Peterka Petersen (CPP) to perform the wind tunnel test and to produce the accompanying report. The firm holds the distinction of being the first U.S. company to provide the construction industry consulting services on the wind effects on building and structures.

With the aid of this exhaustive study, the structural engineer, Thornton Tomasetti, was able to provide a structural frame meeting the architects’ design requirements. This same wind tunnel study was also invaluable for the curtain wall specialty engineer, Broadway Engineering, to compute cladding loads for the curtain walls.

The wind tunnel test effectively eliminates the wall and roof uplift zones one normally finds in the structural drawings. For a wind tunnel test, a scale model of the building is created and sensors are placed along the model’s exterior that read negative and positive pressures throughout the tests. For example, 1023 pressure taps were used in the Exploration Tower model, and the data from 36 wind-direction tests was compiled.

Figure 1 reveals the wind pressures determined by the wind tunnel test. Being an uncommonly shaped building, the wind loads at various zones are equally unusual. In the example given, the test provided ultimate wind speeds; this allowed the glazier to modify the wind speeds by a factor of 0.60 to create an allowable stress design.

Figure 1

FIGURE 3 CPP Wind Tunnel Pressures of Port Canaveral Exploration’s Tower

Incomplete same of wind tunnel pressures of the tower’s north elevation. Image courtesy Cermak Peterka Petersen

To incorporate the wind tunnel test within the project, GWWO included it under the following MasterFormat categories:

  • 05 40 00−Cold-formed Metal Framing;
  • 05 75 00−Decorative Formed Metal;
  • 07 42 13−Metal Wall Panels;
  • 07 62 00−Sheet Metal Flashing and Trim;
  • 08 44 13−Glazed Aluminum Curtain Walls;
  • 08 80 00−Glazing; and
  • 08 90 00−Louvers and Vents.

This allowed the report to be entrenched into the manual, and subcontractors were brought into compliance with its findings.

Hurricane curtain wall
Based on the wind tunnel test, the curtain wall needed to withstand a maximum 380.8 kgf/m2 (78 psf). The tall freestanding spans on the first three floors led to the use of a 260-mm (10 ¼-in.) deep profile curtain wall. Additionally, the glazier was required to use steel within the vertical mullions to withstand reactions reaching as high as 1496 kgf (3300 lb). On the fourth through seventh floors, the glaziers were able to incorporate a standard 185-mm (7 ¼-in.) profile. While the profiles have different depths, the systems appear identical from the exterior of the building as they feature the same pressure bars and face caps.

GWWO incorporated not only standard 63.5-mm (2 ½-in.) wide face caps, but the frames north from the sail wall and across the cantilevered decks featured 133-mm (5 ¼-in.) wide mullions, horizontals, and face caps. The glazing subcontractor extruded special dies for the face caps to help the architects realize their vision for the building.

The finish for the curtain wall and the various trims was specified to be a fluoropolymer polyvinylidene difluoride (PVDF) resin (70 percent Kynar). GWWO inspected the curtain wall paint samples to ensure the colors were going to be as closely matched as possible. The color chosen for the curtain wall and the aluminum entrance doors was a standard fluoropolymer finish.

Unitized curtain wall
The Port Canaveral curtain wall system is a barrier, unitized curtain wall assembly, which translates into benefits in terms of construction speed, lifecycle costs, and field safety. Unitized curtain walls are assembled, glazed, and sealed in the factory, but erected in the field. They are transported to the jobsite on steel racks and then to the floor where they are snapped together (typically with the help of mini-cranes).

FIGURE 4 Two Curtain Walls Waiting on Glazing

Curtain wall frames awaiting delivery and glazing. Photo courtesy Patrick Condon

Each of the insulated glass frames at Port Canaveral was prefabricated to increase construction speed by allowing field workers to insert these units into the building structure. This article’s authors estimate a unitized field takes half the time of stick-built assemblies, which must be assembled and glazed onsite. If all glass and metal is available for manufacturing, it is possible to pre-build an entire building envelope prior to openings being ready. Therefore, this strategy can reduce the general contractor’s overhead and carrying costs. If the glass cannot be ready for preglazing, if opening measurements cannot be guaranteed, or if the project is relatively small, it may be better to opt for a stick-built system, rather than unitized.

Lifecycle costs arise out of long-term energy costs and air quality. Energy cost is related, in part, to air leakage and air quality related to water infiltration. Both stick-built and unitized curtain wall assemblies are caulked the same around the perimeter. This type of caulking is called a ‘barrier’ because the perimeter sealant stops water and air intrusion. Such a silicone perimeter barrier can carry a 20-year warranty from the sealant manufacturer.

The unitized system described in this article applies this barrier strategy to all external joints, whether they are metal to metal or glass to metal. A simulation study by the National Institute of Standards and Technology (NIST) has shown continuous air barriers reduce air leakage by 83 percent. Further, the research suggests reducing air leakage can result in up to a 40 percent savings on energy consumption.

Stick assumptions
As opposed to a barrier, unitized assembly, a typical stick system employs an internal water management strategy to seal internal joints. It includes three elements:

  • exterior glass and aluminum, which shed most of the water;
  • confined ‘air space’ (or ‘wet’ or ‘pressure-equalized’ zone); and
  • internal barrier.

The assumption is the internal barrier is 100 percent sealed, otherwise the pressure-equalized space fails and there are both air and water leaks. In curtain walls, the internal details of this system are complex and often missed during assembly or installation. The theory is any water that gets beyond the trim covers and gaskets will drain out the weep holes. However, because of the many complex details, there is a high probability some details will not be completed correctly.

FIGURE 2 CPP Wind Tunnel Mockup of Port Canaveral Exploration’s Tower

Wind tunnel mockup of Port Canaveral’s Exploration Tower. Photo courtesy Cermak Peterka Petersen

Stick problems
It is extremely difficult to follow 100 percent of the requirements for screws, sealants, joint plugs, and gaskets—a situation that can lead to leaks. For example, typical stick curtain walls rely on gaskets, both exterior and interior. They hold the glass and, in the case of pressure-equalized systems, keep the water out. On the other hand, a barrier, unitized system employs a relatively simple system of wet sealing all exterior joints. Common problems for stick curtain walls occur from ultraviolet (UV) degradation or stretching gaskets during installation of the glass. Over time, these gaskets shrink and corner intersections become another source of leaks.

Weakest link
A curtain wall’s ability to resist water and air leaks depends on the weakest elements. One example is a curtain wall, field-tested at 718 Pa (15 psf), but located next to a composite panel system tested at 287 Pa (6 psf). In this case, the building façade can only resist 6-psf water pressure. A second example  is a stick-built curtain wall with a perimeter silicone and 20-year warranty, but an internal water management system limited to one year.

For all intents and purposes, the system therefore has only a one-year warranty.

Field testing

Curtain walls, whether unitized or stick-built, should be field-tested for air and water. State-of-the-art field testing of curtain walls is defined by American Architectural Manufacturers Association (AAMA) 503, Voluntary Specification for Field Testing of Newly Installed Storefronts, Curtain Walls, and Sloped Glazing Systems. The AAMA field air test standard is 1.5x the project specified rate or 0.45 L/s∙m2 (0.09 cfm/sf)—whichever is greater. A 0.45-L/s∙m2 (0.09-cfm/sf) field test implies a 0.30-L/s∙m2 (0.06-cfm/sf) project requirement. The field water test is 2/3x the project’s water-resistant requirement (normally, 20 percent times design pressure).

For example, a project with a 488.2 kgf/m2 (100-psf) design pressure, field water testing should be 64.9 kgf/m2 (13.3 psf) and air testing 0.45 L/s∙m2 (0.09 cfm/sf). The unitized, barrier curtain wall discussed in this article exceeds these AAMA 503 numbers because it has silicone sealants applied to all joints in the system.

Safety
A unitized curtain wall system should also have a better field safety record than a stick system because fully glazed panels are lifted onto the building and secured (unlike stick-built systems that require installers to lift the framing into place and then return in multiple runs to install glass). With less overhead lifting and less handling of glass, safety can be improved.

Glazing
Since it is located in an area that feels the effects of hurricanes, the design of Exploration Tower required GWWO to consider the effects of wind-borne debris. Therefore, the project incorporated insulated-laminated glass for the large- and small-missile zones on the building, including clear lamination on the interior lite to stop a Missile D.

FIGURE 5 Point-Load Canopy

Point-load canopy in steel structure. Photo courtesy Slade Henson

Additionally, GWWO also had energy concerns regarding solar heat gain coefficients (SHGCs) and U-value. (The former is the amount of solar energy striking the glazing that ends up warming the building, while the latter is the rate of heat transfer through the glazing.) The low-emissivity (low-e) insulated glass chosen for the project exceeded the specification’s requirements.

On the west elevation of Exploration Tower is an eye-catching canopy—a flat plane of glass supported by steel and cross members. The point-loaded canopy was suspended over the steel structure by 66 brushed stainless-steel column-mount spider fittings. Of the 15 units of 14.3-mm (9/16 in.) laminated glass in the canopy, only two are rectangular in shape. All others are trapezoids, offset parallelograms, or have a 559-mm (22-in.) diameter radius cut from the edge.

While the design is beautiful, Broadway Engineering and the architect needed to ensure the project met the loads from the Wind Tunnel Study. This standard was met while the glass fabrication, with various installation features, exceeded the highest glazing standards.

Sunshades
Another feature that is both energy practical and attractive is the use of sunshades. The assembly offsets sun glare while providing another aesthetic architectural detail. The sunshades are aluminum extruded 254 x 76-mm (10 x 3-in.) air foils painted using a standard bone-white fluoropolymer to match the curtain wall. They wrap around the corners and encroach slightly onto the building’s north elevation, but are firmly integrated in the curtain wall. A special bracket was fabricated for these 109-degree mitered corners.

With the assistance of the Wind Tunnel Test report, Broadway Engineering calculated the load the sunshades would place on the curtain wall. With that information, additional steel placed in the mullions allowed the glazing subcontractor to incorporate the sunshades into the curtain wall, allowing GWWO to realize its overall vision for the building.

Conclusion
While it may be easy to rely on standard practices to determine wind loads on a building, performing a wind tunnel test removes the guesswork and uncertainty inherent in a building that is more than a simple rectangle. The wind-load effects of a beautiful canopy will be clear to the glazing subcontractor who can now install a safe product able to withstand the worst Mother Nature has to offer.

Patrick Condon, PhD, LEED AP, is the president of West Tampa Glass Company. He holds a doctorate in industrial engineering from Arizona State University. He has been responsible for company projects across Florida and Southeast United States. He can be reached via e-mail at pcondon@westtampaglass.com.

Slade Henson is the project manager for the curtain wall package at Port Canaveral’s Exploration Tower. He has worked in the construction industry since 2001. Henson transitioned from field to shop fabrication, then from purchasing agent to project management—the position he now holds with West Tampa Glass Company. He can be e-mailed at shenson@westtampaglass.com.

New guide examines North American Fenestration Standard

Modern corporate building

The user guide for the 2011 North American Fenestration Standard/Specification for Windows, Doors, and Skylights (NAFS) provides information on the standard’s applications.
Photo © BigStockPhoto/Leung Cho Pan

After multiple years of collaboration, the American Architectural Manufacturers Association (AAMA), CSA Group, and Window & Door Manufacturers Association (WDMA) have released a ‘user guide’ to the 2011 AAMA/WDMA/CSA 101/I.S.2/A440, North American Fenestration Standard/Specification (NAFS).

The resource was developed as a non-mandatory, advisory document meant for specifiers, architects, code officials, manufacturers, and testing laboratories. Containing commentary, illustrations, and examples, it provides information on the proper application of the standard, which deals with windows, doors, and skylights, explained Joe Hayden, WDMA’s Joint Document Management Group (JDMG) co-chair.

“This is the latest product of the ongoing effort to harmonize fenestration standards in North America,” he said. “This effort started nearly 20 years ago and is evidence of the fenestration industry’s desire to offer a single, unified performance specification across borders.”

The 2011 version of NAFS is already referenced in the 2012 editions of the International Building Code (IBC) and International Residential Code (IRC). It is performance-based and material-neutral, permitting the architect or specifier a great deal of latitude in design and products, while maintaining compliance, explained Dean Lewis, AAMA’s educational and technical information manager.

“In lieu of tedious, time-consuming listings of compliance test methods, the standard permits the designer to specify windows, doors and skylights by providing just the product’s operator type, Performance Class, and Performance Grade,” he told The Construction Specifier. “Related performance requirements, including wind resistance, water penetration and air leakage resistance, forced-entry resistance, and material and component qualifications, have all been included.”

“At the same time, the design professional is not constrained by the minimum performance requirements included in the document. Specific, higher-performance criteria may be designated if necessary, such as a higher percentage water penetration resistance or requiring lifecycle testing for Performance Classes where this is optional,” Lewis continued. “A secondary benefit of such a universal specification has been the development of certification programs that publish directories of pre-qualified products based on third-party testing, inspection and validation.”

For more information, visit here.

Overlooked Considerations for Windows and Curtain Walls

Photo © BigStockPhoto/Zhiwei Zhang

Photo © BigStockPhoto/Zhiwei Zhang

by Derek B. McCowan, PE, and Douglas R. Pac, EIT

The primary factors most designers consider when selecting window and curtain wall assemblies for their projects are well-understood: cost, aesthetics, and thermal performance, to name a few.

There are some other important, though often overlooked, considerations that can be important to weatherproofing, performance, and durability. This article will discuss some of these lesser-known factors and some of the related issues and challenges.

Material selection for windows
Since nearly all curtain wall and storefront systems exposed to weather are fabricated from aluminum, this article’s section focuses on window materials. The common material types selected for commercial windows include vinyl, aluminum, wood, aluminum-clad and vinyl-clad wood, fiberglass, and steel. Each has several advantages and disadvantages, along with considerations that affect performance and durability.

Vinyl
Vinyl windows are typically the most inexpensive of the aforementioned material options; they are often chosen when appearance is less of a concern. Aside from their look, vinyl windows have proven to be an economical and rot-resistant option for residential construction applications where small punched openings are required.

Nevertheless, the authors recommend specifying commercial-grade windows over residential ones, even on wood-framed residential projects. Vinyl is not a high-strength material and therefore typically cannot accommodate large openings where high wind loads are present. (Vinyl is often reinforced with aluminum for improving strength.)

Welded vinyl window frame corner and nailing fl ange. Images courtesy Simpson Gumpertz & Heger

Welded vinyl window frame corner and nailing flange. Images courtesy Simpson Gumpertz
& Heger

Key benefits of vinyl windows often include:

  • heat-welded frame corners that provide good resistance to water penetration (Figure 1);
  • wept glazing pockets that allow water bypassing glazing seals, as well as condensation, to drain to the exterior; and
  • continuous nailing flanges at the perimeter of the windows for easy installation and reliable connections of surrounding weather barriers to the window frame.

Another attractive feature available with some systems is the use of a fully welded ‘master frame’ around the perimeter of multi-unit assemblies, such as two or three windows ganged together. The continuous, welded perimeter frame can help eliminate the sensitivity of gang mullion end conditions that are often problematic with respect to leakage.

Aluminum
Like vinyl windows, aluminum windows are attractive to building owners because they are highly resistant to corrosion, and are also strong and durable. With this increase in strength and durability generally comes added cost.

Although there are exceptions, aluminum windows rarely come equipped with wept glazing pockets; unlike many of their vinyl counterparts, they rely on sealant at mitered or coped frame corners (Figure 2), making a sill pan flashing an essential feature of a window system design—even shop-applied sealant is sensitive to workmanship (i.e. workers in a factory) and sealant can degrade and lose adhesion over time.

The added strength of aluminum, combined with the various options that are often available (e.g. receptor framing, reinforcing mullions, and gang mullions) generally make aluminum windows a good option where large multi-unit and ribbon assemblies are needed.

Wood
Wood windows are often used in residential and historic applications, but are susceptible to rot, especially when less durable species like pine are used without being maintained. Durable wood products like mahogany are a great option, but may be cost-prohibitive. Aluminum-clad or vinyl-clad wood windows are often specified when the owner and/or designer desires the appearance of wood on the interior while obtaining the durability/rot-resistance of an aluminum or vinyl window on the exterior. The concept is appealing, but there are weaknesses of some clad wood windows that often go unrecognized.

Sealed aluminum window frame corner.

Sealed aluminum window frame corner.

Joints in the cladding are either treated with sealant or left unsealed (Figure 3). If seals at the cladding joints and/or glazing seals are poorly fabricated, not well-maintained, or not installed, water can bypass the cladding and reach the wood core. Once water reaches the wood core, it can become trapped due to a lack of weep provisions within the cladding itself (with many systems, no weep paths are provided). This can lead to rotting and leakage to the interior.

Many manufacturers treat the wood with a preservative that can help extend the material’s life. However, the preservative does not solve the underlying problem. Other products like aluminum may carry similar risk of leakage due to failure of glazing seals, but they do not pose the same risk of frame deterioration.

Perimeter detailing
Well-considered fenestration perimeter details are critical for the window, curtain wall, and storefront system’s air and water penetration resistance. Industry professionals who conduct leakage investigations on problem buildings know the perimeters of glazed systems are often the source of problems. Flashing and perimeter sealant detail options can vary depending on the type of fenestration chosen and, in particular, the shape of the frame members at the system perimeter. If the perimeter detailing is not coordinated with the intended fenestration type, issues can arise. The failure of coordination may arise as a result of inconsistency between the drawings and specifications, or due to a substitution.

For instance, the authors have reviewed designs where the drawings showed pressure-glazed curtain wall mullions and related flashing details, but the specification called for an interior-glazed storefront system. Storefront systems have different mullion geometries and work differently from curtain walls—therefore, flashing options are quite different.

Situations like this can go unnoticed until it is time to fabricate and install the storefront system, at which time the installer may realize it is not possible to install the perimeter details shown in the drawings. Similar situations arise when fenestration systems are changed after the design is complete (i.e. value engineering or substitution requests), or when an acceptable ‘or equal’ system is selected by a subcontractor rather than the ‘basis of design’ (BOD) one.]

Unsealed joint in aluminum cladding.

Unsealed joint in aluminum cladding.

Continuing with the curtain wall versus storefront example, a common and reliable perimeter flashing condition for curtain walls consists of a flexible sheet membrane, such as silicone sheet or ethylene propylene diene monomer (EPDM) membrane, extending from the surrounding weather barrier into the glazing pocket of the curtain wall (Figure 4). The perimeter mullions of storefront systems generally do not have an open glazing pocket to facilitate direct access for tying in the perimeter flashing, so the glazing pocket flashing detail is not feasible. If the project team does not realize the discrepancy until the glass-and-metal systems are already fabricated and installation is imminent, issues can result, and the opportunity to achieve a reliable perimeter flashing can be lost.

Similarly, if a stick-built, pressure-glazed curtain wall system and associated glazing pocket flashing concept is specified, but a pre-glazed unitized curtain wall system is provided, the flashing design will no longer work (even with an excellent high-performance unitized system). When units are pre-glazed, the glazing pockets will not be accessible, and a continuous starter sill will be present along the sill condition, necessitating a re-design of the flashing system.

This topic is not only applicable to storefront and curtain wall systems, but it also holds true for the different window types discussed in this article. For another example, if flanged windows are envisioned by the design team though block-frame windows are purchased for the project, the original detailing, sequence of installation, and even the intended weatherproofing materials may no longer work.

Performance data
Designers often rely on manufacturer-published performance data when selecting fenestration systems. Some of the more common test data in which specifiers are interested include thermal performance, air and water penetration resistance, and structural performance. The data is useful for relative comparisons between similar base systems, but does not ensure the exact glazing assembly specified for a project will have the same performance level.

For instance, data is often published for a particular window model of a particular size with a standard configuration (i.e. a single window unit of a standard size is tested, not various sizes or any multi-unit assemblies). In most cases, this exact window size will not be installed on any given project. The data may not be applicable for:

  • larger windows (often, bigger windows have more trouble meeting performance requirements);
  • windows of a different configuration (e.g. multi-unit assemblies ganged together, with or without a receptor frame);
  • windows with additional features such as muntins; and
  • windows with different glass types/thicknesses.
Flexible membrane fl ashing at perimeter of curtain wall is shown in the upper photo. Flexible membrane fl ashing extending from weather barrier into curtain wall glazing pocket is depicted in the lower photo.

Flexible membrane fl ashing at perimeter of curtain wall is shown in the upper photo. Flexible membrane fl ashing extending from weather barrier into curtain wall glazing pocket is depicted in the lower photo.

Often, untested, unrated components—such as gang mullions, sill receptors, and muntins—are where problems arise during field testing and in service.

For example, on a recent project, a window advertised as an “80-psf window” (i.e. 3830 Pa) was barely able to meet a 40-psf (i.e. 1915-Pa) design wind load. This was mainly due to window sizes being larger than the assembly tested by the manufacturer, along with the inclusion of components like muntins that were advertised as standard, but were not included in the tested assembly. The authors have also observed frequent water test failures at ganged mullions that were advertised as standard components, but also not included in the tested assembly.

Conclusion
Special care is needed when specifying and detailing a fenestration system—be it a window, storefront, or curtain wall. One should not only consider appearance, economics, and thermal performance, but also give consideration to other factors with special focus on project-specific detail considerations.

Are large punched openings or ribbon openings desired? How important is rot-resistance when compared to appearance? How proactive will building owners/managers be with respect to maintenance? What is the likelihood a different glazed system will be proposed after the design work is complete, and how will this process be managed? These are just a few questions to consider on future projects.

Derek B. McCowan, PE, is a licensed professional engineer in Massachusetts with national engineering firm Simpson Gumpertz & Heger. He has 12 years of experience investigating building envelope failures, consulting on new design projects, and providing construction administration and performance testing services. He has a graduate degree in civil engineering and is member of the American Society of Civil Engineers (ASCE) and of ASTM International Committee C24 Building Seals and Sealants. McCowan writes and presents frequently on various building enclosure topics and has experience as a guest lecturer at various local universities. He can be reached at dbmccowan@sgh.com.

Douglas R. Pac, EIT, works for Simpson Gumpertz & Heger. He has six years of experience investigating envelope failures, designing envelope repairs for restoration/renovation projects, providing construction administration services, and consulting on new design projects. Pac has a master’s degree in civil engineering from the University of New Hampshire. He can be reached at drpac@sgh.com.

A deeper look at ‘breathable’ curtain walls

Glass-clad buildings are often designed to be airtight for energy efficiency, but some design experts feel new thinking on ventilation could have important benefits for indoor air quality (IAQ). Photo © BigStockPhoto/Oleksiy Mark

Glass-clad buildings are often designed to be airtight for energy efficiency, but some design experts feel new thinking on ventilation could have important benefits for indoor air quality (IAQ).
Photo © BigStockPhoto/Oleksiy Mark

In the August 2013 issue of The Construction Specifier, we included a Horizons column—“Introducing ‘Breathability’ to Curtain Walls—by Raymond Ting, PhD, PE.

As a complement to our more straight-ahead technical features, Horizons examines still-emerging technologies and new ways of assembling buildings. In this particular case, Ting dealt with the issue of glazed assemblies and their impact on both energy efficiency and indoor air quality (IAQ). He acknowledged building codes advocated for airtightness and air barriers, but called for new thinking about how ventilation—a good dose of fresh air, in other words—might be more critical for the occupants living or working inside.

Due to space constraints, a short sidebar article was kept out of the magazine. For a further look into the rationale behind Ting’s call for breathable curtain walls, it is included here. Continue reading