by brittney_cutler_2 | November 25, 2021 10:55 am
By Amy Roberts
Successful selection, specification, and construction of curtain wall systems requires an understanding of project-based goals and site conditions, and a commitment to teamwork. Supporting performance-based specifications, it helps to first define the terminology and then the methodology.
A curtain wall system—relieved of the required strength necessary to support the weight of upper floors, as was the case for vintage construction methods—imposes a relatively minimal dead load and has become primarily a filtering envelope for the building. In this role, it controls the passage of heat, light, water, air, and sound, and has opened many options for architectural esthetic expression.
In its basic form, a glass and metal curtain wall consists of a lightweight metal, usually aluminum, gridwork assembled as individual pieces on site (stick-built system) or as part of factory preassembled panels (unitized system), and a combination of transparent and opaque infill panels (spandrels). The grid is typically attached at discrete points to the floor-slab edges, hanging like a curtain on the building.
Glass forms the most popular choice for the vision area, whether fixed or partially operable to admit ventilation. Insulating glazing units (IGUs) improve thermal performance with double or triple panes of glass, usually tempered or laminated, tinted, or with low-emissivity (low-e) coatings, separated by a space filled with desiccated air or with an inert gas(es), or at the cutting edge, a vacuum. Spandrel glass, which is darkened or opaque, typically is located between the areas of vision glass aligned with the floor edge of a building. Spandrel glass is the area of glass panels that conceals structural building components such as columns, floors, HVAC systems, electrical wiring, plumbing, etc. Unlike vision glass, which is meant to be transparent, spandrel glass is designed to be opaque to help hide features between the floors of a building, including vents, wires, slab ends, and mechanical equipment. Other common infills include stone veneer, metal panels, louvres, composites, plastics, and operable windows or vents.
When designing metal-framed curtain walls, there are generally five key design considerations:
Although made of lightweight materials, a curtain wall must withstand lateral live loads imposed primarily by the wind, transferring these loads along the load path from the curtain wall, to the main building structure, through anchoring devices attached to floor or columnar elements. Quantifying and designing to accommodate these loads is a routine procedure for structural engineers, albeit aggravated due to the height and geometry of the building, as well as by surrounding structures or natural topography. In addition, there are loads imposed by building sway induced by wind, seismic forces, and by thermal expansion and contraction.
It is strongly recommended design loads (in Pa) specific to all relevant areas of the building be stated in the specifications. Special requirements for snow or ice loads on projecting wall elements, such as sunshades, are often applicable. Appropriate design wind loads for most situations may be selected by the architect from ASCE/SEI 7, Minimum Design Loads for Buildings and Other Structures by the American Society of Civil Engineers and the Structural Engineering Institute, or in Canada, the National Building Code of Canada (NBC).
Compliance shall be demonstrated by testing in accordance with American Architectural Manufacturers Association (AAMA) 501.4 “Recommended Static Test Method for Evaluating Curtain wall and Storefront Systems Subjected to Wind and Seismic Induced Inter-Story Drifts.” CWM-19 (page 47 – 184.108.40.206.6) says:
“Determination of seismic movements and load(s) is the sole responsibility of the building’s Engineer of Record, considering code interpretation issues and/or prescriptive requirements not included in Contract Documents. The curtain wall manufacturer is not responsible for determination of these movements or loads, and AAMA strongly recommends that design criteria specific to all relevant areas of the building be provided by the Specifier.”
Deflection of glass-supporting frame members
The more the frame deflects under the load, the more stress is placed on the glass and the greater the likelihood of breakage. The general industry recommendation is to limit frame member deflection to maximum of L/175 of the unsupported span length (L) of up to 4.1 m (13.5 ft) in length, maximum L/240 + 6.3 mm (0.25 in.) at spans more than 4.1 m. The required deflection limit is determined by the specifier, who may discuss with the project’s structural engineer, but is often limited to 9.53 to 12.7 mm (0.375 to 0.5 in.).*
In Canada, aluminum work should be according to the Standards Council of Canada’s (SCC’s) CAN3 S157 Strength Design in Aluminum, with deflection under full design loads limited as noted above. Glass design should be according to CAN/CGSB-12.20 Structural Design of Glass for Buildings.
2. Provision for movement: Thermal expansion
Temperature differences must be considered, as they relate to differential expansion and contraction of various materials. Aluminum, the most common material choice for curtain wall framing, exhibits a relatively high coefficient of expansion in comparison to glass.
The potentially wide daily and seasonal fluctuations in the metal’s surface temperature can induce thermal expansion of typically 6.35 to 7.95 mm (0.25 to 0.3125 in.) in a 3.05 m (10 ft)* framing member. A contiguous sheet of glass will expand by less than half that amount. This disparity causes relative movement that must be accommodated without causing undue stress on glass, joints, and anchors, or without excessively reducing the frame’s ‘bite’ on the glass.
*SI/IP equivalent measurements are as quoted from the reference document or mathematically converted using standard formulae.
3. Weather tightness
Weather-tightness means protection against water leakage and excessive air infiltration. It depends in large measure on adequate provision for movement and is closely related to proper joint design.
Two methods have been developed for preventing leakage through the wall. One is referred to as the ‘internal drainage’ or ‘secondary defense’ system, wherein minor leakage can be shunted by a system of flashing, collection, and drainage devices. The other is the more sophisticated ‘pressure equalization’ method, which is based on the ‘rain screen principle.’ A rain screen system requires the provision of a ventilated outer wall surface, backed by drained air spaces in which pressures are maintained equal to those outside the wall.
The specifier may optionally specify pressure-equalized rain screen wall cladding (PRWC) systems that meet the requirements of AAMA 508-14, Voluntary Test Method and Specification for Pressure Equalized Rain Screen Wall Cladding (Panel) Systems, when tested in accordance with ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference. AAMA 508 and other AAMA references are documents of the Fenestration Glazing and Industry Alliance (FGIA).
Site checks for watertightness conducted during installation are highly recommended to verify the design and installation are satisfactory or to reveal deficiencies in the sitework. Consequently, they should be made early in the installation process, so any faults discovered may be remedied before much of the wall is in place. On tall buildings, the check usually is repeated once or twice at higher levels as an added assurance.
The newly installed curtain wall shall be site tested by an AAMA-accredited independent laboratory, as soon as practical after installation, in accordance with AAMA 503, Voluntary Specification for Field Testing of Newly Installed Storefronts, Curtain Walls and Sloped Glazing Systems.
AAMA Accredited Laboratories are qualified to conduct performance testing for air leakage, water penetration, deflection, and structural strength. Some of these laboratories are also qualified to conduct condensation resistance, thermal transmittance, and other performance tests. A current listing of Accredited Laboratories is available from AAMA.
Resistance to water penetration performance requirements will vary depending on the building’s height, geographic location, and exposure classification. In Canada, the CAN/CSA-A440 Standard includes a user’s guide, which recommends minimal performance levels for each major Canadian city based on geographic location and installation height, while in the United States, the resistance to water penetration rating is typically established as a function of the design wind pressure.
Specifiers should remember to check the NBC (Part 5) to see whether it differs from the provincial code or local jurisdiction. Exact project requirements must comply with, and may exceed, the applicable building codes based on the project’s location.
Industry recommendations limit air infiltration through the wall shall not exceed 0.3 L/s•m2 (0.06 cfm/sf) of fixed wall area, plus the permissible allowance specified for operable windows or doors when tested in accordance with ASTM E283, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Skylights, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen, at a static air pressure difference of 300 Pa (~6.26 psf).
Canadian specifications typically require a maximum allowable rate of air leakage not to exceed 0.10 m3/h/m2 (0.06 ft3/min/sf) of the sample curtain wall area when tested at an air pressure difference of 75 Pa (1.57 lbf/sf). These values are per the Glazing Contractors Association of British Columbia (GCABC) Glazing Systems Specifications Manual.
NBC Part 5, Section 220.127.116.11.2.b, states a requirement of 0.2L/s•m2 for fixed portions, including opaque portions and 1.5 L/s•m2 for operable portions. Any jurisdiction that has adopted the 2015 NBC would have these requirements. Section 18.104.22.168.3 lists exceptions to these requirements. Specifiers should be sure to verify the specific requirements of the local jurisdiction.
4. Energy efficiency
Although metal and glass are materials with inherently low resistance to heat flow, improving thermal performance can be accomplished by minimizing the proportion of metal framing members exposed to the outdoors, eliminating thermal short circuits by means of ‘thermal breaks,’ using high-performance insulating glass, and providing adequate insulation in large spandrel areas. In addition, the large glazed areas allow natural light to penetrate deeper within the building to supplant electric lighting and save energy, while optimizing solar heat gain using low-e coatings.
In Canada, minimum national energy performance is determined according to the Model National Energy Code for Buildings (MNECB) for Part 5 Buildings, or ASHRAE 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings criteria, however, some provinces and territories may have their own provincial energy codes.
According to the Public Works and Government Services Canada’s and the Canada Mortgage and Housing Corporation’s (CMHC’s) Glass and Metal Curtain Wall Best Practice Guide, the fixed light area of the curtain wall must have an overall thermal transmittance (U-factor) [W/(m2•K)] not exceeding that stipulated by the specifier. Note, the U-factor so selected will be defined by applicable codes based on project location and may be guided by other program criteria, such as Leadership in Energy and Environmental Design (LEED).
The overall coefficient of heat transfer (U-value) is an important property of a curtain wall system. When compared with the solar heat gain coefficient (SHGC) of the glazing, an overall energy performance level can be determined. A test for thermal transmittance means heat flow due to conduction, radiation, and convection.
U-factors are tested per AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, or simulated per AAMA 507, Standard Practice for Determining the Thermal, Performance Characteristics of Fenestration Systems in Commercial Buildings, or (optionally) applicable U.S. National Fenestration Ratings Council (NFRC) testing, modelling, and validation protocols.
In Canada, using the methods described in SCC’s CSA-A440.2, Energy Performance of Windows and Other Fenestration Systems, allows U-factor to be determined either by computer simulation or through testing per referenced ASTM methods, per the GCABC Glazing Systems Specifications Manual.
The resistance of highly conductive curtain wall framing to condensation under winter conditions is important in a cold climate. Testing or analysis to assess the condensation potential of a curtain wall system is carried out by one of three different means, each with its own limitations. These means include a simple, large chamber test, a formal thermal chamber test, and computer simulation.
The fixed light area of the curtain wall, including glass and metal framing, should have a condensation resistance factor (CRF), not less than that selected by the specifier based on climate zone when tested in accordance with AAMA 1503, which yields the CRF. In Canada, this is done using the test method specified in CSA-A440.2. Although not a mandatory part of this standard, the results are used to calculate the Temperature Index (I). Note, CRF and I cannot be mathematically reconciled to each other.
5. Sound Control
Insulating and/or laminated glass generally improves sound attenuation. Where a high degree of sound insulation is required, such as near airports and in metro areas, air leakage through the wall, and resonance of rigidly supported glass lites, also should be minimized. For more details, refer to AAMA TIR-A1-15, Sound Control for Fenestration Products, or 1801-13, Voluntary Specification for the Acoustical Rating of Exterior Windows, Doors, Skylights and Glazed Wall Sections.
Building tolerances and clearances
The terms ‘tolerance’ and ‘clearance’ are often confused. A tolerance is a permissible amount of deviation from a specified or nominal characteristic; in general, tighter tolerances equal higher costs. A clearance is a space or distance purposely provided between adjacent parts, either to allow for movements or for anticipated size variations, to provide working space, or for other reasons. Both are critical because curtain wall construction involves covering a site-constructed skeleton with a factory-made skin, involving the work of numerous trades, which implies numerous sources for variance.
The greatest quality issues related to glass and metal curtain wall installation are tolerances and clearances. Failure to properly control them is the reason for most curtain wall installation problems. Architectural detailing can fail to recognize the full significance of standard tolerances and may provide inadequate clearances for installing the wall, necessitating changes, and adjustments on site. This delays the work and usually results in unnecessary additional costs. Therefore, it is essential specifications require the proper alignment and location of all materials related to the wall. Four different tolerances must be considered: building frame, installation, material, and fabrication and assembly.
Clearances are essential to allow proper working of sealant joints for differential movement and for access for modifications. Clearance issues are most often noted at non-typical anchors. In general, these should be at least 50 mm (2 in.) plus outward tolerance provided, per the CMHC’s Glass and Metal Curtain Walls Best Practice Guide.
Industry-recommended curtain wall system tolerances (excluding installation and substrates) for the overall sizes of factory-assembled framing and trim or for singular rectangular curtain wall units cannot deviate from the specified dimensions drawings by more than the following:
(a) ± 1.5 mm (0.0625 in.) for all dimensions 1.83 m (6 ft) and under;
(b) ± 3 mm (0.125 in.) for all dimensions between 1.83 m and 3.66 m (12 ft); and
(c) ± 4.5 mm (0.1875 in.) for all dimensions greater than 3.66 m.
Recommended allowable maximum deviations during installation are called out for level, plumb, and true characteristics, as well as straightness and variation from plane. Typical Canadian tolerance recommendations for variation from nominal level, plumb, square (diagonal measure), and true (straightness) positioning of the overall unit is in all cases 3 mm/m, (~0.12 in./ft).
Guide specification offers roadmap
In North America, the comprehensive resource for manufacturers, architects, specifiers, contractors, and testing agencies is AAMA CWM-19, Curtain Wall Manual, which addresses the above summarized aspects of curtain wall design, specification, testing, and installation. Newly revised in 2019, it is the most current source of information and features a comprehensive guide specification to aid architects and specifiers in developing job-specific curtain wall specifications.
Formatted to be compatible with the three-part CSI Master Specification, it covers performance and testing requirements, plus fabrication and installation methods, referencing a broad range of accepted industry standards. In writing the specifications for a project, an architect or specifier fills in the appropriate blanks for specific performance indices, deletes inapplicable paragraphs, or adds paragraphs to meet special or optional requirements. Explanatory notes aid in use. The guide specification is laid out as follows:
Section 1: General
This section contains descriptions of product, scope of the system, and a list of work to be included in the specification. It references additional items that should be specified, such as qualifications, performance, and testing requirements (including possible use of mock-ups), required submittals (e.g., samples, structural calculations, and test reports), quality assurance measures, and warranties.
Performance and testing requirements
At some stage during their design and development, all curtain wall systems should be tested for air infiltration, water penetration, and structural performance (including frame deflection limits) at the wind loads applicable for the building site. Particularly for custom designs, a preconstruction mock-up test should be scheduled well in advance of the final production schedule for a building, affording ample opportunity to make corrections relatively easily and less expensively.
Section 2: Product
The product section describes the framing material (metals, finishes, and protective coatings), glazing materials, system configuration, and components such as anchoring fasteners. Fabrication requirements also should be specified to include requirements for the desired degree of factory assembly and/or glazing, mechanical fastening, and sealing of joints. Third-party specifications for such items are listed.
Section 3: Execution
This section covers specifications requirements for matching and anchoring the wall to the building structure, such as erection tolerances and clearances, plus requirements related to site installation, such as welding, sealing, glazing, insulating, protecting, cleaning, and other on-site work. To guide the wall contractor, the guide specification covers the provision of building perimeter offset lines on each floor and benchmarks to be scribed on each floor on a designated column.
Language for a variety of optional project-specific requirements is included in the guide specification. Some examples include unitized system requirements, green documentation (sustainable design), pressure-equalized design requirement, sound transmission, site testing requirements, as well as use of building information modelling (BIM) software.
Other publications specific to the Canadian market, are:
• Glass and Metal Curtain Walls Best Practice Guide – Building Technology
Referenced above, this is one of a series of CMHC’s technical publications. It covers materials, types of curtain wall systems, basic performance aspects, test methods, and quality control. The guide includes a guide specification NMS089020 (2000).
• Glazing Systems Specifications Manual (National Version)
Originally published by the GCABC in the early 1990s, the Fenestration Association of British Columbia (FEN-BC), has initiated a revision to comply with the 2020 NBC. It should be noted the Glazing Systems Specifications Manual is not specific to Part 9 Buildings. It is not based only on CSA A440 and on the NBC.
The guide specifications provided with these publications are quite like that in AAMA CWM-19, but reference traditional Canadian versus international standards for specific materials and testing.
Glass and metal curtain walls are highly engineered, and factory-built to close tolerances. Installation requires the placement of these precision-built parts on a structure are built to much greater dimensional tolerances. As such, proper and timely communication among the members of the project team—the architect, the specifier, manufacturer, general contractor, the wall manufacturer, and the installation contractor—is the essential requirement of a successful installation. For best results, design and installation considerations cannot be relegated to their respective silos.
Communication is key. Each member of the curtain wall designing-building team—the architect, specifier, curtain wall system manufacturer, general contractor, and installation contractor—must be on the same page regarding the needs and requirements of the system and any potential problems that could affect other team members. Doing so from start to finish anticipates and prevents problems. To avoid such situations, AAMA CWM-19 lists specific installation responsibilities that link the architect or specifier to the installer.
Amy Roberts oversees the Fenestration and Glazing Industry Alliance (FGIA) Canadian standards and regulatory building/energy codes, as well as the Insulating Glazing Manufacturers Association of Canada (IGMAC) Certification Program for Insulating Glass (IG) Units. She has more than 20 years of industry experience in glass and IG manufacturing, and in both residential and commercial window manufacturing. She can be reached via email at firstname.lastname@example.org.
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