Establish System Track Record
Select a curtain wall with a demonstrated track record in similar applications and exposures. Verifying track records may require significant research by the designer. ASTM E1825 provides guidance.
Review laboratory test results of systems or similar custom systems for air, water, and structural resistance, heat transmission, condensation resistance, sound transmission, and operability. Verify that tests pertain to the system under consideration and not a version of the system with the same product name but of different construction.
Designing for Waterproofing Performance
Curtain wall design should start with the assumption that external glazing seals, perimeter sealant joints and curtain wall sills will leak. The following summarizes recommended features:
Select frames with wept glazing and pocket sills sloped to the exterior to collect water that penetrates the glazing and drain it to the exterior. Do not use vertical mullions as drain conductors. Each glazing pocket should be fully isolated from adjacent glazing pockets. Provide a sill flashing with end dams and with an upturned back leg turned up into the glazing pocket at the base of the curtain wall to collect and drain curtain wall sill leakage; provide jamb flashings to direct perimeter leakage down to the sill flashing.
Key frame drainage features include slope to the exterior at surfaces that collect water (slope top of exposed horizontal mullion surfaces, slope at flashings), large (3/8 inch diameter or a slot 5/16″ x 3/8″ minimum) weep holes closely spaced (three weep holes per each section of horizontal mullion between vertical mullions, typically), and drainage at every horizontal frame (do not use vertical frames to drain past horizontal frames). Use as many 1/4-inch by 2-inch slots as required for pressure-equalized systems. Design the drainage system to handle condensation as well as rain.
Curtain wall perimeters should have flashings (sill, jambs and head) that are sealed to the air and water barrier at adjacent walls. Slope head and sill flashings to the exterior to promote drainage. Integrate curtain wall sill flashings with sill flashings or base of wall flashings of adjacent walls.
Curtain wall should have a primary air/water seal between the shoulder of the tube at the plane of the glazing pocket and the air barrier of the adjacent construction.
Perimeter sealants are useful as a rainscreen for limiting air and water penetration through the outermost plane of the wall, but should not be relied upon as the sole air/water penetration barrier.
Coordinate placement of setting blocks with weep holes to avoid blocking drainage paths.
Glazing Methods and Their Impact on Performance
Pressure Plate Glazing: In this system the glass and infill panels are installed from the exterior, typically against dry gaskets. The outer layer of gaskets is installed and the gaskets are compressed against the glass by the torque applied to fasteners securing a continuous pressure plate. The plate is later typically covered with a snap-on mullion cover. This system provides reasonable performance but is susceptible to leaks at corners or joints in dry gaskets. For improved performance four-sided gaskets can be fabricated at additional cost or wet sealants can be installed to provide a concealed interior toe bead or exposed interior cap beads. Pressure plate glazing allows the easiest method to seal an air barrier from adjacent construction into the air barrier of curtain wall system.
Interior Dry Glazing: In this system the glass and infill panels are installed from the interior of the building, eliminating the need for substantial scaffolding and saving money.
The frame is fixed and exterior dry gaskets are installed. Typically only the top interior mullion has a removable stop. The glass unit is slid into a deep glazing pocket on one jamb far enough to allow clearing the opposite jamb and is then slid back into the opposite glazing pocket and then dropped into the sill glazing pocket. The removable interior stop is installed and finally an interior wedge gasket is forced in. Sometimes this method is called “jiggle” or “wiggle” glazing because of the manipulation necessary to get the glass into place. Performance is slightly reduced because dry metal to metal joints occur at the ends of the removable stop at a point that should properly be air and watertight. Wet sealant heel beads will improve performance and some systems include an extra gasket to form an air barrier seal. Installation of spandrel panels may need to be installed from the exterior.
Structural Silicone Glazing
In this system the glass or infill unit is adhered to the frame with a bead of silicone. Outer silicone weather seals supplement the structural seal. Unitized systems are frequently structural silicone glazed, especially if four-side SSG is desired. Two-sided SSG, with pressure plate glazing or wiggle glazing on the other two sides is acceptable to be field installed.
SSG is frequently mistakenly referred to as butt-glazing. True butt-glazing has no mullion or other back-up member behind the joint and relies solely on a sealant, typically silicone, between the glass units to provide a perfect barrier seal.
Designing for Condensation Resistance
AAMA’s Curtain Wall Design Guide provides guidance on window selection for condensation resistance. Establish the required Condensation Resistance Factor (CRF) based on anticipated interior humidity and local climate data and select a curtain wall with an appropriate CRF. Designers should be aware that the CRF is a weighted average number for a curtain wall assembly. The CRF does not give information about cold spots that could result in local condensation.
Projects for which condensation control is a critical concern, such as high interior humidity buildings, require project-specific finite element analysis thermal modeling using software such as THERM. Careful analysis and modeling of interior conditions is required to accurately estimate the interior temperature of the air at the inside surfaces of the glass and frame. Curtain walls that are set well outboard of perimeter heating elements will have air temperatures along their interior surface that are significantly lower than the design wintertime interior temperatures. Thermal modeling of the building interior using Computational Fluid Dynamics (CFD) software can help establish a reasonable estimate for air temperatures at the inside surfaces of the glass and frame. These interior air temperatures are inputs for the thermal modeling software. Include lab mock-up thermal testing in addition to CFD modeling for analysis of project-specific conditions. Unusual or custom details, such as copings, deep sills, projected windows, spandrel areas and shadow box can dramatically alter performance.
Use thermally broken or thermally improved aluminum frames for best performance. At the perimeter of the curtain wall, the thermal break must be properly positioned with respect to the wall system/insulation to avoid exposing the aluminum frame inboard of the thermal break to cold air (“short circuiting” the thermal break). Special insulation provisions may be required where curtain walls project beyond adjacent cladding systems (e.g., an insulated perimeter extrusion or metal panning).
Consider frame geometry for thermally conductive aluminum frame materials. Minimize the proportion of framing exposed to the outdoors.
Refer to AAMA 1503 for descriptions of test method, parameters and equipment for determining U factors and CRF’s for window products. Refer to NFRC 100 for U Factor and NFRC 500 for condensation resistance.
Designing for Solar Heat Gain Control and Solar Optical Properties
The use of glazed curtain walls can present challenges in balancing the desire for more natural daylight versus addressing the heat gain typically associated with such systems. Occasionally, there are concerns relating to having too much uncontrolled daylight, sometimes referred to as glare. The challenge is to strive for the highest visible light transmittance (VT) and the lowest solar heat gain coefficient (SHGC) while not preventing the glass from being too reflective when viewed from both the exterior and the interior, while controlling glare. This glass performance data are obtained from data using the Lawrence Berkeley National Laboratory (LBNL) Window 5.2 program with Environmental Conditions set at NFRC 100 criteria. NFRC 200 is used to determine the VT and SHGC values while the solar optical properties are determined using NFRC 300. Typically, for products more widely available on the market, the aforementioned values are readily available from glass manufacturers/fabricators.
Designing for Finish Durability
Aluminum: Class I anodic coatings (AAMA 611, supersedes AAMA 606, 607 and 608) and high performance factory applied fluoropolymer thermoset coatings (AAMA 2605) have good resistance to environmental degradation.
Unitized systems are typically custom designed. There is a wide range of systems on the market from manufacturers that provides varying levels of reliability. Unitized systems range in performance ability from industry standard to high performance walls. It is thus recommended that projects specifying unitized curtain wall systems incorporate a team member who has a breadth of experience in designing and working with unitized systems.
Unitized systems are typically pressure equalized rain screen systems. The units should be completely assembled in a factory and shipped to the site for installation on the building. The units are placed on the floors, bundled in crates, using the tower crane and lowered into place using a smaller crane or hoist owned by the glazing contractor.
The mullion dimensions tend to be slightly larger than a stick system due to their open section as compared to the tube shape of a standard stick curtain wall section. The advantages of the unitized system derive from the more reliable seals achievable from factory construction and the reduced cost of labor in the factory versus that of high rise field labor. Units can be assembled in a factory while the structural frame of the building is being constructed. Where stick systems require multiple steps to erect and seal the wall, unitized walls arrive on the site completely assembled allowing the floors to be closed in more quickly. Unitized systems also require less space on site for layout thus providing an advantage for urban sites with space limitations.
Unitized systems generally rely on rain screen design principles and gaskets and/or the interlock of mating frames for moisture protection at joints between adjacent modules. The interlocking vertical mullions will typically have two interlocking legs.
One leg will be in the plane just behind the glazing pocket and the other at the interior face of the mullions. The interlocking leg in the plane of the glazing pocket will be sealed by gaskets and is the primary line of defense against water and air infiltration. More robust systems will also include a gasket at the interior interlock. Systems whose connecting legs lock also compromise the ability of the system to accommodate movement. Some unitized designs are sensitive to small irregularities in the spacing of adjacent modules; for example, if the module joints are slightly out of tolerance, gaskets may not be properly compressed and moisture protection may suffer. Robust designs include multiple lines of defense, realistic tolerances and adjustability for erection of modules.
The four-way intersection refers to the location where four adjacent units meet. This is where field labor must seal between adjacent units to achieve a weather tight wall. The interlocking legs of the horizontal mullions are the most critical interface of a unitized system. Water that infiltrates the interlocking vertical mullions drains to the interlocking horizontals that must collect and divert this water to the exterior.
The top horizontal mullion of a unit incorporates upstanding vertical legs that mate with cavities in the bottom horizontal of the unit above. These upstanding legs have gaskets that seal against the walls of the bottom horizontal. Some designs provide one upstanding leg that provides one line of defense against air and water infiltration. More robust systems will provide two upstanding legs with gaskets on both legs. A splice plate or silicone flashing that is installed at the top of the two adjacent units as they are erected on the building is typically required.
The vertical mullions of unitized systems typically anchor to the slab edge as they pass by. The stack joint is the horizontal joint where units from adjoining floors meet. Placing the stack joint at the sill of the vision glass (typically 30″ above the floor) will minimize the dimension of the vertical mullions. This positioning utilizes the back span of the mullion above the anchoring point at the slab to counteract the deflection of the mullion below the slab. Also placing the stack joint above the floor provides a more convenient location for field workers to achieve the critical seal at the four-way intersection.
While two story spans are feasible, the weight of the unit is doubled which may require increased structural capacity to accommodate the increased load.
Wind load bracing should be incorporated at the single span height to avoid increasing the vertical mullion dimension to accommodate the increased span. Steel can be added to a unitized system to increase its spanning capability. However, unlike a stick system which has an integral hollow shape, the split mullions must be allowed to move independently to accommodate the building movement thus complicating the introduction of steel. Large units may also increase transportation costs from the factory to the site and erection costs of placing the units on the building.
Thermally broken unitized systems are available, utilizing similar technology as that used in stick curtain wall systems.
Logistical and Construction Administration Issues
The service life of even the most durable curtain wall may be shorter than that of durable adjacent wall claddings such as stone or brick masonry. Therefore, the design of the curtain wall and perimeter construction should permit curtain wall removal and replacement without removing adjacent wall components that will remain.
The service life expectancy of components that are mated with the curtain wall into an assembly should match the service life expectancy of the curtain wall itself. Require durable flashing materials, non-corroding attachment hardware and fasteners, and moisture resistant materials in regions subject to wetting.
Laboratory testing: For projects with a significant amount of custom curtain wall, require laboratory testing of a mock-up curtain wall prior to finalizing project shop drawings. Have a curtain wall consultant present to document mock-up curtain wall construction and verify mock-up performance. Specify that laboratory tests are to be conducted at an AAMA Accredited Laboratory facility.
Field Mock-up: For all curtain walls, stock or custom, require construction and testing of a field mock-up representative of the wall/window assembly. This is best scheduled prior to the release of shop drawings for window production, so that there is an opportunity to make design changes based on the test performance of the field mock-up. Specify that field tests be conducted by an independent third party agency accredited by AAMA.
Field testing of curtain walls: Require the field testing of curtain walls for air infiltration and water penetration resistance, for quality assurance of curtain wall fabrication and installation. Require multiple tests with the first test on initial installations and later tests at approximately 35%, 70% and at final completion to catch problems early and to verify continued workmanship quality. Require additional testing to be performed if initial tests fail.
Shop drawing coordination: Require curtain wall installation shop drawings showing all adjacent construction and related work, including flashings, attachments, interior finishes, and indicating sequencing of the work.
Curtain wall systems, especially unitized systems, require expertise on the part of the building designer, the manufacturer, the fabricator, and the installer. For all but the simplest of systems, the designer should consider engaging an outside consultant, if such expertise is not available on the staff.
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