Thermal expansion
Photo courtesy Architectural Testing, Inc.
Temperature differences must be considered in curtain wall design, as they relate to differential expansion and contraction of various materials. Aluminum is a common material choice for curtain walls due to its strength and light weight. However, its relatively high coefficient of expansion, in comparison to glass, means the potentially wide daily and seasonal fluctuations in the metal’s surface temperature can induce stresses from differential thermal expansion that can cause stress on glass, joints, and anchors, or reduce glass ‘bite.’
AAMA CWM guide specification in section 5 recommends curtain wall systems design should provide for such expansion and contraction of component materials due to an exterior metal surface temperature range of –17 C (1 F) to 82 C (180 F) without causing buckling, undue stress on glass or structural elements, failure of joint seals, damaging loads on fasteners, reduction of performance, or other detrimental effects. This range comes directly from the AAMA CWM specifications and prior documents. While local conditions may require more stringent design criteria, this is the anticipated starting point. More stringent requirements would need to be determined by the local design professional and stated in the building specifications.
Weather tightness
Controlling water and air movement at the building envelope is important to the long-term integrity of the structure and the comfort of its occupants.
Water penetration
Two methods have been developed for preventing leakage through the curtain wall system. One is referred to as the ‘internal drainage’ or ‘secondary defense’ system wherein minor leakage can be prevented from penetrating by providing within the wall itself a system of flashing and collection devices, with ample drainage outlets to the outdoor face of the wall. The other is the more sophisticated ‘pressure equalization’ method, based on the ‘rainscreen principle,’ which 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 rainscreen wall cladding (PRWC) systems meeting the requirements of AAMA 508, 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. The static air pressure difference used in the test is set at 20 percent of the specified maximum inward acting ASD wind load pressure, but not less than 300 Pa (6 psf) nor more than 720 Pa (15). No uncontrolled water penetration may occur when the curtain wall is tested to the required specification.
Air infiltration
The guide specification requires that air infiltration through the curtain wall should not exceed 0.3 L/s•m2 (0.06 cfm/sf) of fixed wall area and 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, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen, at a static air pressure difference of 300 Pa.
Energy efficiency
With increased attention being placed on the impact of large buildings on the environment by policies like those recently introduced by the mayor of New York, energy efficiency of curtain wall systems will continue to grow in importance.
Condensation resistance
The fixed light area of the curtain wall, including glass and metal framing, should have a condensation resistance factor (CRF), not less than the one selected by the architect based on climate zone when tested in accordance with AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors, and Glazed Wall Sections.
Thermal transmittance
The fixed lite area of the curtain wall must have an overall thermal transmittance U-Factor (W/m2•K [BTU/hr-sf-F]) not exceeding that specified by the architect. It is important to note the selected U-factor may be defined by applicable codes based on project location or desired environmental design credits, such as offered by the Leadership in Energy and Environmental Design (LEED). U-factors should be tested per AAMA 1503, or simulated per AAMA 507, Standard Practice for Determining the Thermal Performance Characteristics of Fenestration Systems in Commercial Buildings, or (optionally) applicable National Fenestration Ratings Council (NFRC) testing, modeling, and validation protocols.
Building tolerances
A primary feature of AAMA CWM is more attention is paid to the issue of tolerances and clearances. Since they may be closely related, the two terms are often confused. However, they have different meanings. 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 anticipated size variations, to offer working space or for other reasons. Both are critical. Since curtain wall construction involves covering a field-constructed skeleton with a factory-made skin, the designer must consider how the curtain wall system connects to many other parts of the building, thus involving the work of numerous trades.
