W
ater penetration is one of the most persistent performance problems with all wall types. The materials used in traditional walls of masonry and stone have significant capacity to absorb water. This feature often masks a water penetration issue. Most materials in a metal curtain wall are impervious to moisture. This greatly reduces the area subject to water penetration but increases the importance of joints and seals. The impervious nature of the curtain wall materials also eliminates any absorptive storage capacity, therefore any wetted opening rapidly leads to visible water entry. While lacking absorptive water storage capacity typical curtain wall sections do contain cavities that can potentially accumulate water. Accumulated water can damage seals and, in particular, will promote premature failure of insulating glass units. Three different concepts have been and are still used in metal and glass curtain wall system design to control water penetration.Design Concepts and Features
Exterior Face Seal • Common through 1960s, since that time use has decreased
• Used in four-sided SSG and in retrofit of older systems
• Relies on integrity of exterior sealant and gaskets
• Exterior plane is air barrier
Internal Drainage • Used through 1960s and into 1970s, rarely seen in Canada after 1980, still common in USA
• Recognizes difficulty in maintaining perfect exterior seal, provides backup drainage to exterior
• Intentional openings in air barrier can lead to condensation.
Two-Stage
Weathertightening or Pressure Equalized Rainscreen (PER)
• Most common contemporary design approaches in Canada
• Outer screen, vented air space, air sealed interior barrier
• Used since mid-1970s, common on most major buildings since 1980.
• PER employs intentional delineation of specific cavities with specific properties (size, stiffness, venting).
4-18
Many major office towers built in the 1960s and early 1970s incorporate interior glazed single lites of glass. They rely on exterior sealant or tape seals to control water penetration. The original seals were often butyl or
polysulphide based materials which degrade over time. While some seals glass to frame are accessible, framing seals may not be. Many of these towers have been resealed more than once in their lives and newer sealant products are used–particularly inorganic silicone products. Properly installed these products promise a much longer life than their predecessors. Unless a major refit is proposed to the facades, it is often not practical to change from the original face sealed design to a more contemporary design.
While two-stage weathertightening, PER or interior drainage can be
incorporated into 4SSG systems they present a design challenge. Small areas of 4SSG are often designed as face sealed systems and reliance is placed on the silicone-to-glass seals. Fortunately the glass edge provides an ideal substrate and the silicone a very durable material for this type of sealing. Cladding of larger buildings should incorporate two-stage weathertightening or PER into any 4SSG system.
Of all of the wall types the metal curtain wall is the most ideal wall to properly apply pressure equalized rainscreen design as advocated by NRC/IRC. The curtain wall is typically composed of a regular grid of small separated rigid walled compartments, convenient to air seal and easy to vent/drain. Typical thermal break and aluminum pressure plates act as compartment seals for each glazing unit/spandrel.
For curtain walls where profiled spandrel panels or column covers are used, specific measures must be taken to compartmentalize the larger cavities. The size of the cavities and the inherent flexibilities in these assemblies often make formal application of PER difficult. But a two-stage weathertightening approach is feasible.
While pressure equalized rainscreen design provides an overall concept to water penetration control, it still occurs more frequently than desired. This is not due to a flaw in the PER concept but to poor execution of details and the failure of designers to properly deal with forces other than air pressure differences that can also cause water penetration.
While PER reduces or eliminates water entry due to air pressure differences, curtain wall design must also incorporate in its design a means to eliminate entry due to capillarity, kinetic energy, surface tension and gravity. The design must also provide a means to drain water that will enter the spandrel or glazing cavities. The design intent is to provide a reasonably watertight exterior seal system coupled with adequate venting and drainage paths from
PER and Water Penetration
It is important to remember that PER design addresses only one component of water penetration–that is it reduces or eliminates the air pressure difference that might drive water through an opening. It does not address other forces such as capillary, kinetic energy, surface tension, and gravity. Good detail design must address these factors regardless whether a PER concept is used or not.
the cavities behind. Small quantities of water entering a glazing cavity are re- directed to the first horizontal member below the point of entry and then to the exterior via weep slots. Compressible thermal breaks and corner blocks serve to compartmentalize the cavity and direct water to the weep slots. Weep slots are typically provided in the horizontal pressure plate so there is drainage between the setting blocks and also between each setting block and the end of the framing member.
The actual size and shape of weep/vent slots is subject to some debate. Most manufacturers use the same size of weep/vent slots regardless of cavity size. While this is not in accordance with PER theory, once a minimum size is provided adequate water penetration resistance is often achieved. This suggests that full pressure equalization, while an admirable target, is not necessary to achieve adequate resistance to water penetration.
A curtain wall system’s ability to control water penetration is evaluated by a number of different standard test methods. While some tests have been developed for field use most are meant to be applied to a representative sample of wall installed in a chamber at a testing facility.
ASTM E331 – Standard test method for Water Penetration of Exterior
Windows, Curtain Walls, and Doors by Uniform Static Air Pressure Difference
ASTM E547 - Standard test method for Water Penetration of Exterior
Windows, Curtain Walls, and Doors by Cyclic Static Air Pressure Difference
ASTM E1105 - Standard test method for Field Determination of Water
Penetration of Installed Exterior Windows, Curtain Walls, and Doors by Uniform or Cyclic Static Air Pressure Difference
AAMA 501.1 – Standard test method for Metal Curtain Walls for Water
Penetration using Dynamic Pressure
The standard test methods all basically involve applying a water spray to the sample while applying some pressure difference across the wall sample. It is important to recognize that the referenced test methods provide a common procedure useful in comparing different wall systems. The test methods do not always provide specific test parameters (pressure difference, test duration, number of cycles, wind speed, etc.) or an acceptable pass/fail criteria that might relate the test to expected climatic conditions (see Chapter 5–Design Criteria). Also some tests and sequence of testing are more
appropriate to certain wall types. Manufacturers quoting performance
supported by specific tests must provide details of the test parameters and the pass/fail criteria for the results to be meaningful.
Unfounded Reliance on Testing
Too frequently designers only specify test procedures without providing the necessary test parameters, or specify test parameters that are not rationally related to the building type or location. An incorrect reliance is placed on a test method to predict in adequate in-service performance.
4-20