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 KNOWLEDGE CENTRE: Articles

DECODING GLASS & ALUMINIUM CURTAIN WALL SYSTEMS
12/20/2011


 
 
The curtain wall comprises a complete cladding and exterior wall system with the exception of the indoor finishes. It is generally assembled from aluminum frames, vision glass and spandrel glass (or metal or stone) panels to enclose a building from grade to the roof. It is available in three system types to include the stick built system, the unitized (or panel) system and the structural glazing system (capless vertical joints).

The glass and aluminum curtain wall is designed to resist wind and earthquake loads, to limit air leakage, control vapour diffusion, prevent rain penetration, prevent surface and cavity condensation and limit excessive heat loss (or heat gain). It is further designed to resist noise and fire.

STICK BUILT SYSTEM

The oldest curtain wall type is the stick built system. It is a cladding and exterior wall system which is hung on the building structure from floor to floor. It is assembled from various components to include steel or aluminum anchors, mullions (vertical tubes), rails (horizontal mullions), vision glass, spandrel glass, insulation and metal back pans. In addition, there are various hardware components to include anchors, aluminum connectors, setting blocks, corner blocks, pressure plates, caps, gaskets and sealants
The stick built system can be constructed very air tight and resistant to water penetration. When the aluminum frames are assembled, a corner block is installed at the junction of the vertical mullion and rail. This corner block separates the glazing cavity of the sealed units from the glazing cavity of the spandrel area.

It acts both to divert water into the sill cavity of the rail and as a compartment seal for pressure equalization performance. It is important to seal the corner block to the vertical mullion and rail and that it fit tightly behind the pressure plate to prevent water from draining to the IGU or spandrel cavity below.

To prevent excessive heat loss at the vertical mullion or rail pressure plate and cap connections in winter, a thermal break of EPDM rubber or other material, is placed between the pressure plate and the screw spline (slot with linear threads inside) of the vertical and horizontal mullions.

This rubber-like material is not insulation, but it does provide sufficient thermal resistance between the cold pressure plate on the outside and the indoor mullion to allow the indoor temperature to warm the indoor part of the mullion above the dewpoint temperature (condensation temperature) of the indoor air.

 
UNITIZED CURTAIN WALL

A glass and aluminum curtain wall fabricated and installed as a panel system is referred to as a unitized curtain wall system. A unitized curtain wall will have the same components as a stick built curtain wall system. It will comprise aluminum mullions, an IGU and a spandrel panel mounted in a prefabricated aluminum frame.

However, instead of assembling the glass and aluminum curtain wall in the field, most of the system components are assembled in a plant under controlled working conditions. This promotes quality assembly and allows for fabrication lead-time and rapid closure of the building.

The unitized system is assembled on the building as panels. The mullions and rails are fabricated as half sections instead of tubular sections, which mate at assembly time to form the curtain wall system. The panels are installed in shingle fashion, starting from the bottom of the building and going around each floor and up the building.

 STRUCTURAL GLAZING SYSTEMS

Structural glazing curtain wall systems may be found in many cities in Canada and in the United States. It may be found with two or four sided capless glazing applications. In Canada, only two sided applications are used with the vertical joints of the IGUs being capless but there are a few specially designed four sided examples.

 A structural glazing curtain wall system uses the same aluminum mullion components as the stick built curtain wall except that the mullion nose (neck) of the verticals is omitted to create a capless vertical joint system. The vertical joints between the IGUs are sealed on the outside with silicone sealant for a flush exterior appearance. The horizontal mullions are constructed with standard pressure plates and caps.

 
 
 
AIR LEAKAGE CONTROL
 
The leakage of air through a curtain wall system in winter may result in excessive ice build up on aluminum caps, at parapets or soffits. The ice can grow to become a safety hazard to persons and property below. Air leakage also causes condensation in glazing cavities to the detriment of the IGUs and it can corrode backpans and fasteners. Excessive condensation in a glazing cavity may expose an IGU to prolonged edge immersion in water.

While sealed unit edges can resist wetting and drying cycles, immersion in water, even for a few months, will destroy an IGU in a few years. Rain will penetrate a curtain wall that is not air tight and sealed correctly and air tight. When wind and rain impinge on a facade, rain water is pushed through imperfections in the outside seals and into glazing cavities by the difference in wind pressure between the outside and the glazing cavity pressure. The rain may accumulate in a cavity until it overflows into the building to appear at a floor, window head or ceiling.
 

VAPOUR DIFFUSION CONTROL

Vapour diffusion through an exterior wall is sometimes the cause of cavity wall wetness or condensation in winter. However, because a modern curtain wall, whether stick, unitized or structural glazing, has components which are resistant to vapour diffusion (aluminum extrusions, glass, sheet metal panels, gaskets), cavity moisture due to diffusion is not a concern, except for silicone sealants on the inside surface of the IGUs which can be protected with a butyl cap sealant.

RAIN PENETRATION CONTROL

Like any wall system, the curtain wall system must prevent the penetration of rain or melt water to the inside of the building. Being constructed of glass, aluminum, steel, fibrous insulation and sealants, the components have no ability to absorb and release even the most incidental amounts of water penetration. While some types of insulation can absorb moisture, very little moisture enters the backpan area.

Also, because most of the materials comprising the curtain wall are corrosion resistant, water does not damage the system materials, except the seals of the IGUs if they remain wetted for long periods of time. To enhance the rain penetration control of a glass and aluminum curtain wall system, the rainscreen principle is applied.

CONDENSATION CONTROL

The curtain wall is also designed to be resistant to surface condensation. To be resistant however, the aluminum curtain wall must incorporate various features such as quality thermal breaks for the aluminum frames, double or triple glazing for the vision area and an insulated spandrel pan area.

Connections and fasteners may also include thermal breaks or thermal separators. The condensation resistance of the aluminum curtain wall in winter is governed by the indoor conditions of temperature and relative humidity and the outdoor temperature. In summer, the condensation resistance of the aluminum curtain wall is governed by the outdoor temperature, the outdoor relative humidity and the indoor air conditioned temperature.

Condensation occurs on the glass or aluminum surfaces when the humidity of the surrounding air comes in contact with a cold surface to change from a vapour to liquid. The temperature at which this change occurs is known as the dewpoint temperature of the surrounding air.

To determine the dewpoint temperature of the surrounding air, two conditions must be known or selected (conditions are selected when designing a new project), the (dry bulb or ordinary) temperature and the relative humidity. Using a psychrometric chart the dewpoint (or condensation) temperature of any mass of air at a given temperature and relative humidity may be determined.

GLASS AND GLAZING

Glass for curtain walls is available as float, tinted (heat absorbing), wired glass, patterned and cathedral glass. Float glass may be heat treated to become heat strengthened glass or tempered glass to provide greater resistance to thermal and mechanical stresses. For greater safety, laminated glass is also available.

Vision glass is usually fabricated from float glass. However, if additional strength or safety is required, then heat strengthened, tempered, laminated or wire glass may be used. Vision glass may be heat absorbing (tinted) or heat reflective (coated). Laminated glass or wire mesh glass are used for impact strength and fire resistance. Vision glass for a curtain wall may be single, double or triple glazed.

While glass breakage may occur occasionally, the most frequent cause of failure of an IGU is moisture. When the bottom edge of an IGU is immersed in water for an extended period of time, the water attacks the seals and finally allows glazing cavity air to leak into the IGU cavity space, eventually fogging or streaking the surfaces between the glass panes.

When this occurs there is no recourse except to replace the IGU. The most frequent causes of excessive wetness are the absence of a drained and vented cavity and/or excessive amounts of sealant in the glazing cavities which block drainage paths to the outside.

TESTING CURTAIN WALL SYSTEMS

The performance of aluminum curtain walls is often tested at a laboratory. Testing is undertaken to determine the strength and service deflections, the maximum air leakage rates, the rain penetration resistance under static conditions, the rain penetration under dynamic conditions and the resistance to surface condensation.

To determine the strength and deflection of mullions, rails and glass as well as the failure mode at ultimate load, a curtain wall system may be tested in accordance with a

"Standard Test Method for Structural Performance of Exterior Windows, Curtain Walls and Doors by Uniform Static Pressure Difference", an ASTM E-330 procedure. In this method, a sample curtain wall, representative of the materials and spans to be used, is attached to one side of a pressure chamber. 
 

Air is then supplied to or extracted from the chamber to exert a pressure difference across the curtain wall system to a prescribed schedule of conditions and exposure times. During the tests, the sample curtain wall is observed to record deflections, deformations and the nature of any distress or failure of the sample curtain wall components.

There are two other rain penetration tests for curtain walls. These include the ASTM E-547,

Standard test method for Water Penetration of Exterior Windows, Curtain Walls ands Doors by Cyclic Static Air Pressure Differential. This method is the same as method ASTM E-331 except that the wall will be subject to four cycles of air pressure difference, a cycle being 5 min. on and 1 min. off. It also includes the AAMA 501-4 Dynamic Rain Penetration Test. In this test, an aircraft engine is used to create high wind conditions at the surface of a wall specimen subjected to wetting by spray rack.
 

IN CONCLUSION

The glass and aluminum curtain wall system is a marvel of engineering and architecture. A totally non combustible system of glass and aluminum requiring minimal maintenance and providing years of aesthetic quality and building envelope performance. It is the most advanced exterior window wall system available for buildings. Most curtain wall suppliers and glazing companies provide the necessary expertise and production capabilities to construct a quality building.

However, no architect/designer should design or prescribe a curtain wall system without a general understanding of the characteristics of glass and aluminum curtain wall technology, in particular the assembly requirements, scheduling and testing of the curtain wall in situ or in a laboratory.

(Article written by Rick Quirouette, B. Arch.)

REFERENCES:

 

1. Building Envelope Design Using Metal and Glass Curtain Wall Systems, Building Practice Note No. 37 of the National Research Council of Canada, by R. L. Quirouette, Ottawa, September 1982.


2. Pressure Equalization Performance of a Metal and Glass Curtain Wall, by U. Ganguli and R. L. Quirouette, Appeared in the Proceedings of the 1987 CSCE Centennial Conference, Montreal, Quebec, IRC Paper No 1542, NRCC 29024.
3. Murs Rideau; Guide de conception et d’installation, par: le Conseil de l’enveloppe du bâtiment du Quebec (CEBQ), Janvier 1997.

4. Kawneer Product Manual, An Alcoa Company, the Kawneer Company Canada Ltd., 1051 Ellesmere Road, Scarborough, Ontario, M1P 2X1, Tel. (416) 755-7751.

5. American Architectural Manufacturers Association, World Wide Web: www.aamnet.org, 1827 Walden Office Square, Suite 104, Schaumberg, Illinois, 60173.

6. Aluminum Curtain Wall Design Guide Manual, (CW-DG-1-96), see AAMA book store at web site on Curtain Walls and storefronts,

7. The Rain Screen Principle and Pressure Equalized Wall Design , (CW-RS-1-96), see AAMA book store at web site on Curtain Walls and storefronts.

8. Methods of Test for Exterior Walls (AAMA 501-94), see AAMA book store at web site on Curtain Walls and storefronts,
9. ASTM E-283-91, Standard Test Method for Determining the Rate of Air Leakage Through Exterior Windows, Curtain Walls and Doors Under Specified Pressure Differences Across the Specimen, from the Book of ASTM Standards, Vol. 04.07.

10. ASTM E-330-90, Standard Test Method for Structural Performance of Exterior Windows, Curtain Walls and Doors by Uniform Static Pressure Difference, from the Book of ASTM Standards, Vol. 04.07.
11. ASTM E-331-93, Standard Test Method for Water Penetration of Exterior Windows, Curtain Walls and Doors by Uniform Static Air Pressure Difference, from the Book of ASTM Standards, Vol. 04.07.
12. www.wbdg.org-The Whole Building Design Guide



 
 

 
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