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Curtain Walls

Curtain walls are common today in commercial construction, adding simplicity and design options for buildings of all types. There are various definitions of curtain walls, but at the most fundamental level, they are a building façade or cladding system, generally of glass, steel and/or stone, that is nonbearing and provides no structural support except for itself (not insignificant). They are supported by imbeds or supports in the building frame.

Often lightweight, they are relatively easy to construct. However, they perform a most critical function – keeping the weather out. If not installed or maintained properly, they can also be a source of never-ending problems.

The term “curtain wall” extends from medieval times when exterior walls were connected like a “curtain” between posts. Prior to the 20th century, exterior walls supported the entire building load. In the 19th century, walls were still massive but often did not support the load. It wasn’t until the 20th century that lightweight walls began to be “hung” on the structural frame.

The exterior wall system generally accounts for 25 percent of the total construction budget, second only to the structural system. Curtain walls maximize occupant visibility, enhance usable floor space, bring in natural light, and afford designers the opportunity to create architectural drama and excitement. As we become more energy-conscious, curtain walls properly designed may help reduce energy consumption (e.g. lighting) and with proper shades may control solar gain.

Types of Curtain Walls

There are three basic types of curtain walls: field assembled, panelized, and glass wall systems. Field assembled systems consist of vertical and horizontal mullions filled with clear or spandrel glass or metal or stone panels. They are the least expensive to build and most tolerant of scheduling variations. In panelized systems, the components are fabricated and assembled in a shop. They are best suited to buildings with repetitive wall designs. Quality control is more easily maintained with panelized systems. Glass wall systems employ various structural support systems to attach directly to the glass. They are the most expensive systems but offer various aesthetic options.

Care in Design and Installation Avoids Problems

Curtain walls must defend the building against wind, rain, ice, heat and humidity. Generally speaking, as with most building envelope systems, moisture is the enemy. It must be kept from penetrating the envelope or entering spaces where it may remain and cause damage. Buildings must be designed and constructed to shed moisture effectively. Problems generally arise from the failure of sealants, the push to make buildings airtight to reduce energy costs, and the use of moisture-sensitive materials in wall construction.

Curtain wall systems must have an integrated drainage system as well as gaskets and sealants or moisture protection and acoustical control. Small deviations from plans in installation or variations in the dimensions separating panels can create major problems. Sealants should not be relied upon as the sole water penetration barrier – yet they often are. Weep holes must be left uncovered. Attachment details should not penetrate flashings. The drainage system should be capable of handling condensation as well as rain.

If the building is newly constructed, there should be (or have been) a series of post-construction inspections to confirm performance. These should occur at various intervals throughout the first year to correspond with seasonal changes. As an owner, you should have a set of as-built drawings, manufacturer’s warranties, and maintenance requirements. Proper maintenance is essential to minimize future failures. Ongoing periodic inspections should be integral to the building maintenance program and in some cases, may satisfy requirements under local ordinances.

How Do Curtain Walls Fail?

Curtain walls most commonly fail due to gaps in continuity, failure of drainage systems, poor repair/replacement procedures, or failures at the interface with window treatments, sills, and ceilings/soffits.

Typical areas to look for breaks in continuity are at intersections between:

  • Roof/parapet
  • Support connections to structure
  • Soffits and canopies
  • Site grade
  • Entrances and storefronts
  • Transitions between different materials within the envelope
  • Window washing accessories and tracks
  • Interfaces with other exterior wall materials

It is also important to maintain separation of aluminum curtain wall framing members from dissimilar metals and substrate materials, such as masonry, wood, and concrete.

Caulks and Sealants

Sealants should last 20 years if properly designed and installed, but many do not. Two critical concerns are that the correct sealant be identified for the application, and that the gap to be filled is consistently of the proper dimension. If a product is going to fail, it will probably do so after the first year and before three years. Sealants fail when they cease to adhere to the substrate, are installed improperly, create chemical reactions with adjacent materials and deteriorate due to weather,or from fatigue. Resealing a curtain wall system can be very expensive.

Frames, Mullions and Attachments

If panels are to be mounted within a frame, it is critically important that the design of the frame incorporate the ability to shed water, both from the exterior (rain) and from the interior (condensation). Weep holes must be unobstructed.

In an existing building, the method of attachment of the curtain wall to the structure is often hidden. Methods of attachment and sealing cannot be readily observed and are therefore not part of a standard Property Condition Assessment. However, if problems are suspected, certain components of the curtain wall may be removed to observe those critical elements.

If removed, one can then see if the structural silicone glazing or pressure plate installation properly secures the glass. Field-applied splice joints between mullions, head tracks, and sill starter tracks must be installed per manufacturers’ specifications to allow for each bay to expand and contract vertically and horizontally. These splice joints are often visible with the removal of trim accessories. EPDM gaskets installed between sash frames and glass panels should either be continuous or butted tightly to provide sufficient thermal resistance between the cold plate on the outside and the indoor mullion.

As for the structure, field dimensions of wall openings between floors and wall partitions must match plan dimensions to allow for the curtain wall to expand and contract with the change in weather, yet allow it to maintain its weather tightness. Anchor bolts are visible with the removal of perimeter sealant to inspect the integrity of the curtain wall. Coping transition details from the curtain wall to the structure’s roof parapet allow for continuity of the exterior wall and are visible from the roof overhang.

Volume 19, Number 2

April 2008

Copyright © 2008

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