The content of this article was originally published in The Puget Sound Business Journal on Jan 19, 2004.

The advent of the curtain wall is one of the most influential developments in the history of modern architecture. Without proper design and installation, however, this common form of exterior for high-rise buildings is a source of costly problems.

The radical idea that the exterior skin could be divorced from load-bearing perimeter walls was nothing short of a paradigm shift.

Modern high-rise buildings are constructed of innovative curtain walls comprised of lightweight glass, stone, aluminum, marble, metal or composite materials — the architectural and aesthetic possibilities are almost unlimited, as are potential problems.

There are four basic curtain-wall systems used today. The barrier system seeks to keep water outside the envelope with no redundancy or backup against possible water infiltration. The drainage system has an additional line of defense, capturing and releasing water that enters the system, through an internal drainage channel. The rain screen system has a redundant, pressure-equalized waterproofing barrier — effectively, an air space that reduces pressure drive.

The latest system is the bioclimate, or “green” double curtain wall, where two glazed walls are separated by a one- to three-foot space.

With this system, computer-controlled and user interfaced devices may be integrated to encourage the use of fresh air and natural lighting, improving user comfort and air quality and reducing energy consumption. Although the benefits of this system are primarily energy-related, the redundant second glazed wall also provides effective protection from air and water infiltration.

If the curtain wall is such an elegant concept, what could possibly go wrong? When they work as intended, these walls prevent the infiltration of water and air leakage while allowing for more transparency and a spectacular array of cladding options.

But when curtain walls fail, the results are often dramatic and disastrous: glass or stone panels can detach and fall off, endangering lives below; water infiltration can accelerate degradation of the building’s skin and structure; incompatibility of materials can cause electrolysis, staining or disintegration; and building envelopes may need wholesale replacement — the cost for which can exceed the cost of rebuilding the entire project.

Poorly designed or improperly installed curtain walls leave a trail of litigious misery that take years and sometimes millions of dollars to diagnose and repair.

Curtain wall problems can result from poor design, product failure, deficient installation, or a devastating combination of all three.

Although in many cases product failures and deficient installation are to blame, curtain wall failures often happen at the drawing board, the result of four problems:

• Failure to accommodate required tolerances for deflection and building movement.
• Poorly designed interface conditions where diverse and sometimes chemically inappropriate materials are married or where flashings are not properly designed.
• Sealant adhesion failures or excessive reliance on sealants.
• Failure to properly test and modify design details in response to field or laboratory-tested assemblies.

Buildings expand, contract and move in response to loads, wind, earthquakes, temperature, freeze-thaw cycles and other forces.

Curtain-wall elements must be designed to absorb, transfer and withstand these loads through carefully designed vertical and horizontal support elements.

Thermal expansion/contraction of curtain wall components require attention to ensure that joints can withstand as much as a three-eighths inch of deflection across a typical spandrel beam. Interfloor story drift can actually twist panels into a parallelogram shape. Joints and connections must accommodate these tolerances.

Forensic waterproofing experts and materials scientists often point out that many curtain-wall problems occur where divergent materials intersect.

Materials that are not suitable for contact can react chemically, creating material degradation. Poor flashing details can result in water infiltration contributing to problems including wallboard-fastener corrosion, loosening of mortar joints, loss of insulation value, the “sick-air building problem” due to the growth of toxic molds in the wall cavity, and even catastrophic structural failure.

Sealant joints and gaskets that are not designed to absorb shear distortion can lead to failure of curtain wall panels in the form of buckling panels or falling glass. Additionally, permanent staining, or streaking, can occur when sealants are not compatible.

Curtain wall assemblies must be tested by the American Society for Testing Materials to confirm compatibility and building envelope integrity.

Architects must resist pressure from owners and contractors to eliminate this vital process, which is essential for ensuring long-term building performance.

Although in many cases product failure or improper installation is to blame, many failures occur on the drawing board.

To maintain the integrity of the building envelope, a design team must ensure proper construction tolerances and pay extraordinary attention to sealants, flashings and weeps.

Finally — and most importantly — unique assemblies must be tested in accordance with ASTM standards and refined in response to any detected weaknesses. Best efforts and due diligence in all of these areas will help ensure project success.