A curtain wall is a non-structural system that covers the exterior of a building, with the purpose of isolating the indoor environment from outdoor conditions. Curtain walls typically use an aluminum frame with an in-fill of glass, metal panels, or thin stones.
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Thanks to their flexible design, curtain walls have become popular in high-rise construction. This article provides an overview of the main design features of curtain walls.
Type of Rainscreen Systems
Curtain walls use three types of rainscreen systems: face-sealed, water-managed, and pressure-equalized.
Pressure-equalized systems usually provide the highest water resistance and air tightness.
- The inside faces of the glass, the glazing pocket, and the wet seal are designed as an airtight barrier.
- On the other hand, the outside face of glass, exterior glazing materials, and the outer framing surface create a rainscreen to shed water away.
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Between the exterior rainscreen and the interior airtight barrier, a pressure-equalization chamber is created in the glazing pockets. This reduces water penetration, by eliminating pressure differences across the rainscreen.
Water-managed systems have no air barrier, and some water penetration is inevitable. This water is controlled by using drains and weeps within the glazing pocket.
Face-sealed systems depend on continuous and complete seals between glass units and frames, these systems are not commonly used because the long-term reliability of the seals is questionable.
Structural Support
A curtain wall is designed to transfer any load to the floor structure below, or any intermediate framing. The loads on curtain walls consist mostly of positive and negative wind loads, and there can also be snow, seismic, or maintenance loads.
Curtain walls experience slight movements caused by temperature changes and wind loads. Therefore, all connections that anchor the wall must be designed to withstand movement, while resisting and transferring loads.
Thermal Performance
A curtain wall is part of the building envelope, affecting energy efficiency. The overall thermal performance of a curtain wall depends on the frame, glazing infill, construction area, and perimeter details. The curtain wall frame conductance depends on the frame material, geometry, and fabrication. For example, aluminum has a very high thermal conductivity. Thermal breaks of low-conductivity materials are often incorporated to improve thermal performance.
Moisture Control
Water can infiltrate the exterior wall system under the action of five different forces: gravity, pressure differences, surface tension, kinetic energy, and capillary action. To control water infiltration, all these forces must be considered in the design.
Water resistance depends on the glazing detail, frame construction, drainage details, weatherstripping, frame gaskets, interior sealants, and perimeter flashings. For a curtain wall to achieve a suitable level of water resistance, it must have proper drainage for the glazing pocket and a watertight frame construction.
Visual Design
Curtain walls stand out in groups of buildings, thanks to their unique glazing appearance. Among the key visual features of a curtain wall are the sightlines, defined as the visual profile of the horizontal and vertical mullions. The final appearance also depends on the width and depth of the curtain wall frame, and the frame depth in particular depends on the lateral loading requirements.
Acoustics
The acoustic performance of curtain walls is determined by their glazing and internal seals. There are several methods to improve acoustic performance: using sound-attenuating infills, making the system as airtight as possible, incorporating glass of various thicknesses, or using noise-reducing layers like polyvinyl butyral.
Back Pans
Back pans are metal sheets installed behind the opaque areas of a curtain wall, usually made of aluminum or galvanized steel. These elements provide a second line of defense against water infiltration, in areas of the curtain wall that are not visible from the interior.
Safety Features
Smoke seals between the floor slab and the back of the curtain wall are crucial for fire protection. They divide the walls into sections, slowing down the movement of fire, smoke, and combustion gases between floors.
Uncontrolled leakage of air and water can lead to air quality issues. When water accumulates directly or by condensation, it can lead to mold growth. Many construction materials are damaged by mold, and its spores can cause irritation and allergic reactions.
Durability
Like any other building element, curtain walls can deteriorate over time, especially when they lack proper maintenance.
Curtain wall glazing can suffer visual obstruction from condensation or dirt, and opacifier films can be damaged over time by wear and condensation. Glazing can also lose its insulating properties without adequate maintenance, increasing energy consumption in the building. The gaskets and sealants in a curtain wall can also fail due to wall movements, prolonged exposure to moisture, and ultraviolet radiation.
Maintenance
Curtain walls and perimeter sealants require planned maintenance to maximize their life cycle. When perimeter sealants are properly installed and serviced, they can last for 10 to 15 years. Aluminum frames are protected with special coatings, which provide resistance to environmental degradation. However, these coatings must be cleaned properly at regular intervals.
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Sustainability
The following are some best practices to make curtain wall systems more sustainable:
- Maximizing their service life to reduce material waste. This can be accomplished with a professional design and installation, combined with adequate maintenance.
- Designing curtain walls with thermal breaks and high R-values, to improve energy efficiency in buildings. Low-E glass coatings can greatly reduce summer heat gain and winter heat loss.
- In case of demolition, recycle materials like aluminum and steel frames.
Since a curtain wall is part of the building envelope, it affects energy efficiency during its entire life cycle. A well-designed curtain wall can save plenty of energy in the long run, by improving natural lighting and reducing HVAC loads.
Thermal expansion
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 , 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 , 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.
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