August 1, 2001
by John Straube
Curtain walls are a popular building enclosure for many commercial buildings, and are often considered synonymous with Modern architecture. Curtain walls are non-loadbearing building enclosures formed from a grid of aluminum (and occasionally steel) elements with clear glass and a variety of infill materials–spandrel glass, metal panels, even stone. Most curtain wall systems are suspended in front of the primary structural elements, which can be a new or existing structure of concrete, steel, masonry or even wood. Hence, they offer a flexible enclosure system that can be used in new or retrofit construction.
The earliest modern curtain walls were based on the stick system, which is basically assembled in situ piece-by-piece, with all glazing and other infills installed on the building site. This approach allows for flexibility and cost savings in smaller projects, but requires a high level of on-site quality control. Unitized curtain walls, which became available in the mid-1970s, are assembled off-site under controlled conditions in one-storey high units. This tends to result in more reliable quality control.
Curtain wall systems have evolved rapidly since their introduction, especially with respect to enclosure performance. The early systems had a notorious history of problems: rain penetration, condensation on interior surfaces, glazing seals pumped out of the rabbet, glare and overheating. Most of these difficulties can be overcome with improved detail design and materials. Today, most curtain wall manufacturers offer a line of components that can be used to create excellent overall exterior wall systems. Unfortunately, many curtain wall installations still do not live up to their excellent potential as high-performance enclosures.
The key technical issues that should be considered in the design and specification of a curtain wall can be listed under the headings of structure, movement, thermal performance, rain penetration, and fire and acoustic control.
Structural strength and movement issues are related. The design must accommodate movement of the building structure to which it is attached (creep over time, temperature changes, sway of frames, and sag of floors). The curtain wall itself moves as it deflects under wind loads, and aluminum components expand and contract as the temperature changes. Connections to the main structure must accommodate the combined effect of these movements without imposing load on the system, while transferring wind, self-weight, and earthquake loads to the primary structure. The greater the unsupported curtain wall height the deeper the frame section must be. Sections can be as small as 75mm but are typically 150 to 200mm in commercial projects.
Thermal performance is one of curtain wall’s weakest attributes. A typical opaque wall on a commercial building will easily achieve a thermal resistance of R12 (U=0.45 W/m2 C). The R-value of typical double-glazed insulated glazing units is about R2 (U=2.6 W/m2 C), and low-E coated, argon filled units with special spacers can achieve an R-value of 4 (U=1.35 W/m2 C). Although some manufacturers specialize in curtain wall systems that achieve R-values of over 8 (U=0.7 W/m2 C), these are exceptions. The opaque parts of a curtain wall can achieve reasonably high thermal resistance values (over R20), as long as high quality thermal breaks are installed.
An opaque wall will have a Solar Heat Gain Coefficient of less than 0.05. (The SHGC is a measure of the fraction of solar energy incident on a wall that passes into the building). Clear double glazing has an SHGC of about 0.7 and reflective glass an SHGC of as low as 0.1. In any case, the solar energy that enters a building through the glazing of a curtain wall is often many times greater than through an opaque wall. Hence, the glazing area and type (i.e., body tint and coatings) should be carefully selected to minimize air conditioning loads and reduce glare and comfort problems. There is an amazing range of high-tech coatings available today that can provide excellent solar control (SHGC<0.30) while remaining almost transparent to the occupants (i.e., visual transmission of over 50%).
Cold weather condensation on the interior surface of curtain walls is usually a symptom of poor thermal performance. Because aluminum is an excellent conductor of heat, all curtain walls should use good thermal breaks. Since some heat can still flow through a thermal break, even a thermally broken system will tend to have a lower thermal resistance than the glazing units alone. Not all thermal breaks are created equal, however, so designers should investigate and always specify the desired condensation resistance of the frame and glass (by referencing a standard test method) and U-value of the actual system being considered for the project.
Air leakage increases energy costs and reduces comfort in both the winter and summer. The air barrier system used to control air leakage in a curtain wall is usually comprised of glass, metal framing, metal back pans, and the seals that connect all of these components. Care is required in detailing and construction to ensure that the air barrier is as tight as possible. To guarantee performance, designers should specify a maximum allowable air leakage rate and the relevant test standard.
Rain control is a constant concern in all enclosure systems. In the past, curtain walls made extensive use of exposed sealants to control rain penetration. Because such seals demand the impossibilities of perfect workmanship and materials that do not deteriorate, they often failed and caused rain penetration problems. Quality curtain walls today make use of drained (or pressure-equalized) joints that redirect water that penetrates the outer seal to the outside. Problems that still occur are often at three-dimensional intersections, so these should be given special consideration. Rain penetration testing of full scale mockups is generally recommended, as are drainage capacity tests.
Fire and acoustic control are often specific to a project and municipality. The most common issue in fire control is the provision of fire stopping at floor intersections. A reasonably high level of acoustic control can be achieved by carefully specifying glazing and by providing a very tight air barrier system. Thicker sheets of glass and asymmetrical air spaces in triple glazing can provide excellent sound control.
Experience has shown that special care must be taken in the detailing of the interface between the curtain wall at parapets, at grade, and between other enclosure systems. Movement, air or water leakage and condensation problems can easily occur at such regions, typically labelled “by others” on the manufacturer’s design drawings.
Curtain walls can provide durable and energy-efficient enclosures and a wide range of aesthetic choices. However, regardless of the style, cost, or type of system, good performance can only be achieved if care is taken by the designer working with the manufacturer on detailing, specification, testing and inspection.
John Straube teaches in the Department of Civil Engineering and the School of Architecture at the University of Waterloo.