Curtain Wall Window Systems

A curtain wall system is a complete exterior envelope facade system which provides a non-structural, relatively lightweight, weather-tight covering on buildings. They impact the design appearance of the exterior of course, but it should not be forgotten that they are also notable design elements on the interior of the building as well— typically visible from floor to ceiling. In the case of small, low-rise projects, the system may be field fabricated or "stick built" and glazed using standard components similar to a storefront system. However, curtain wall components are notably different in design and performance characteristics with typically much better results compared to storefront components. For larger, multi-story projects with repetitive and large-scale surfaces, pre-fabricated and factory-glazed or "unitized" systems may be warranted to create a monolithic assembly. In either case, curtain wall systems are generally installed outside of the structural system of a building running past floor slabs and other structural elements. They are then attached via tiebacks directly to the building structure at floors, columns and beams. This installation process means that all wind loads and dead loads imposed on the system are compartmentalized and transferred directly to the building structure. Hence the curtain wall system carries only its own weight and loading while the building structure absorbs all imposed loads. Factors influencing the selection of this type of system include the following:

A proper use of curtain wall extending multiple floors and correctly incorporating doors into the façade with supports aligned Photo courtesy of Manko Window Systems, Inc.

 

Curtain Wall Structural Performance: Since curtain wall systems are attached directly to the building structure, their size is virtually unlimited horizontally or vertically as long as the building is designed to carry the transferred loads. The wind load resistance or Performance Grade of curtain wall systems can be notably increased over the ratings of storefront systems. In part this is because the internal structure of the system can be increased in size or enhanced with structural pieces that are then concealed in aluminum sections. Typically curtain wall is designed to meet or exceed class CW or AW ratings and can accommodate design pressures well over 100 psf, making them ideal choices for high-rise installations subject to higher wind loads or even hurricane prone environments. However, they also perform extremely well at lower PG ratings on lower floors and in less extreme environments.

Curtain Wall Thermal Performance: Curtain wall systems typically are capable of superior thermal performance depending on the choices made for the components of the system. First, select high-performance glass that is appropriately coated and treated to suit the thermal needs of the building and the local environmental conditions. In so doing, the glazing U-factor and Solar Heat Gain Coefficient can be optimized without necessarily compromising the desired Visible Transmittance. From there, the spacer used in insulated glass layers should be upgraded to a structural tri-seal foam spacer to minimize thermal bridging between glass layers along the edges. Finally the frame members must have true, continuous, thermal breaks between the interior and exterior portions of the frame. This will not only reduce thermal bridging between the inside and outside, it will contribute to reducing the likelihood of condensation forming on the frames. In cases where air infiltration is a particular concern, the curtain wall system will likely maintain a higher degree of consistent quality control with tighter resistance to air infiltration, particularly if they are factory glazed.

Curtain Wall Water Management: Since curtain wall systems can extend for many stories, a very different water management system is employed compared to storefront systems. Rather than using horizontal and vertical members to channel water internally to a single sill flashing condition, curtain walls employ weep holes at every horizontal member to discharge any excess water that may enter the system. In most designs, the tighter sealing and fabrication reduce the amount that may enter in the first place, but this incremental weepage approach means that water does not have the opportunity to collect anywhere and cause potential damage to the building.

Curtain Wall Fabrication: As already noted, curtain wall systems can be fabricated either on site for small low-rise projects or in the factory for projects of any size or type. The choice between these two will have other impacts on the total fabrication and installation process. Site-fabricated systems can accommodate on-site variations, modifications and more complex shapes while factory-fabricated systems are pre-engineered making them better suited for larger and simpler monolithic installations. Of course, site-fabricated systems require more on-site time for installation and have correspondingly higher site labor costs than factory-produced units. In part this is because site-fabricated systems are intended to be installed from the exterior of the building (another reason to limit them to low-rise installations) while unitized systems are installed from the interior of the building. Naturally any pre-fabricated curtain walls will require time to be manufactured and shipped meaning that lead times need to be scheduled. Finally, the choice of system type has an impact on the type of joint control and movement capabilities of the components. Site-fabricated systems are commonly very limited in this regard while factory-produced systems typically come with interlocking frames that accommodate normal building movement.

An external solar control device integrated into the curtain wall system allows for the proper structural transfer and coordination of components, particularly if they are made by the same manufacturer. Photo courtesy of Manko Window Systems, Inc.