Curtain Wall Strengths: Curtain wall systems perform in a superior manner over storefront systems. This is true in terms of available wind performance, water management and thermal performance. From a general standpoint, they also exhibit a considerable number of design strengths. They are easy to customize, are available with a variety of interior and exterior aesthetic appearances, and allow a virtually unlimited range of installation locations, configurations and opportunities. The typically single-source product design and fabrication means that specific decisions can be made early related to anchoring options, accommodation of specified glazing thickness, and other details. Further, many curtain wall manufacturers offer accessory items such as sun shades or light shelves to enhance daylighting approaches for the overall building. The significance of acquiring these accessories from the same source should not be overlooked since these components add notable stress and strain not only to the curtain wall system, but to the building structure as well. Coordination and integration into the overall curtain wall design will help avoid potential problems later. Finally, from a building operation standpoint, it should be noted that many curtain wall systems use dry gasket systems which allows for simplified re-glazing should it be needed for any reason in the future.

Curtain Wall Limitations: For factory-made curtain walls, proper scheduling and planning are required to allow for the production and delivery of the system. Certain standardized systems are readily available, but with greater customization often comes greater lead times that need to be allowed for. Beyond the schedule, the cost of curtain wall systems is often cited as higher than other systems. Field fabricated curtain wall systems may cost about 40-60% more than storefront systems while factory glazed and shipped systems may, in some cases, be up to twice the cost of storefront systems. However, none of this is surprising considering the higher quality and higher performance that is usually included in curtain walls. Of course, cost will predictably vary based on the Performance Class and Performance Grade specified. In some cases it may be worth doing a life cycle analysis on the system compared to less expensive systems to identify potentially higher long-term costs for maintenance, repairs and replacement, not to mention higher potential building energy costs compared to poorer performing systems.

Curtain Wall Applications: Site fabricated curtain wall systems are best suited for smaller, low-rise projects that need high performance but seek the uniform appearance of continuous glazing systems. They can also be used on first-floor locations instead of storefront systems to achieve the desired performance and help control costs as well. Since a local fabricator/ installer can provide the system on site, custom variations and complex shapes are more readily accommodated with this type of system. By contrast, factory-made unitized curtain wall systems are best suited for low- to high-rise buildings that require superior performance. They are also well suited to situations where customization of the system itself is desired while the general shape of the system is kept simpler and more monolithic.

Pre-glazed Window Systems

Pre-glazed windows as described here are simply manufactured products that are designed to be inserted into "punched" openings in building walls. They can be a fixed glazing unit which is sealed shut or can be an operating unit offering open and closed positions for fresh air and ventilation. Pre-glazed windows can be used as single installations in a wall or if used in combinations, they can be ganged together to create larger openings sometimes called "strip windows, ribbon windows, or window walls." Window units are typically set into the wall openings with the perimeter head, jambs and sills anchored to adjoining wall section materials.

Selecting the appropriate glazing system is important for good performance. In this building, a curtain wall system with integrated doorways is used above and below the canopy in the large openings. In the smaller punched openings above, pre-glazed windows have been designed and installed successfully. Photo courtesy of Manko Window Systems, Inc.

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Common among them, are the characteristic of being factory made with the associated levels of consistent quality control and performance levels that come from a manufacturing environment. Factors influencing the selection of this type of system include the following:

Window Structural Performance: Pre-glazed windows are anchored into openings in the exterior wall construction and subsequently transfer their loads back to the building structure. Their size, nonetheless, is limited by the manufacturing process, the restraints of glass size, the wind loads that can be carried by the window unit itself, and in some cases by the performance of hardware used on the window. If window units are ganged together, they are limited by the ability of the intermediate building structure between or around the windows to carry the transferred loads. The wind load resistance or Performance Grade of pre-glazed windows generally falls between the levels of storefront and curtain wall systems. Class R products are appropriate for low-rise residential buildings, class LC for low-rise commercial needs, and class CW or AW products can achieve PG ratings of up to 100 psf or more, making them quite suitable for a wide range of installations.

Window Thermal Performance: Also similar to curtain wall systems, pre-glazed windows are also capable of superior thermal performance. High-performance glass can be specified based on availability from a particular window manufacturer to achieve desired U-factors and SHGC values. While window units are commonly made with clear glass with high Visible Transmittance, it is also possible to specify other types of glass and coatings in individual windows. In regards to the spacer in insulated glass layers, it is reasonable to require structural tri-seal foam for manufactured windows. If the window frame is made from aluminum, then the frame members are likely available with proper thermal breaks and should be specified. Ultimately, the unit should be NFRC certified and the product should demonstrate the tested performance to show compliance with the specifications.

Window Water Management: Pre-glazed window units are inherently smaller than storefront or curtain wall, which means the surface area to collect water is less. Nonetheless, horizontal members can be a source of unwanted water seepage particularly in operable units. Hence, should any water seep into the window frame, a sub-sill is typically incorporated into the window unit to allow water to collect away from the window or the building and then weep out of the bottom.

Window Fabrication: By our definition, pre-glazed windows are fully factory fabricated and glazed then shipped to the site for installation. They may or may not require as much lead time as curtain walls particularly if standardized sizes are selected that are readily available. Further, they have the benefits of factory fabrication in that they require less on-site installation time and have lower site labor costs, particularly if the installation takes place from the interior rather than the exterior of the building. The installation details will vary based on manufacturer and wall construction but window units can typically be installed in most wall types and sealed appropriately around their perimeter.

Window Strengths: Pre-glazed windows generally perform structurally quite well on a level approaching curtain wall performance levels. Thermally, windows can achieve true high performance that can be customized and pre-engineered. From a general design standpoint, they also offer some other strengths. Manufacturers offer a wide range of both operable and non-operable selections in terms of size, type, style and performance ratings. That also means that a variety of appearance features can be selected and specified which can contribute to the overall design appearance of the building. If natural ventilation or emergency egress is a building design requirement or desire, then this window system offers the most choice in those regards. Further, different depths of walls or different locations of windows can generally be accommodated through coordinated detailing and anchoring options. Finally, from a cost perspective, pre-glazed windows offer the high-performance potential associated with pre-manufactured products but at a cost notably below the cost of curtain walls.

The evolution of thermal improvements in pre-glazed windows is depicted by the improvements in frame, glazing and spacer design that led to better energy performance, warmer interior temperatures and greater resistance to condensation. Photo courtesy of Manko Window Systems, Inc.

 

Window Limitations: As with other manufactured products, appropriate lead times need to be scheduled, although for window units that time is commonly short. Budgeting for window systems will be dependent on making decisions in design and specifications early and selecting properly from the wide variety of choices to maintain accurate cost estimates. Heavy customization of some pre-glazed window systems will either increase the cost notably or will not be fully available. Alternating between fixed and operable units will create differences in the mullion profiles and appearance of the windows in many cases which will need to be resolved as a design issue. In terms of operations and maintenance, re-glazing and repairing can be hampered by removing and re-installing a wet glazing compound as is commonly used.

Window Applications: Pre-glazed window systems can be specified and designed for buildings and locations of virtually any type or location. They are particularly appropriate in wall systems that are conducive to punched openings and that require a smaller ratio of wall area to window area. This may be dictated by the requirements of the building structure or by the thermal performance of the building where highly insulated wall areas are needed more than glazed areas. Window wall designs that use multiple units will have all of the same opportunities for successful solutions as single window installations as long as the rest of the building is appropriately designed to accommodate them. This can be true regardless of the floor level where they are installed provided they are designed and specified to the appropriate performance class and grade.

Conclusion

Selecting the best type of glazing system for a particular building requires that the design professional involved has a solid understanding of the fundamental choices available within the categories of storefront, curtain wall and pre-glazed window systems. To go further and achieve true high performance, it is imperative that the appropriate system and component parts are understood, designed, specified and installed properly. Selecting a system that does not match the building and location criteria can lead to the unwanted consequences of increased cost, poor performance, or damage to the building. Proper design and selection of high-performance glazing systems will contribute to the long-term integrity and trouble-free operation of the building for many years following their installation.

MANKO Window Systems, Inc. provides contract glazing companies with a reliable, efficient source of projected and fixed commercial windows. They currently produce 29 types of commercial window systems, 18 types of entrance doors, 6 storefront systems and 6 curtain wall systems.   www.mankowindows.com