Photo courtesy Tubelite Inc.
Thermal and condensation performance
To achieve the intended energy goals and improve occupant comfort of an educational facility, the curtain wall’s aluminum framing members must be physically separated to reduce thermal bridging and conduction. Heat loss is transferred through a window by four mechanisms—conduction, convection, radiation, and air leakage. U-factor measures the rate of heat transfer and indicates how well the window insulates, generally in a range between 0.20 and 1.25. With this measurement, a lower U-factor indicates better performance and less transmittance.
In aluminum curtain wall framing, the reduced transmittance is accomplished with a thermal isolator, a low conductive material that separates the pressure plate and back member. To improve energy performance and lower the U-factor, an additional second thermal barrier is created by utilizing thermal struts (insulating strips) within the back member.
Thermal isolators for various climate zones are as follows:
- equal to or greater than 1.6 mm (63 mils), but less than 5.3 mm (209 mils) is used in ‘thermally improved’ systems sufficient for warm-humid Climate Zones 1 to 3;
- greater than 5.3 mm is used in ‘thermally broken’ systems typically sufficient for mixed Climate Zones 4 to 6;
- combining a thermal isolator with polyamide thermal struts, or with a poured-and-debridged barrier, creates a double thermal break to further enhance performance;
- a double thermal break with a larger strut or with dual poured-and-debridged barriers, allows for triple insulating glass, offering an even more effective barrier in very cold climates, and these systems can be used up to Climate Zones 7 to 8 when utilized with appropriate glass; and
- pressure plates made from alternate materials, such as glass-reinforced polyurethane, polyamide, and fiberglass can be employed in the above combinations to enhance the curtain wall’s thermal performance.
Along with providing a comfortable, consistent, year-round interior temperature, a thermally broken, high-performance curtain wall can play a critical role in promoting safer and more resilient buildings. The thermally broken, metal framing also helps control condensation that otherwise could promote the growth of mold, mildew, and microorganisms, which can harm the health of building occupants.
Hazard mitigation
The I-codes are intended to benefit local communities through adoption in accordance with the laws and procedures of a governmental jurisdiction. When adopting a model code, some jurisdictions amend the code in the process to reflect local practices and laws.
For example, the Florida Building Code (FBC) has more stringent guidelines for high velocity hurricane zones (HVHZ) required in south Florida’s Dade and Broward counties. A National Institute of Building Sciences (NIBS) research study reported hurricane wind mitigation in new buildings has a benefit-cost ratio of 5:1 with every $0.72 invested yielding a $3.80 benefit.
The American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, describes the means for determining design loads including dead, live, soil, flood, tsunami, snow, rain, atmospheric ice, earthquake, and fire, as well as how to assess multi-hazard load combinations. The NIBS research study estimated implementation of recommended I-code mitigation strategies could “prevent 600 deaths, 1 million nonfatal injuries, and 4000 cases of post-traumatic stress disorders (PTSDs) in the long term.”
High-performance curtain wall systems can not only help protect people and property from natural disasters, but can also mitigate manmade risks such as forced entry or blast hazards. Often, curtain wall systems engineered for hurricane and blast mitigation must be specified with reinforced steel to avoid excessive deflection and rotation. When specifying protective curtain wall systems for blast, hurricane, forced entry, and human impact, it is important to ensure the appropriate glass and glazing method are clearly identified in appropriate specification sections.
Manufacturing and installation
The curtain wall’s extruded aluminum, finished framing members, and glass can be installed and glazed in the field or pre-assembled in sections. In some instances, they can be pre-glazed in the manufacturing facility or by the glazing contractor. Curtain wall framing members are often assembled using either shear-block or screw-spline construction.
Shear-block assembly
The shear-block assembly method for curtain wall framing members involves connecting the horizontal member to the vertical member using an aluminum shear block with integral screw-splines and as well as non-exposed fasteners. Starting from one side of the opening, the vertical mullion or vertical jamb is installed and the corresponding horizontal members are attached to the vertical. Then, the next vertical is attached to the horizontals in a stick-by-stick method.
