Subject: Curtain Wall Systems in The 90.1 ASHRAE Era
Author: MSC Eng Btech Adi-Ben-Senior
All rights reserved to the Author
Curtain wall systems are defined as the exterior wall (skin) of the building, built off the edge of the slab, covering the building and protecting it from the elements. Curtain walls are usually a load bearing element. They are only required to carry their own dead load weight with loads being transferred to the structure, in most cases through an anchoring system to the slabs, beams, and columns. Since curtain walls are generally non-load bearing they can be made of more flexible and lighter weight materialsl than structural elements.
In the last eight decades more midrise and high-rise buildings are applying curtain walls. Undoubtable, glass is the most common material used. Glass allows for more natural light to penetrate deeper into the building, it is made of lightweight aluminum frames and it claims to be energy efficient reducing utility bills; but, is that really the case?
Glass curtain wall applications has developed a great deal in the last four decades. Low Energy, double pane, better seals between each glass units, stronger frame members, etc. However, although implementing newer technologies like non-conductive materials, the thermal resistance achieved is very low in comparison to top high-performance rigid insulation and new exterior walls assemblies meeting new code compliance. In favor of existing constructions methodologies, one can say – “this is the best technology they had then”.
What about 2018 codes, and how are these applied today?
Although the IECC and ASHRAE codes are not a binding legal requirement yet, they are the leading guidelines for design in the construction industry. While IECC is mostly for residential use and federal government, the ASHRAE (90.1 aiming) is for commercial use and is an independent organization. It begins with the climate zone levied by the IBC, which are dictating the design criteria per code, and one would expect the ASHREA and the IEC to develop same perspective of the guidelines, however that is not the case. Various approaches and Independent methods between the two, lead to different recommendations and separate solutions. As far as adoption of codes, in the commercial industry, California and Nevada are the only two of fifty states that had adopted the ASHRAE 90.1 -2016 or equivalent; nineteen states had adopted the ASHRAE 90.1 -2013 or equivalent; and the rest, twenty-nine states, had adopted the ASHRAE 90.1 -2010 or older. In the residential industry, California and Nevada are the only two of fifty states who had adopted the IECC 2018 or equivalent; seventeen states had adopted the IECC 2015 or equivalent; and the rest, thirty-one states the IECC-2012 or older. In February 2018 the DOE has issued a determination for increased energy efficiency in the commercial buildings across the nation with the expectation of energy savings: 8.2% energy cost savings, 7.9% source energy savings, and 6.7% site energy savings. Naturally, for the sheer size, the high-rise and mid-rise buildings, are the first on this list.
When more than half of the country is left behind in adopting new energy codes and design guidelines, how can we make progress and
a achieve the energy savings as mentioned in the determination of the DOE? Some solutions are already existing. In the world of “continuous insulation” and “thermal bridging” wall assemblies, a lot of efficiencies have been achieved. In residential and commercial application, wood framing is giving way to light gage steel, and consequently the SIP market and the prefabricated C-SIP (composite structural insulated panels) claims its position.
Thermasteel, a pioneer of the C-SIP, provides a curtain wall system solution based on a composite panel system, manufactured to size, pre-assembled, with an R-value of 35-47 completely thermally broken. The technology connects four by twelve feet structural panels to one continuous curtain wall that integrates all glass panes, vapor barrier, insulation and structural capacity to one unit. The connection points and anchoring systems are assembled in the production facility in Radford Virginia. The entire curtain wall module is transported to the construction site and then lifted up by a crane from a flatbed truck onto the building.
On a typical fifteen-foot floor section, the openings are not more than ten feet, leaving five feet for insulation, (seal and header) for insulated material, to create energy savings. Where glass curtain wall systems cover four sides of high-rise buildings, a lot of energy savings can be achieved with such solution. The highly insulated panelized system creates the structure and insulation (30%) around the glass panes. Most high-rise buildings are fully covered with glass curtain walls to their entire height, so the application of such system, over great surface, provides significant benefits to the builder and the owner.
First, the light opening is not affected, therefore the amount of effective light coming in the building stays the same.
Second, the structural frame along the slab lines keeps the original look and remains untouched.
Third, five-foot frame between the slabs provide 30% of high energy efficiency of a thermally broken envelope that will potentially exceed the determination of the DOE.
Fourth, the highly insulated area (slab lines five feet tall) will reduce the greenhouse effect caused by heat caught inside the building, but more so, to down size the required mechanical equipment and generate further savings on energy and costs.
Fifth, Thermasteel’s pre-fabricated composite insulated panel system, is the structure for the curtain wall, and the attachment points to the building’s structure, allowing much more flexibility in the locations of the anchoring mechanisms to the structure of the building.
The design of such curtain wall systems allows for the anchoring points to the structure to be connected to the columns or to the slabs depending on the type of structure.
Six, where a standard curtain wall weighs 8-10 psf five feet of fifteen can be reduced by 50%, hence a reduction of 15-20% of the entire envelope, significantly reducing the load of the entire curtain wall application. Further, prefabricated systems are more environment friendly, more precise, and save time on construction. While the implementation of energy codes and guidelines of the ASHRAE and the IECC remains to be seen, more technical solutions for better application of curtain wall systems are out there, that would lead to achieving the energy savings determination and even more.