August 9, 2018
by Jonathan Carreiro, P. Eng., and Patrick M.B. Chan
In the age of high-performance design and construction, thermal bridges at balconies, slab edges, canopies, parapets, and rooftop connections cause these building elements to act like cooling fins, allowing heat to flow unimpeded from the interior through these uninsulated penetrations in the building envelope to the cold exterior environment.
While heating exterior penetrations wastes energy and increases carbon emissions and operating costs, chilling interior structures reduces comfort and allows condensation and mold to form on adjacent surfaces (see “Airtight Vapor Barriers Worsen Thermal Bridging Problems at Structural Penetrations” sidebar). Therefore, one must look for an alternative solution.
Structural thermal breaks (STBs) address these thermal weaknesses in the building envelope by ensuring continuous insulation (ci), thereby improving the overall thermal performance. They are changing the design philosophy to an “envelope-first approach.”
How STBs insulate
An STB is a fabricated longitudinal assembly of the same approximate width as the insulation layer of the building enclosure. It is a pre-engineered assembly containing loadbearing elements and insulation as opposed to thin insulation layers such as pads requiring additional design to make them work. It creates a structural insulated break between an exterior structural penetration (e.g. balcony, slab edge, canopy, parapet, and rooftop support) and the interior structure supporting it, minimizing thermal conductivity between the interior and exterior structures, while providing the specified loadbearing capacity.
Many building types and applications can employ STBs, with the most common being concrete cantilevered balconies. These engineered STBs contain high-strength stainless steel rebar tying in to both the interior and exterior concrete slab reinforcement. Stainless steel is approximately 70 percent less thermally conductive than regular carbon steel rebar. In addition to acting as a thermal separator in the concrete slab, the STB transfers the balcony dead and live loads back into the interior structural concrete slab. Other concrete STB applications include thermally separating concrete parapets on roofs, exposed slab edges, and architectural eyebrows.
STBs are also employed in steel construction to address thermal bridging from steel beams penetrating the building enclosure to support canopies and sunshades, as well as vertical steel applications like roof snow fences and roof anchors.
Lesser-known STB solutions address thermal bridging in concrete-to-steel connections (i.e. bolt-on steel balconies to concrete slab), concrete-to-wood, and steel-to-wood applications.
Compared to non-insulated connections, STBs can reduce thermal conductivity in the connection area by 90 percent for standard loadbearing scenarios, translating into average reductions in energy use and carbon footprint of up to 14 percent for the overall building per year. (For more information, click here.)
The reduction in BTUs required to heat the building also allows corresponding reductions in heating system size/capacity, resulting in savings on capital equipment and ongoing operation and maintenance of mechanical systems.
In the case of cantilevered balconies, STBs typically increase the warmth of adjacent interior floors by up to 15 C (27 F). (For more information, click here.)
By allowing the interior side of structural penetrations to remain warm, STBs also prevent condensation, mold, and rust problems associated with today’s airtight vapor barriers and elevated interior humidity.
Since STBs are still a relatively new concept in North America, it is important suppliers provide local technical expertise, onsite support, and signed and sealed drawings by a professional engineer who is registered in the project’s state or city. Suppliers should also be able to furnish a comparative energy performance benefit of using STBs through 3D thermal modeling.
In Europe, gaping thermal penetrations through the building envelope are rare in high-rise construction because STBs are installed to simultaneously insulate and carry the loads of structural penetrations. The opposite has been true throughout North America where the adoption of STBs lags by decades. The reason for this is building codes.
Unlike European codes requiring continuous envelope insulation, North American “performance” codes contain loopholes of sorts allowing thermal deficiencies in one area to be offset by efficiencies in another.
Since thermal insulation of structural penetrations in North America remains discretionary, the adoption of STBs has been led primarily by:
However, change is on the horizon as building codes regulating ci and energy efficiency tighten in the United States and Canada at the national, regional, and local levels.
As building codes evolve to mandate higher energy efficiency, thermal penetrations through the building envelope may soon go the way of single-pane windows and incandescent fixtures. Thus, design and construction professionals will benefit by keeping pace with current codes and understanding where new ones are headed.
Evolving building codes are narrowing the gap between decisions based on economic constraints and the ones reflecting broader, greener priorities.
The American National Standards Institute/American Society of Heating, Refrigerating and Air-conditioning Engineers/Illuminating Engineering Society (ANSI/ASHRAE/IES) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, was updated in 2016 to require specific energy efficiency modeling for uninsulated assemblies in building envelopes, including balconies, perimeter edges of floor slabs, and parapets. This goes beyond past versions of ASHRAE 90.1, which did not effectively address, or allowed design/construction teams, under common interpretations, to ignore, these types of assemblies.
However, model building codes—established by organizations like ASHRAE, ANSI, IES, the International Code Council (ICC), the American Concrete Institute (ACI), along with Canada’s National Building Code (NBC) and National Energy Code for Buildings (NECB)—do not have the force of law until local authorities having jurisdiction (AHJ) adopt them. Therefore, the schedules and details of code adoption vary, and depend on regional climatic, economic, cultural, and political variables.
According to the July 2017 U.S. Department of Energy (DOE) code maps, no state has advanced to ASHRAE 90.1-2016. (For more information, click here.) Most state commercial building standards are based on pre-2007 codes. However, California, Massachusetts, and Washington employ regulations that are more efficient than ASHRAE 90.1-2013. The 2018 version of ICC’s International Energy Conservation Code (IECC) is expected to correspond to ASHRAE 90.1-2016 in many respects. States like New York and Illinois usually incorporate IECC revisions into their codes within the year the standards appear.
|AIRTIGHT VAPOR BARRIERS WORSEN THERMAL BRIDGING PROBLEMS|
In addition to wasting energy, uninsulated structural penetrations chill steel and concrete support structures on the interior side of the insulated building envelope, raising the potential for condensation and mold growth on adjacent surfaces, with steel structures additionally subject to rust.
Condensation on interior support structures is a relatively recent phenomenon. Prior to the advent of airtight vapor barriers, high-rise buildings leaked air profusely, causing interior humidity levels to equalize with low exterior humidity levels (typically 18 to 25 percent) during winter. Forced hot air commonly vented at or near structural penetrations further ensured interior humidity remained too low to reach dewpoint and form condensation.
While the adoption of airtight vapor barriers reduced heat loss through air exchange, it also raised interior humidity levels roughly two-fold, to as high as 50 percent during winter.
Although high humidity benefits energy efficiency and human comfort, it also allows air surrounding cold interior penetrations to reach dewpoint and form condensation on the inside face of sheetrock, studs, and insulation. Occupants can be exposed to airborne mold for months or years before it spreads to visible surfaces, exposing developers to remediation and liability costs—problems that can be prevented by insulating structural penetrations.
Three paths to compliance
The 2016 iteration of ASHRAE 90.1 offers three paths to compliance. The prescriptive method specifies details of building elements such as:
The energy cost budget (ECB) method is an alternative performance-based system for determining compliance, and available as a free web program. (For more information, click here.) ECB compares two models of a building: the proposed building as designed and the budget building design (a structure of the same size and constructed to minimum ASHRAE 90.1 prescriptive requirements), calculating costs and identifying areas needing change. The program simulates the building’s proposed energy costs, comparing them with those of the code-compliant structure, and indicating whether the proposed costs are less than or equal to the baseline and, thus, compliant. ECB can also summarize a building’s energy performance as a percentage of ASHRAE 90.1 standards.
In the performance rating method (PRM), the project team applies software modeling tools to prove the building will perform at least as well as under the prescriptive requirements with an equal or lower annual energy cost. PRM is also used for “beyond code” programs such as LEED or the International Green Construction Code (IgCC).
Canada has introduced similar “objective-based” codes, providing a consistent means of evaluating and accepting alternative solutions to code requirements. Each technical requirement now includes objective, functional intent statements accommodating innovative approaches.
The performance-oriented ECB and PRM paths allow energy tradeoffs comparable to carbon credits (e.g. more roof insulation or solar panels to offset energy loss from weakly performing glazing or other thermal bridges). These flexible paths are preferable for designs with less-efficient features such as floor-to-ceiling windows, or to allow new best practices not previously covered in prescriptive codes. STBs can enhance an envelope’s ability to meet performance criteria.
The implementation of ASHRAE 90.1 or IECC standards is not straightforward. As energy standards evolve, architects, developers, and even code officials may be unclear about whether local codes require STBs. For a project on the ECB or PRM performance path to compliance, accurate energy modeling can be an art form. Today’s 3D modeling software is very precise mathematically but the practitioner needs to put the correct and realistic information into the model to derive accurate output. For example, it is essential to include balcony slabs among inputs rather than treating it as part of the continuous exterior surface of the building. If one ignores thermal bridges, even though they appear small, they will compound and have a significant effect. Precise input is, therefore, critical to accuracy of the model.
Vancouver in the vanguard
Canadian codes have imposed higher standards than those in most U.S. jurisdictions. The 2017 NECB requires all new buildings to be “net zero-energy ready” by 2030. Vancouver, B.C., takes performance, particularly seriously. The city has an interesting market. There is not a lot of free space for new construction, and property is expensive. Most of the projects are infills, redevelopments, and construction of condos on parking lots. These types of projects need rezoning, and application approval requires very efficient construction such as Passive House certification or meeting a new metric called thermal energy demand intensity (TEDI). Measured in kWh/m2 annually, TEDI is the amount of heat required to keep a building warm after offsets from building envelope losses and heating of ventilation air regardless of how efficiently the heat source is produced. This metric directly reflects the building enclosure performance.
Vancouver building officials, aware of the performance challenges facing their densely-built environment, have instituted a green buildings policy—starting with rezoning of high-rises—requiring all rezoning applications after May 1, 2017, to meet standards for “near zero-emissions buildings” (equivalent to Passive House or the International Living Future Institute’s [ILFI’s] Zero Energy Building [EZB] certification) or “low-emissions, green buildings,” which address building envelope performance. (For more information, click here.)
A building ahead of its time in addressing building envelope performance is Cedar Springs Retirement Residence, a 146-unit, 11-story building in North Vancouver owned by Pacific Arbour Retirement Communities. Completed in 2012, architects Besharat Friars and structural engineers Glotman Simpson achieved LEED Gold certification by paying close attention to design and materials throughout the building. At cantilevered concrete “eyebrows” shading the building’s south and west façades on each floor, STBs help control energy loss, earning the building LEED credits for Innovation in Design and Thermal Comfort.
Additionally, STBs helped the Smithsonian’s National Museum of African American History and Culture in Washington, D.C., conserve energy and reduce humidity to protect artifacts from the effects of condensation.
The museum’s façade was designed to provide ci, but thermal bridging was a significant concern where the rooftop cooling towers are connected. Steel posts supporting the cooling towers penetrate the roof. Since an exhibit space lies directly below the cooling towers, thermal bridging had to be eliminated to prevent condensation from forming on the warm interior side of the building envelope, thereby preventing the prospect of mold growth and moisture damage that could threaten the museum’s collection.
STBs for steel construction were installed at 16 connection points per cooling tower to simultaneously insulate and support the columns penetrating the rooftop. The energy savings from the thermal breaks reduce heat loss at each point of penetration by up to 50 percent, contributing to sustainable design and LEED certification.
Thermal modeling may influence codes
B.C. Hydro has published the Building Envelope Thermal Bridging Guide containing 3D models of common envelope details and heat-transfer patterns. The guide refines industry-wide benchmarks for whole-building thermal performance, and may possibly influence future code development. The authors
are optimistic if the work from these guides can be incorporated into the ASHRAE standards, then ci could really gain leverage because building professionals across the board recognize ASHRAE and this standard.
The latest version of ASHRAE 90.1 requires specific modeling of “uninsulated assemblies” (e.g. projecting balconies, slab edges, and roof parapets where ci is not covered) in the envelope trade-off option (Appendix C1.2.6) and the ECB method of compliance (Appendix G Table G3.1 row 5.a.1). The next IECC iteration slated for publication later this year is expected to follow many of the changes from the 2016 version of ASHRAE 90.1. The next version of 90.1, to be released in 2019, is expected to provide more explicit requirements to address structural elements causing thermal bridging.
The future should see more STB use, driven by evolving codes and increased awareness and desire for energy savings, moisture avoidance, and thermal comfort. Agencies setting codes, and architects, engineers, and constructors are increasingly recognizing how leaving penetrations in a building envelope uninsulated “is like wearing a down jacket but leaving it unzipped,” as once described by Hao Ko, managing director of Gensler.
Jonathan Carreiro, P. Eng., is product manager for Schöck North America. He has been in this position since 2015 after moving from the builder side of the industry. Carreiro has a bachelor’s degree in civil engineering from Royal Military College of Canada and an MBA from Carleton University. He is a member of the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE). Carreiro can be reached at firstname.lastname@example.org.
Patrick M.B. Chan is regional engineering and sales manager with Schöck North America. He manages the Pacific Northwest, Alberta, and the Prairies territories. Chan holds a civil engineering degree from the University of British Columbia. Based in Vancouver, B.C., he focuses on simple solutions to reduce the environmental footprint of structures by addressing building envelopes. Chan can be reached at email@example.com.
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