Thermal Bridging and Building Facades
The Missing Key to Low Energy Building Design
Presented on October 13, 2022 at Facade Tectonics 2022 World Congress
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Thermal bridging through building façades have been overlooked by designers and building energy codes and standards in the past, which has led to higher space heating and cooling loads, occupant discomfort, and higher risk of condensation. There are numerous examples of energy efficient buildings consuming more energy than what has been predicted in energy models that may be attributed to a lack of understanding of thermal bridging.
Thermal performance of a building façade can drop significantly when thermal bridging is accounted for. A typical 40-story high-rise residential building with 40% window-to-wall ratio, and balconies equal to 20% of the floor perimeter can have 71% of the total heat loss from thermal bridging based on typical construction details, with the majority of the heat loss at the window to wall transitions (e.g. window heads, jambs, and sills).
While thermal bridging is mostly evaluated with 2D and 3D thermal simulations, there are many resources available. Thermal bridging catalogues such as the Building Envelope Thermal Bridging Guide (BETB) and thermalenvelope.ca have over 600 details and an integrated calculator for calculating building envelope overall thermal performance that makes it easier to account for thermal bridging.
Recognizing and mitigating thermal bridging is an important part of the strategy to meet low energy targets for buildings. Many jurisdictions are starting to incorporate thermal bridging into their codes and standards. The NECB 2017 already requires thermal bridging to be considered, the NYC 2020 Energy Conservation Code now requires thermal bridging values be reported in designs, and both ASHRAE 90.1 and IECC will likely include thermal bridging requirements in their next update cycles. With net-zero energy targets in the horizon and more strict building energy requirements coming, it is time for designers to pay more attention to thermal bridging as this is a bridge we must cross for a better future.
It is widely known that thermal bridging through façade components and details degrade building energy performance and cause issues such as condensation and thermal discomfort for occupants. Until recently, accounting for the impact of thermal bridging in façade design has been rare due to difficulty in measuring and evaluating its impact. As a result, the effect of thermal bridging in building façades were under reported leading to many buildings with higher energy consumption and energy costs than predicted by code or energy models due to higher space heating and space cooling loads. Mechanical designers also compensated for the added space conditioning loads by oversizing mechanical units that were more expensive and not always operating at peak efficiency.
Advances in computer simulations have made evaluating thermal bridging in façades easier for designers, first through two-dimensional heat transfer analysis made popular by programs such as THERM which is widely used for evaluating thermal performance of glazing systems. In recent years, three-dimensional thermal modeling has become popular as this analysis can better evaluate thermal bridging of assemblies with discrete thermal bridging components and account for complex heat flow through highly conductive components of curtain wall and window wall systems often within a few percent of measured heat flow through hot box measurements (Norris et al. 2015). Thermal bridging resources such as the Building Envelope Thermal Bridging Guide (Morrison Hershfield 2021) has made accounting for thermal bridging in façade design much more accessible to designers with a comprehensive catalogue of details and assemblies that are typically found in North American building construction.
Building energy codes and standards have also made thermal bridging more prominent in façade design by placing a greater emphasis in including the impact of thermal bridging in façade U-value calculations and whole building energy simulations. Progressive building energy codes and standards that are targeting Net-Zero Energy or Net-Zero Energy Ready performance levels, such as Passive House, the BC Energy Step Code, Vancouver Zero Emissions Building Plan, and Toronto Green Building Standard, all stipulate the inclusion of thermal bridging in façade thermal performance calculations. Other codes that has less stringent energy requirements, such as the National Energy Code for Buildings Canada (NECB) and the NYC Energy Conservation Code 2020, are also starting to require thermal bridging be accounted for in assessing façade thermal performance.
Despite the changing requirements for designers to account for thermal bridging, many are not fully aware of the impact of thermal bridging on building energy performance, how to integrate it in their analysis, and how to mitigate the impact of thermal bridging. This paper discusses the impact of thermal bridging and mitigation strategies at common façade details through examples and tools that are readily available to façade designers.
Thermal Bridging Types and Mitigation Strategies
Thermal bridging in building façade assemblies may be categorized into three types which are typically found on most buildings. Façade details such as balconies, intermediate floors, parapets, corners, soffits, penetration
Façade Thermal Performance of 40-Story Multi-Unit Residential High-Rise Building
It is important to note designers do not have to mitigate thermal bridging at all façade details. Instead addressing thermal bridging at a few key details may be enough to
The façade thermal performance evaluation presented in this paper may be easily applied to any façade and building by using Equation 1. Resources such as thermalenvelope.ca includes a built-in calculator
Impact on Whole Building Energy Performance
The impact of thermal bridging on whole building energy performance and operational greenhouse gas emissions may be determined through whole building energy modeling. To show potential savings in energy and
Façade Thermal Performance of Different Mitigation Strategies
The analysis from the 40-story high-rise residential building showed the majority of heat loss of the façade is from thermal bridging, specifically at the window-to-wall details and cantilevered balconies. This
Recognizing the impact of thermal bridging on façade thermal performance is an important step in determining effective energy performance of the façade and building. As seen in the residential high-rise
Rights and Permissions
ASHRAE 2016, ASHRAE 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings, Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc.
BC Housing, 2020, Thermalenvelope.ca, https://thermalenvelope.ca/
City of New York, 2020, 2020 Energy Conservation Code, New York City, NY
City of Toronto, 2018, Toronto Green Standard Version 3, Toronto, ON
Hay, K. Pattrick, B., Roppel, P. 2020, Thermal, Structural, and Cost Optimization of The Building Enclosure for Net-Zero Construction, Proceedings of International Conference on Building Envelope Systems and Technologies (ICBEST)
International Code Council, Inc. International Energy Conservation Code (IECC), Country Club Hills, IL
Morrison Hershfield, 2021, Building Envelope Thermal Bridging Guide, Vancouver, BC: https://www.bchousing.org/research-centre/library/residential-design-construction/building-envelope-thermal-bridging-guide
Morrison Hershfield, 2018, Guide to Low Thermal Energy Demand for Large Buildings, Vancouver, BC: https://www.bchousing.org/research-centre/library/residential-design-construction/guide-low-energy-demand-large-buildings
Natural Resources Canada, 2015, National Energy Code of Canada for Buildings 2015, Ottawa, ON
Natural Resources Canada, 2017, National Energy Code of Canada for Buildings 2017, Ottawa, ON
Norris, N., Carbar, L.D., Yee, S., Roppel, P., Ciantar, P., 2015, The Reality of Quantifying Curtain Wall Spandrel Thermal Performance: 2D, 3D and Hotbox Testing, Proceedings of BEST Conference Building Enclosure Science and Technology (BEST4), Kansas City, MI
Open Technologies, 2020, buiildingpathfinder.com, https://buildingpathfinder.com/
Passive House Institute, 2015, Criteria for Passive House, EnerPHit and PHI Low Energy Building Standard, Darmstadt, Germany
Province of British Columbia, 2017, BC Energy Step Code, Victoria, BC: https://energystepcode.ca/
Thermal Bridging Solutions, 2022, Energy Codes and Thermal Bridging, Mattapoisett, MA: thermalbridgingsolutions.com/thermal-bridging/energy-codes-thermal-bridging/