10 • IIBEC Interface April 2022 In addition to heat loss and greenhouse gas emissions, thermal bridging in balconies, which is the focus of this paper, causes other issues, such as cold floors opposite balconies and condensation adjacent to cold penetrations, possibly resulting in damage or mold at interior finishes. Thermal bridging can be defined as localized areas with a higher thermal conductivity than adjacent areas. A typical thermal bridge in a building enclosure occurs where a material of high conductivity, usually a structural component, penetrates the insulation layer. Such elements may be spread over large areas of the building enclosure, such as facade clips, which may be continuous along a line; balconies or slab edges; or point concentrations (like a steel canopy beam). In all cases, the higher conductivity of the material of these elements would decrease the effective R-value of the surrounding enclosure. The insulation layer would therefore not be continuous, and additional heat would dissipate into the outer environment. TYPES AND CHARACTERISTICS OF THERMAL BRIDGES Linear Thermal Bridges Linear thermal bridges are disturbances in the enclosure that occur along a line. A typical example of this is a balcony connection with the floor slab (Fig. 1). The energy loss incurred through a linear thermal bridge is described by the linear thermal transmittance ψ in W/mK (Btu/h ft °F). Point Thermal Bridges Point thermal bridges are disturbances in the enclosure that occur in one spot. A typical example is a steel beam penetrating the enclosure to support a balcony or canopy (Fig. 2). The energy loss incurred through a point thermal bridge is described by the point thermal transmittance χ in W/K (Btu/h °F). Figure 1. Thermal image of a linear thermal bridge at a balcony connection. Figure 2. Thermal image of a point thermal bridge at a canopy steel beam connection. April 2022 IIBEC Interface • 11 TYPICAL LOCATIONS OF STRUCTURAL THERMAL BRIDGES IN BUILDINGS Structural thermal bridges in buildings occur wherever a structural element made of conductive materials such as concrete or steel penetrate the building enclosure, as on: • Balconies and canopies—conductive elements that penetrate the vertical enclosure • Slab edges—shorter cantilevers but just as conductive as balconies, and often more • Parapets—conductive elements that penetrate the horizontal enclosure (roof insulation) • Rooftop-mounted equipment—conductive connections penetrating the horizontal enclosure (roof insulation), causing point thermal bridges TYPES OF STRUCTURAL THERMAL BREAKS BY MATERIAL Structural thermal breaks may be classified according to the material of the structural elements they connect, referring to the material of the primary structure and that of the uninsulated element on the outside of the enclosure. Concrete-to-Concrete Structural Thermal Breaks Concrete-to-concrete thermal breaks (Fig. 3) connect, for example, concrete balconies to concrete structures. They typically consist of the following primary components: • Reinforcement, consisting of stainless steel welded to carbon steel reinforcing bars. Stainless steel is used at the insulation location because of its lower thermal conductivity (about 30% of that of carbon steel). • Pressure blocks, consisting of ultra-high-performance concrete (UHPC) with steel fibers. UHPC is used to minimize the amount of concrete because it is a conductive material. • Graphite enhanced expanded polystyrene, a material of very low conductivity, is used as insulation material. • Fire plates and intumescent strips, which provide the required fire resistance. The reinforcing bars extending on both sides of the insulating block are tied to the reinforcing bars of the balcony and inner slab. Concrete-to-Steel Structural Thermal Breaks Concrete-to-steel thermal breaks connect, for example, a steel balcony to a concrete building (Fig. 4). They typically consist of: • Stainless steel threaded rods welded to carbon steel reinforcing bars • Insulation consisting of expanded polystyrene The threaded rods enable bolting of the steel cantilever, while the reinforcing bars are tied to the reinforcing bars of the inner slab. Steel-to-Steel Structural Thermal Breaks Steel-to-steel thermal breaks connect, for example, a steel balcony to a steel building (Fig. 5). They typically consist of: • Stainless steel components—hollow sections and endplates • Bolts and other accessories The steel elements on each side of the thermal break are bolted onto the thermal breaks. Figure 3. Example of a concrete-to-concrete structural thermal break. Figure 4. Example of a concrete-to-steel structural thermal break. 12 • IIBEC Interface April 2022 The use of structural thermal breaks provides the necessary structural integrity while considerably reducing heat losses. In addition, structural thermal breaks improve occupant comfort, eliminate condensation, and mitigate potential mold formation caused by thermal bridging. STRUCTURAL THERMAL BREAKS BY TYPE OF FORCE TRANSFER Structural thermal breaks also may be classified according to the type of forces they can resist. Thermal Breaks that Provide Flexural and Shear Strength Under Gravity Loading Only These thermal breaks with top reinforcement are used for the typical application of cantilevered balconies, subject to gravitational loads (Fig. 3). Thermal Breaks that Provide Flexural and Shear Strength for Flexural Loading These thermal breaks consist of reinforcing bars on the top as well as the bottom (Fig. 6). Thermal Breaks that Provide Shear Strength Only, Allowing for Rotation at the Support These thermal breaks are used, for example, in propped cantilevers (Fig. 7). Thermal Breaks that Provide Lateral Strength These thermal breaks are used in seismic areas, in combination with other thermal breaks, to address the other forces (gravitational, etc.) (Fig. 8). IMPACT OF STRUCTURAL THERMAL BREAKS ON THE ENCLOSURE PERFORMANCE Impact on Energy Savings Let us consider a typical thermal bridge in a concrete building: a balcony. A conventional balcony can be characterized by the linear transmittance shown in Eq. (1). A balcony incorporating a structural thermal break can be characterized approximately by the linear transmittance shown in Eq. (2). The total amount of energy saved will depend on the difference between the indoor and outdoor temperatures. For a difference of 30°C (54°F) over an hour, the energy saved (per meter length of balcony) would be 25.5 Wh/m (26,450 Btu/ft), as shown in Eq. (3). The energy saved over one year may be calculated using the heating degree hours (HDH), available from various weather Figure 5. Example of a steel-to-steel structural thermal break. Equation 1. Equation 2. Figure 6. Concrete-to-concrete thermal break—flexural and shear strength, downward and upward loading. Figure 7. Concrete-to-concrete thermal break providing shear strength only, for supported cantilevers. April 2022 IIBEC Interface • 13 data sources. For example, for Montreal, the HDH would be 111,000 degree-hours, and the energy saved would therefore be 94.35 kWh/m (97,900 Btu/ft), as shown in Eq. (4). If the above numbers are multiplied by the total length of the balconies, the result will provide the total energy saved for that particular project. Impact on the Effective R-Value If the enclosure R-value is R0 and the linear transmittance of the balcony penetrating this enclosure is ψ, the effective R-value would be calculated as shown in Eq. (5), where h is the floor height. For the R0 shown in Eq. (6) and the linear transmittances shown in Eq. (7) and (8), respectively, for the conventional and thermally broken balconies, the effective R-values would be calculated as shown in Eq. (9) and (10), for an assumed floor height of 3 m (10 ft). The R-value improvement in this case is about 50% (1.82/1.20 = 1.52). The effect of thermal breaks would be higher for higher-performing enclosures. For an RSI-value of 3.0, the R-value improvement would be 74%. Impact on Condensation/Mold Risk and Comfort The potential for condensation is a function of the indoor air relative humidity and low temperature in the proximity of thermal bridges. Based on Morrison Hershfield’s study,1 the temperature at the edge would be –1°C (33°F) for the conventional concrete balcony and +17°C (63°F) for the thermally broken case. The modeled outside and inside temperatures were, respectively, –10°C (14°F) and +21°C (70°F) for the conventional and thermally broken cases. The relative humidity of the indoor Equation 3. Equation 4. Equation 10. Figure 8. Concrete-to-concrete thermal break for seismic conditions. Equation 9. Figure 9. Psychometric chart of relative humidity. Equation 8. Equation 6. Equation 7. Equation 5. 14 • IIBEC Interface April 2022 Figure 10. Installation of concrete-to-concrete structural thermal breaks. Figure 11. Installation of concrete-to-concrete structural thermal breaks at balcony connection. air at which condensation at the cold edges would occur would be 23% and 77%, respectively. Considering that the comfortable range of relative humidity in a living space is considered to be between 30% and 70%, it may be concluded that the risk of condensation is eliminated when structural thermal breaks are used. To eliminate condensation in the conventional case, uncomfortably dry indoor air conditions (relative humidity below 23%) would have to be accepted or created as a tradeoff. The impact on comfort is clearly quantifiable from the difference in temperature at the edge, –1°C (34°F) versus +17°C (63°F). INSTALLATION OF STRUCTURAL THERMAL BREAKS For the installation of concrete-to-concrete thermal breaks, a gap is left within the slab reinforcing bar cage at the enclosure location. The thermal breaks are then dropped into place, and their reinforcing bars are tied to the slab reinforcing bars. When concrete of different strengths is used for the inner and outer balcony slabs, the thermal break also provides the benefit of a concrete pour stop (Fig. 10–12). April 2022 IIBEC Interface • 15 Figure 12. Poured concrete balcony with thermal breaks installed. For the installation of the steel-to-steel thermal breaks, the steel section is cut at the enclosure location and two end plates are welded on each end. The thermal break is then bolted between these to the end plates, providing the structural continuity, while reducing the heat transfer along the steel element (Fig. 13). Installation of the concrete-to-steel thermal break involves the embedment of its reinforcing bars into the slab on one side and the bolting of the steel element on the other side (Fig. 14). Various product configurations available to address specific installation needs include: • Retrofits, when balcony replacements or additions to an existing building are necessary • Balcony step-downs or step-ups • Reduced space/depth for the embedment of straight reinforcing bars • Balcony slabs embedded in concrete walls rather than a continuous slab For specific applications, new configurations may be designed. Figure 13. Installed thermal break at steel beam connection. Figure 14. Concrete-to-steel thermal break installation. 16 • IIBEC Interface April 2022 PROJECTS The industry experience with thermal breaks spans more than 40 years, with millions of product installations all over the world. A few projects that have used thermal breaks in the past are shown in Fig. 15–17. With a greater emphasis on improving building enclosure performance and increasingly stringent building codes, today’s construction market cannot ignore thermal bridging. Region by region, we are seeing the adoption of codes that require continuous insulation, including thermal modeling of all potential thermal bridges. Although thermal breaks may be one part of a much larger whole, their significance in creating more sustainable building practices cannot be underestimated. REFERENCE 1. Morrison Hershfield. 2013. Thermal and Whole Building Energy Performance of Thermal Break Technology for Concrete Balconies in High-Rise Multi-unit Residential Buildings. Please address reader comments to chamaker@iibec.org, including “Letter to Editor” in the subject line, or IIBEC, IIBEC Interface Journal, 434 Fayetteville St., Suite 2400, Raleigh, NC 27601. Dritan Topuzi is the product manager of Schöck North America, an adjunct faculty member at Norwich University in Northfield, Vermont, and a registered professional engineer in Ontario, British Columbia, Alberta, Quebec, and Nova Scotia. Topuzi received his PhD from the University of Waterloo in 2015. He is a member the American Concrete Institute, Canadian Standards Association, and other technical associations. He is also an accredited PMP and LEED AP. Dritan Topuzi, PhD April 2022 IIBEC Interface • 17 Figure 15. The designers for the Comandante Ferraz Research Station on King George Island in Antarctica used steel-to-steel thermal breaks to insulate its interior from one of the most extreme climates on Earth. Figure 16. The designers for Candela Lofts, a luxury multifamily residential building in Hoboken, N.J., incorporated concrete-to-concrete thermal breaks at its balcony and slab edge connections to help meet LEED Platinum and Passive House requirements. Figure 17. One of the tallest buildings in British Columbia, 3 Civic Plaza is a 52-story multiuse building in Surrey. Structural thermal breaks were used to insulate its 300+ balconies. Credit: Simon Chiu, Kwantlen Polytechnic University.
