Structural Thermal Bridging in Buildings Part 1: Structural Penetrations

To view the figures and tables associated with this article, please refer to the flipbook above.

Over the past several years, the need for structural engineers to pay attention to and mitigate structural thermal bridging, especially the most severe conditions, has grown in importance. This is primarily because architects, pushed by more stringent energy codes, have improved the thermal performance of the building envelope in areas such as walls, roofs, and floors. However, many architects and engineers have not accounted for thermal bridging in these insulated building envelope designs, which reduces the overall thermal performance of the building envelope by increasing energy loss. Without considering thermal bridging, many architects and engineers have falsely believed the building envelope has better thermal performance than it actually does. This can lead to underperformance of actual versus modeled building energy consumption, which leads to failure to meet energy targets such as net-zero operational energy or higher costs due to higher-than-expected energy consumption. Structural thermal bridging through exterior walls, roofs, and foundations can impact not only the energy performance of the building, but also occupant comfort and durability of materials. Excessive heating or cooling energy loss from structural thermal bridging can create cold spots which increases the risk of condensation and moisture-related damage to finishes and structural elements. Details that seemed to work fine 10 years ago may cause severe condensation problems today.

Tightening of energy codes alone is reason enough for structural engineers to pay attention to thermal bridging conditions. Some building energy codes now explicitly require thermal bridging conditions, such as balconies and parapets, be addressed by the design team. To the extent that these details are structural, they cannot be addressed without the structural engineer’s collaboration and input—even if they had not considered thermal losses to be part of their responsibility.

Litigation cases involving deterioration of building elements due to thermal bridging-related condensation are on the rise. Remedying an existing building condition to minimize problematic thermal bridging can be challenging and expensive—much more so than incorporating details that mitigate thermal bridging in the original design. If the cause of such problems is identified as thermal bridging through structural elements, they might still be considered legally and financially culpable, if the problem-causing details are on the structural drawings.

Another big factor that is growing in importance is limiting structural thermal bridging energy losses for the reduction of operational carbon from buildings. Recent attention has been paid by structural engineers to understand and reduce the embodied carbon of the structural materials used on their projects, and rightly so. But operational carbon—the carbon emissions from the energy needed to heat and cool the building—is equally significant. Here as well, when caused by thermal bridging energy losses, the carbon emissions can only be effectively mitigated by the structural engineer. Severe structural thermal bridging conditions across a building envelope that is otherwise well-insulated and air-sealed can cause in excess of 20% of the building’s thermal losses. This will occur during every year of the service life of the building for both heating and cooling energy, resulting in a significant increase in operational carbon emissions—which the project’s structural engineer could prevent.

The Structural Engineering Institute (SEI ) Sustainability Committee’s Thermal Bridging Working Group has been working to increase awareness of, and communicate solutions to, the problem of structural thermal bridging to practitioners. In this three-part series on thermal bridging, we’ll identify the various types of problematic thermal bridging conditions that occur on building design projects. For each we’ll illustrate some examples of effective mitigation strategies to reduce, if not eliminate, thermal bridging energy losses. We’ll also provide resources for more information to assist designers who don’t necessarily want to become experts in thermal flow analysis but want to prevent excessive thermal losses and avoid problems on their projects.

Cantilever concrete balconies and other appurtenances that project out across a building’s thermal envelope are problematic thermal bridging details because the penetration through the building envelope is continuous; that is, the thermal bridge is linear, creating a greater magnitude of thermal loss. For a typical high-rise concrete residential building with balconies equal to 20% of the intermediate floor area, thermal bridging from cantilevered balconies can account for 13% of the building envelope heat loss.

An effective remedy is to reframe the balcony with exterior supports so that the bridging elements are discrete, i.e., creating discrete point thermal bridging, rather than a continuous cantilever.

If this is not feasible, the cantilever support condition can be modified to reduce thermal flow by the use of a proprietary manufactured structural thermal break assembly, inserted in the plane of the thermal envelope. These have been used in Europe and other countries for decades. They can be considered design-delegated components, that is, the loading requirements can be specified as well as thermal properties, which should be determined in conjunction with the project architect, and the manufacturer’s engineer can verify that the manufactured connection element can resist the design loads. Other methods include wrapping the balconies with insulation; however, this may be difficult to implement due to durability concerns of the insulation.

The thermal performance of these balcony details is described by the Psi-value which is the incremental heat loss due to thermal bridging from continuous details such as balconies, parapets, and shelf angles. The Psi-values listed in Table 1 were determined using 2D and 3D thermal simulations.

Steel-framed and wood-framed balconies usually can avoid continuous thermal bridging, with a discrete number of cantilever beams providing support. However, the high thermal conductance of steel can result in significant thermal losses unless mitigated. Wood has less conductance; however, it is still more conductive than any continuous insulation used in the building wall, so it is still vulnerable to the development of cold spots, leading to condensation.

Calculating the thermal losses and determining the areas of potential condensation through linear thermal bridging can be determined using thermal simulation software following thermal simulation standards such as the ISO 10211 and procedures in ASHRAE 1365 RP. 2D thermal simulation software, such as THERM, which is a free program available through Berkeley Lab can simulate heat loss for continuous thermal bridging details. However, for details with discrete components, such as the intermittently supported balconies, 3D thermal simulation software will provide accurate results, since 2D simulation results may be too optimistic. Pre-calculated thermal bridging catalogs such as the BC Hydro Building Envelope Thermal Bridging (BETB) Guide (thermalenvelope.ca) and the ISO 14683 Default values are great sources of thermal bridging values for architects and structural engineers in lieu of thermal simulation data.

Discrete thermal bridging elements, such as steel beams or columns penetrating through the thermal envelope, usually do not cause large amounts of thermal losses. However, if the elements have large cross-sectional areas, or if a series or cluster of discrete thermal bridging elements occurs, the thermal losses can add up. In addition, the local areas of these thermal bridges are just as likely to create condensation problems as linear thermal bridges.

A variant of this type of thermal bridging is the connection of sunshades or other projecting architectural elements to the building structure. Such metallic elements can act as thermal collectors or radiators, especially when they are made of aluminum, which conducts heat about five times as well as carbon steel. One of the authors has seen condensation dripping onto and staining ceilings and causing rust on structural members, at locations adjacent to the attachment points of aluminum solar shades (Fig. 2).

To remedy these conditions, some type of thermal intervention is required, ideally at the plane of the building’s insulation. An effective option is, again, a manufactured structural thermal break assembly; there are versions to connect to steel on both sides, as well as steel on one side and concrete on the other.

Also possible is the use of a thickness of low-conducting high-strength structural shim material. When using this approach, keep in mind the following:

Examples of thermal bridging at beam and column penetration details are shown in Tables 2 and 3. The heat loss at these penetration details is quantified by the Chi-value, which is the incremental heat loss from point thermal bridges. These values were determined using 3D thermal simulations from the BETB Guide.

Similar to balconies, a concrete or other type of continuous cantilever canopy can create significant thermal losses and condensation potential. The mitigation strategies are similar to that which we described for balconies.

Canopies supported by diagonal ties above and connected back to the buildings generally have point-type thermal bridges at the connection points—both at the main beam supports and at the ties. If stainless-steel connection elements are used, the thermal losses become much smaller, since stainless steel has a heat transfer coefficient about one-third that of carbon steel. To further mitigate the losses, a properly detailed connection incorporating low-conductance pads, or shims, can be used, such as fiberglass reinforced plastic.

Often overlooked, parapets can cause a large amount of thermal loss, depending on their construction. One common detail that is particularly problematic is when the cold-formed steel studs that comprise the wall structure below the roof plane are extended up past the spandrel beam to form the parapet. With the parapet exposed to exterior temperatures on three sides, both faces and the top surface of the parapet must be insulated—and air sealed—to prevent heat flow out of the building (Fig. 3).

An alternative to enclosing all surfaces of the parapet with a well-detailed thermal blanket is to provide a manufactured thermal break assembly at the base of the parapet—in line with the roof insulation. This relegates the parapet structure to being outside of the building’s thermal envelope, avoiding thermal bridging.

Another option for parapets is to design their structure with discrete vertical cantilever beams, rigidly connected to the spandrel beam with a base plate of low-conductance material such as fiberglass reinforced plastic. Here again, it is important to use a thick enough shim—at least 1 inch—or the reduction of the thermal flow may not be significant.

Table 4 illustrates some thermal bridging details showing the relative performance of some thermal bridging mitigation strategies for parapets. Like balconies, thermal bridging at parapets is also recognized as linear thermal bridges and quantified using Psi-values. These values were also found in the BETB Guide.

The best way to address and mitigate problematic structural thermal bridging is to work with the project architect and envelope consultant, if there is one, early in the design process. Early and active collaboration on building envelope designs can result in details that are less in need of remedial thermal bridging mitigation once the limitations caused by thermal bridging are understood by the team. It’s always easier to steer clear of conditions that would cause severe thermal bridging than to try to mitigate them after the architect has established their details. Also, early engagement with the project’s energy modeler, if there is one, might allow them to provide quantitative feedback on what works regarding thermal energy flow mitigation. This is especially useful, because most structural engineers are not equipped to perform these types of analyses themselves. Working with the team to practice integrative design—possibly thinking of your role as a structural consultant rather than the structural engineer—can lead to better coordinated, and better designed, projects. ■