Structural Thermal Bridging in Facades

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Thermal bridging in buildings has been gaining more and more attention in recent years not only as a potential cause of condensation and moisture related deterioration but also for its role in degrading the thermal performance of the building enclosure. As we strive to reach low and ultimately net-zero carbon goals, mitigating thermal bridging to enhance building energy performance is critical. While many believe thermal bridging and thermal performance of the building envelope is within the domain of architects and building enclosure consultants, structural engineers also have an important role in mitigating thermal bridging since many of these details have a structural component.

The building facade often refers to the components and systems that enclose the vertical surfaces of a building including exterior walls and windows. Its primary functions are to control heat, air, and moisture flow, as well as provide structural support against lateral wind loads and occasionally gravity loads for load bearing walls. Facades also contribute to the aesthetics of the building since it is a recognizable exterior component.

The opaque sections of the building facade can be composed of many different types of systems that include glazed wall systems such as curtain wall and window wall systems and precast insulated concrete panels. Steel-framed exterior wall assemblies are commonly found in many low to mid-rise commercial buildings. Increasingly, these wall assemblies are fitted with more and more insulation. Although many designers and contractors have added insulation between the studs in the past, many building energy codes and standards are asking for a continuous layer of insulation that often is placed outboard of the steel studs to help mitigate thermal bridging. This presents challenges of how to securely attach exterior cladding to these walls such that wind loads can be transferred back to the structure. The solution many designers have posed is to add exterior cladding support systems to these assemblies along with the exterior continuous insulation. Common solutions that we have seen include: vertical Z-girts, horizontal Z-girts, mixed Z-girts, and intermittent brackets or clips.

Of these four types of cladding support systems, intermittent brackets/clips system offers the best thermal performance since it minimizes components penetrating the exterior insulation. Many bracket and clip designs include thermal break pads or thermal breaks within the bracket, while others use low thermal conductivity materials such as stainless steel or fiberglass.

At first glance, choosing a cladding support system made with low thermal conductivity materials may seem appropriate for optimizing the thermal performance of the building facade. However, other factors should be considered when selecting the right system including structural capacity, fire resistance, and cost. A fiberglass clip may provide low thermal conductive performance compared to an aluminum clip but have less structural capacity. As a result, more fiberglass clips may be required to carry cladding dead load and wind loads than an aluminum clip system. This increases the amount of penetrations through the insulation as well as attachments to the exterior wall, which could drive up material and construction costs since more components are needed. An aluminum clip system may be able to achieve the same thermal performance but with greater clip spacing.

Rail penetration depth into the insulation and orientation impact the thermal performance of many clip systems. Because the clips are intermittent, rails are added to help support the cladding and cladding attachment components. Rails penetrate through the insulation in many North American clip systems. As a result, the rails act as fins that draw heat from the insulation. This effect is greater for clips that are much smaller than the insulation depth since the rails are fitted deeper in the insulation. The orientation of the rails also has an impact on thermal performance. Systems that include rails running horizontally often have shorter overall rail length than vertical rail systems and therefore better thermal performance. Many cladding support systems have vertical clip spacing greater than the horizontal spacing, resulting in fewer horizontal rails because the spacing between the rails is dictated by the clip spacing.

Recent building energy codes have not only begun to recognize thermal bridging of cladding support systems, but some have also provided prescriptive derating values. Energy codes such as the Massachusetts Stretch Code have provided derating values that can range from 27% to over 63% for most typical exterior insulation ranges. With good design that balances both structural and thermal performance, many clip systems are able to achieve higher thermal efficiencies and lower derating values than these prescribed values. A holistic approach, with close collaboration between architect, structural engineer, and building enclosure consultant, is often required to find the optimal design. Many cladding support system manufacturers provide engineering reports of 3D thermal simulations, span charts, and load tables for their systems to help aid in the design. Some will provide engineering services to ensure their system is fully optimized for the building.

Brick veneer and masonry panels are common cladding types used in buildings. Many types of masonry cladding systems are connected to exterior wall assemblies using intermittent anchors which are much smaller than typical cladding support clip systems. Although these anchors may have good thermal performance, they are only part of the masonry support system. Shelf angles are often needed at regular intervals to support the weight of the masonry cladding. These angles are commonly structural steel and attached to the primary structure of the building at intermediate floors. Because of the loads these shelf angles must carry they can be hefty and significant thermal bridges in brick and masonry facades.

Most shelf angles are directly attached to the intermediate floor which interrupts the exterior wall continuous insulation and forms a large linear thermal bridge. The solution to mitigating thermal bridging at this detail is to use shelf angles that are offset from the intermediate floor with discrete supports. This reduces the amount of steel penetrating through the insulation as well as direct contact with the primary structure of the building. An offset shelf angle can be 59% to 80% more thermally efficient than a direct anchor shelf angle for exterior insulation ranges between R-5 and R-25.

In cases where offset shelf angles are not feasible, adding low conductivity thermal shims between the shelf angle and intermediate floor may help reduce heat flow by thermally separating the metal angle from the primary structure.

Window to wall junction facade details are often overlooked in terms of thermal bridging. Windows are often installed directly on the interior steel-frame, wood-frame, or concrete back up wall for structural support of the window. Although this is effective structurally, a significant thermal bridge may be created when the exterior wall insulation and the thermal resistive parts of the window are not aligned. For mid-rise and high-rise buildings with punched windows, the window to wall interface length may be several thousand feet long, meaning any small reduction in thermal bridging can significantly improve the facade thermal performance.

To reduce thermal bridging at window to wall junction details, the window should be aligned with the mid-point of the wall’s insulation. For framed wall assemblies with insulation in the stud cavity and exterior continuous insulation, the optimal location is over the exterior insulation.
This installation position poses a problem of how to adequately support the window. One solution is to install an angle beneath the windowsill to support the cantilevered window. Another approach is to use intermittent cladding support clips to carry the window, such as the Akira Window Connection detail. Compared to the conventional window installation detail where the window is directly supported over the steel-frame wall, the angle supported windowsill reduces the heat flow by 64% and the intermittent clip supported window sill reduces heat flow by 95%.
Although both of these window support configurations may seem unconventional, both can carry significant loads when designed properly by a structural engineer. Depending on the structural capacity of the clips and its spacing, the intermittent clip supported window can support triple-glazed windows as tall as 6 feet.

As shown in the examples mentioned in this article, structural engineers have many opportunities to help improve the thermal performance of facade systems through mitigating thermal bridging. The next article of this three-part series will cover thermal bridging at roof and foundation details. ■

About the Authors

Jim D’Aloisio, P.E., LEED AP is a Principal with Klepper, Hahn & Hyatt of East Syracuse, NY, focusing on structural engineering and building envelope consulting services.

Ivan Lee, P.Eng., M.A.Sc., LEED APBD+C, WbLCA AP, is a Building Science Engineer and Team Lead in the Building Performance Analysis department with a focus on thermal analysis, hygrothermal analysis, and Life Cycle Assessment. Lee has worked at Morrison Hershfield now Stantec for over 14 years and has been involved with various projects including 3D thermal modelling to assess thermal bridging details for the Building Envelope Thermal Bridging Guide (BETB Guide).