Exterior Steel Beams that Break Thermal Bridgingfrom Adam Kimble
Steel is the most popular framing material for non-residential buildings in the US. As the AISC’s slogan goes, “There is always a solution in steel.” It is sustainable and readily available, strong in both compression and tension, and allows acceleration of project schedules making it a cost effective construction option.
This being said, carbon steel comes with the challenge of thermal performance. Canopies and sunshades can provide benefits of coverage from outside elements, but when cantilevered beams enter through the insulation layer, three typical problems caused by steel thermal bridging are likely to occur:
- a significantly reduced effective thermal R-value of the wall assembly,
- condensation at interior cold surfaces, and
- the increased risk of mold growth.
This results not only in increased energy costs but also in premature deterioration of enclosure components and therefore additional costs for maintenance.
These steel penetrations are common in locations such as canopies and sunshades, beams connecting to exterior frames and columns, and mechanical penetrations on the roof.
This image is from an abstract by Wagdy Anis, FAIA, LEED AP, Principal, Wiss, Janney, Elstner Associates, Inc., Six Ways for Condensation in Buildings, and is an example of this common problem. These beams on the interior of the building are dripping wet.
Insulation Qualities of Carbon Steel
We identify how well energy is transferred through steel by simply touching a hot skillet (warning: please do not try this, intended only for visual reference). This type of thermal transmission is ideal for cooking but not as part of the insulation layer when designing an energy efficient building.
For the purpose of energy efficiency, carbon steel is a very poor insulation material (R-value 0.0031 per inch) compared to other common structural materials such as concrete (R-value 0.1 per inch) and wood (R-value 2.5 per inch). When not properly insulated, the amount of heat loss through carbon steel impacts the R-value significantly within the local surface area. The heat is lost through the thermal bridge, and at the same time, the interior surface temperature dramatically drops (see below), creating condensation and the ideal condition for mold growth.
Properly Insulated Steel Beams with Isokorb® Type S22 Thermal Break Elements
To properly insulate a steel beam means to cut or minimize the heat flow through the beam at the insulation layer. The Schöck Isokorb® Type S22 thermally separates the beam an simultaneously provides a structural connection. Isokorb® Type S22 connections are used in a modular configuration, adding multiples of Schöck Isokorb® modules based upon the loading criteria to discontinue the beam into two sections.
The high conductive carbon steel of the beam is replaced with an insulating material (EPS) with a thickness of 80mm (3 ½”)-to give an effective thermal separation in the slab by changing the material characteristics and incorporating an insulation layer. This is non-structural and constitutes the main body and surface area of the thermal break module.
The structural material characteristics of the Schӧck Isokorb® thermal break element consists of cold formed 316-stainless steel to both conserve the structural integrity and to minimize the thermal conductivity. Stainless steel increases the thermal resistance up to 70% compared to carbon steel. Adding graphite enhanced eps foam manufactured by BASF raises the thermal resistance up to 85%.
The Schöck Isokorb® Type S22 thermal break element is a full engineered solution, providing all structural elements for a typical bolt connection. Compliance with the according structural codes in the US has been independently assessed.
Alternative Solutions to Structural Thermal Bridging
As building performance standards are raised and more recognition is gained for building envelope details with steel beam penetrations, designers are seeking for further solutions to structural thermal bridging throughout the building.
There are other materials intended for shock control which have been repurposed as thermal breaks, yet unfortunately they cannot fulfill the structural nor adequate thermal performance. Those materials are known as Thermal Insulation Material (TIM) or PTFE pads. They were recently highlighted in a post which covered the viability of thermal breaks for steel to steel connections with a study of Oxford Brookes University in the United Kingdom to validate the claims of thermal resistance at these connections. The study provided undeniable evidence that Schöck Isokorb® type S22 connections achieve significant thermal resistance and prevent the conditions of condensation and mold. In the study, TIM or PTFE failed to provide adequate thermal resistance for the connection and could still create condensation at the insulation layer. In addition, TIM and PTFE products contain only the pad, without structural connection bolts (which would need to be provided by the steel contractor.) To the structural engineer this causes concern, as there is no standard material data for bolts at such a structurally critical connections.
Material data information, structural proposals with signed and sealed drawings by a profession engineer, as well as building physics information for connections should all be expected when specifying structural thermal breaks. A recent project which solved the thermal bridging issues at the canopy with Schöck Isokorb® thermal breaks is the University of Massachusetts at Amherst.
Visit the Schöck North America website for more information about structural thermal breaks for concrete and steel connections. Explore the full range of Isokorb® products for balcony, canopy, steel beam, exposed slab edge, parapet and rooftop connections.
Considering structural thermal breaks for an upcoming project? Have a Schöck Engineer call you to answer your specific design questions.