Structural Steel and Insulation: An Effective Solution?from Dieter Hardock
Steel beams which penetrate the exterior wall (and break the continuous insulation layer) represent a detrimental thermal bridge in the building envelope. This situation often occurs in the structural details when a continuous steel canopy or balcony beam cantilevers out from the interior structure.
This penetration to the continuous insulation (CI) layer, is being further considered and addressed in energy building codes such as the ASHRAE 189.1 and 90.1 and the International Green Construction Code (IGCC), which guides codes and standards for both baseline and high-performance green buildings.
Since steel is a highly conductive material (k=50W/mK) / (R-0.003 per inch), a thermal break solution is necessary to reduce energy loss, prevent condensation on the surface, and avoid damaging results to the building.
Options to Minimize Thermal Bridging in Structural Steel
While there are options for structural steel thermal breaks available on the market, it takes time to evaluate the products to determine the most effective and safe solutions. This research time is not always available to the design team on project schedules.
Fortunately, Oxford Brookes University, Oxford Institute for Sustainable Development (OISD Technology) an accredited research institute in the United Kingdom, has completed this research, with a comparison of structural steel beam connection methods.
The Oxford Brookes Report 060814SCH, Thermal Performance of Steel Beam Junctions using Different Connection Methods, determined the heat loss (characterized by the χ-value), the surface temperatures, and the temperature factor (fRSi) to evaluate the condensation resistance of three situations:
- a steel beam passing straight through the insulation layer, with no insulation solution
- a Thermal Insulation Material (TIM) or PTFE “thermal pad” solution, and
- an Isokorb® manufactured thermal break connection for steel beams
A Closer Look at Thermal Break Solutions for Structural Steel
The two solutions studied in the Oxford Brookes Research Report are considered for structural thermal break solutions, which means the solutions have to both: 1) thermally separate the beam, but at the same time 2) ensure the structural integrity of the connection.
The first solution is Thermal Insulation Material (TIM) / “Thermal pad” which consists of PTFE. The plastic materials separates the compressive stress zone of the connection. This connection has a thermal conductivity of 0.25 W/mK and a thickness of 5mm and 10mm to separate the continuous beam into 2 beams. The connection is held together by 4 bolts, with the option of either steel or stainless steel bolts.
The second solution, Schöck Isokorb® Type S22 modules are designed to take tension forces, compression forces and shear forces. Various load conditions can be met by stacking Isokorb® modules upon each other, as can be seen in the video link. The modules include an insulation layer of expanded polystyrene at 80mm thick, with thermal conductivity of k=0.031W/mK. Stainless steel is used within the Isokorb® module for the structural elements (bolts and a hollow section) to transfer the loadings, while further reducing the thermal conductivity (since stainless steel k=15W/mK, compared to carbon steel 50W/mK.).
The Results of the Structural Steel Thermal Performance
Oxford Brookes used TRISCO software from Physibel to construct three dimensional models of the applications.
Results that Reduce Heat Flow
Schöck Isokorb® provides a solution to the structural steel that significantly reduces the heat flow (from 0.77W/K to 0.43W/K) and increases the surface temperature from 7.7°C to 15.2°C, therefore minimizing the risk of condensation.
A Solution with Damaging Results
The results of the “thermal pads” or TIM made of PTFE show a potential increase in the thermal bridging effect compared to no solution (continuous steel beam.) The heat loss increases from 0.77 W/K up to 1.4 W/K and the surface temperature shows only slight improvements, decreasing in some cases. Why are the results worse? The extra surface area of the beams end plates (compared to the continuous beam) counteracts the thermal resistance of the PTFE layer.
While the report refers to UK and German building code and energy requirements, the results none the less, are conclusive on page 6 of the report.
There is a significant improvement by implementing Schöck Isokorb® compared to traditional continuous steel beams, and PTFE solutions.
In addition to thermal performance, the structural performance of thermal break solutions in terms of tension forces, compression forces and shear forces must also be ensured. Documented structural calculations of the whole connection should be supplied from the structural thermal break manufacturer.
In North America, Schöck provides a solution for steel structures – Isokorb® Type S22 Schӧck Isokorb® provides an effective thermal bridging solution, minimizing structural steel’s impact on building envelope energy transfer.
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.