Q: What is the best way to reduce thermal bridging from a cantilevered steel beam protruding from a building for a balcony or canopy? We aim for high-performance enclosures and have been applying closed-cell spray foam along the length of the beam inside the structure, but we are not sure this completely mitigates the energy loss from this massive member conducting heat through the enclosure.
A: Marc Forget, associate editor of JLC responds: Any piece of steel that’s projecting through a building’s shell is a thermal bridge that degrades the overall value of the enclosure’s insulation. “It becomes even worse,” Kohta Ueno of Building Science Corporation explains, “as you add thicker and thicker insulation levels. You have a tighter bucket, but you have slit the same big hole in it.”
According to Ueno, the ideal way to address the thermal bridge is with off-the-shelf structural thermal break products made by Schöck, Armatherm, and others. For a steel-to-steel connection, these are typically made with a high-density plastic pad that interrupts the steel beam at a bolt-through connection. This requires you to create your cantilever with two segments of steel—an interior section and an exterior section. Is this weaker than the typical unbroken, through-steel cantilever? “Absolutely,” Ueno responded. “The beam must be designed by your structural engineer to account for that.”
If the structure is already built, and a steel cantilever protrudes from the building, a 2-inch-thick coating of closed-cell spray will reduce the transfer, Ueno said. He compared it to wearing a mitten in winter; it’s not ideal, but your hand will be warmer for wearing the mitten. “However, this solution, or coating the steel with an aerogel-incorporated paint, is only going to be as good as it is maintained or protected,” Ueno cautions. “If decking is applied without care, or if [the insulation is] left to the elements, its performance will degrade over time.”
I also consulted engineer Peter Baker, president of Building Science Corporation, to explore how different structural materials might perform. “Different materials are worse at [energy] transfer than others,” he said. “Concrete is worse, steel next, and wood being the least.” A concrete beam or concrete slab projecting from the building typically conducts more energy, he explained, because it needs to be bigger. “The bigger the protrusion, the more energy transferred through the building enclosure. A concrete slab coming out for a balcony or canopy generally has the most mass.”
To minimize the energy transfer, Baker urged that we need to first think of minimizing the cross section of material that is being used. “Simply make the protrusion material as small as possible to reduce the potential transfer,” he said.
Once the materials have been defined, we can add thermal breaks. Thermal break products for steel-to-steel, concrete-to-steel, and concrete-to-concrete connections are available from several sources, as noted above.
“Insulating the material both inside and outside of the penetration is a good strategy, too,” Baker continued. “But this can be a challenge depending on both the material used and what will be applied to the beam outside of the building.” For example, steel beams supporting a cantilevered deck can be insulated, but the connection between the deck and the cantilevered beams will create its own thermal bridge into the beam and through the enclosure. In this case, the steel beam can be fully coated before the deck is attached, and standoffs incorporated into the structure to allow for attaching the deck, minimizing the thermal bridging.
