How to Make a Thermal Break in an Overhang

All materials can transfer heat. Some transfer heat very efficiently, like steel, and some transfer heat inefficiently, such as wood. Conceptually, the challenge of creating a thermal break -- for example, to keep cantilevered I-beams for a balcony from absorbing heat and radiating through to the interior of the house -- you have to maintain the structural properties required, while mitigating the heat transference. One way to do this is to rely on tension strength instead of shear strength.

Plasma or oxy-torch
MIG welder

Instructions

- 1
  Measure and mark the point along your I-beam from the end of the cantilever to the point it will intersect the exterior of the wall. Then, make another mark beyond this mark, the thickness of your wall. Essentially, you are marking the cross section where your cantilever will pass through the wall.
- 2
  Offset both of your cross-section marks by 2 inches, so the cross-section is nearer the end of the cantilevered edge. These are the marks you will cut.
- 3
  Note that cantilevered overhangs are always shorter than the length of of the anchored section of the beam, so the cantilevered section does not require all the shear strength required of the interior component of the beam that will serve other structural purposes.
- 4
  Cut the section of I-beam you have marked with a plasma cutter or oxyacetylene torch as if you were slicing a piece of bread from the center of a loaf.
- 5
  Smooth the cut edges with an angle grinder.
- 6
  Rotate the section of I-beam 90-degrees in either direction so the vertical member of the I-beam is situated perpendicular to the vertical section of the rest of the I-beam.
- 7
  Cut a horseshoe or U-shape in the top cord of the I-beam so it removes all the metal on the top chord except a 2-inch perimeter around the edge and above the vertical I-beam member. From above, this section should look like an "H," oriented so the openings are longitudinally aligned with the rest of the beam. Do not remove metal from the bottom cord.
- 8
  Weld the piece back in place, in line with the I-beam. You will have a break in the otherwise contiguous, vertical center cord of the I-beam to interrupt heat transfer. The top chord will have significantly less heat transfer as you will have created to tension bars, instead of a solid bar. The bottom piece will be fully intact to handle the moment of force from the hinging action of the cantilevered load. The offset will allow some air flow through the member, helping dissipate heat from the I-beam, rather than radiating into the internal component of the I-beam.

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