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Community and Q&A

How much insulation for a steel beam?

Jeremy Turner | Posted in General Questions on

How much insulation do I need over a steel beam to stay above the dew point inside the house?

This is new construction in progress, in NC. The I beam is over a door and window assembly and contained within the wall, sandwiched between the sheathing and drywall. However, no consideration was given to insulating it in the design.

I feel the best approach is to insulate it on the outside only and keep it within the thermal envelope. For most of its length I can add as much insulation as I want but a short five foot leaves little room. In this area I want to start with the minimum to stay above the dew point then see if we have the space to do better.

How do I figure this minimum? Thanks in advance for your suggestions.

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  1. Stephen Sheehy | | #1

    It's hard to insulate an I beam. This might be a good spot for one of those spray foam kits. Even though in your house, I suspect the concern is avoiding thermal bridging, an added benefit of totally insulating a steel beam is to keep it from turning to limp spaghetti in a fire.

  2. D Dorsett | | #2

    As long as more than ~20% of the total R is on the exterior side of the steel you'd be fine from a dew point control point of view, even in the cooler zone 4A parts of NC. But more is always better.

    If it's a 2x6 wall, and inch of rigid foam (any type) on the exterior of the sheathing would be enough with R20 fiber insulation.

    For a 2x4 / R13 wall a half-inch of rigid polyisocyanurate or 3/4" of EPS would get you there.

    The flanges of the I-beam are pretty big thermal bridges, but it's still worth filling the channels on both sides of the beam with fiber insulation, along with installing the exterior continuous rigid foam.

  3. Jeremy Turner | | #3

    Thank you Dana, we can achieve what you describe and do better in some areas.

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