Brandon M. is building a three-story house in Seattle whose design includes cantilevers on the second and third floors. The designer has specified steel I-beams to provide the structural support in this modernist design, and this is what’s giving Brandon pause for thought.
Although Seattle’s climate isn’t exactly arctic, it gets cold enough. The beams will not be exposed to the elements, but are located close to the outside of the building, and Brandon fears the steel might wick in cold temperatures. That could lead to condensation inside the building that eventually may rot the wood framing joined to the steel.
As originally planned, the bottom flanges of the I-beams would get a scant 1/2 in. (R-2.5) of insulation. Would that be enough to head off the problems he envisions? Brandon asks in this Q&A post. Further, are there products designed for this application? And, does anyone have suggestions about the details?
Brandon’s building dilemma is the topic of this week’s Q&A Spotlight.
‘Back to the drawing board’
The design makes GBA senior editor Martin Holladay cringe.
“I would talk to your architect about changing the design,” he says. “You don’t want steel beam cantilevers penetrating your thermal envelope. Back to the drawing board.”
Holladay and others react strongly to Brandon’s original description of the project because it suggests the beams would actually penetrate the building envelope.
This kind of design, writes Jesse Thompson, are “classic leak points on decks and overhangs in a marine climate, and very tricky to seal where they enter the building. They can wick long distances if they get a chance.”
Brandon clarifies: “The steel beams are fully encased by…
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