Many cantilevered joist details that I see show continuous screened soffit vent. Is this really necessary? The only thing that I can think of is that a floor is typically not given a vapor retarder (not a problem over a vented crawlspace). I can find nothing in IRC specific to this situation.
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Venting soffits under a cantilever may or may not be done because the cantilevered portion of the overhang needs to be fireblocked and sealed from the crawl or basement area under the IRC.
This means that soffits under a cantilever do not vent the basement or crawlspace at all because. 2x10 fireblocks must be placed over the basement wall between joists at the point where the cantilvered joists exit the basement.
And since the IRC requires that insulation equal in thickness to that used for your attic be used under any exposed cantilever, there is no requirement to have open soffits.
So don't use them.
This means that soffits under a cantilever do not vent the basement or crawlspace at all because. 2x10 fireblocks must be placed over the basement wall between joists at the point where the cantilvered joists exit the basement.
And since the IRC requires that insulation equal in thickness to that used for your attic be used under any exposed cantilever, there is no requirement to have open soffits.
So don't use them.
