# Basement Open Truss Soffit,Vapor Barrier



## Gary in WA (Mar 11, 2009)

Where are you located?

Gary


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## spring3100 (May 6, 2011)

Zone 5 Gary,sorry about that


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## Gary in WA (Mar 11, 2009)

Thanks. Just seeing if my answer is pertinent or not. The rim board/OSB/plywood will be the temperature of the outside climate. On the inside it will be R-1.25 (per inch of solid wood) degrees less than exterior temp. Any vapor barrier used requires *complete* air-tightness or it will fail, which is why I always suggest foam/with canned foam or caulking; http://www.buildingscience.com/documents/information-sheets/crawlspace-insulation/ 

How much, Zone 5? http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements

It requires great attention to keeping the air barrier intact; http://www.carb-swa.com/articles/in%20the%20news/HomeEnergy_The%20Challenges%20of%20Basement%20Insulation.pdf

Be sure to air seal the framing/drywall ceiling and all plumbing/wiring/heating ducts/etc. holes that can admit air to the floor system cavities. http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

I really don't see how a vapor barrier will work with an open truss floor as yours. IMHO, just add foamboard behind the Roxul (against rim). Pages 14-19 (even used Chicago- Zone 5); http://www.buildingscience.com/documents/guides-and-manuals/gm-guide-insulating-sheathing

Gary


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