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Ventilate stud spaces in earthquake retrofits?

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Hello all,

I'm faced with a quandary concerning ventilation of the stud spaces when installing plywood sheathing on the interior side of wood stud exterior walls of garages to improve their earthquake strength/safety. 

I'm not talking about the exterior walls of crawl spaces, where it is standard practice (around here) to provide holes in the plywood sheathing near the top and bottom of each space, to allow for air circulation within the stud space and thus prevent possible buildup of moisture which could damage the wood framing.  I understand that such moisture buildup may occur in sealed spaces due to temperature cycling, etc., and thus would occur only in exterior walls.  I see this situation as a smaller version of the need to ventilate crawl spaces to prevent moisture buildup for the same reason. 

I'm talking here about the exterior walls of garages integral with a residence, which must have drywall for fire protection and cannot have holes in the drywall and plywood sheathing.  These spaces are not conditioned and normally their exterior walls would not be insulated. 

My usual practice is to specify ventilation holes in the new plywood sheathing of all exterior walls, "unless the wall is insulated and has a vapor barrier" (on the interior side).  I have encountered little resistance to this because it usually involves only one wall, costs little, and improves the "climate" in the garage to some degree. 

But now I have a situation where we'd *really* like to know whether this is necessary, or even desirable.  I know little of the physics of this nonstructural condition, basing my recommendations for this aspect on "hearsay" over the years, and a couple of more formal recommendations.  I image it depends quite a bit on the local climate.  Here in the (San Francisco) Bay Area we have a very mild climate, but a very large earthquake threat, so retrofitting walls with plywood, etc., is very common, and highly recommended for practically all older homes. 

TIA for any advice/guidance anyone can provide.  I'm counting on y'all!  ;)

Ralph Hueston Kratz, S.E.
Richmond CA USA




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