Unconditioned portions should be treated similarly to conditioned portions, creating semi conditioned space, with controlled air exchanges.

https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall

https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

https://www.buildingscience.com/documents/information-sheets/crawlspace-insulation

https://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation

Is there no vapor retarder under the slab?

No. It's a 50 year old house. No vapor retarder under slab.

I am no expert but wouldn't some evidence of moisture shown up by now? And wouldn't this be a regional question. Like AZ is not Alabama?

I personally would think the small space argument is a pretty good one.

I am a bit confused why you would vent this area over the slab. In my mind, you are building a crawlspace with a slab floor. Best practice is to seal and condition that. Some rigid foam on the slab and insulate the walls. But - again, regional....

It can be difficult to prevent moisture from rising through a slab. How well drained is the site at the garage and what would you use to "seal" the slab?

We're in the San Francisco Bay Area. It can rain alot in winter but it hasn't rained since last spring so we wouldn't expect to see anything now. The garage has been studs and all fully vented and existing walls on stem wall - really no chance for water to cause trouble even if it comes up through slab. Under the room with the raised floor, the slab was sealed and is vented. I'm just worried about where they are closing holds in the wall due to relocated doors - that's where it will be in a wall, so not vented, the slab's not sealed, theyre just laying pressure treated 2x4 on slab with studs on it, and flashing outside.

Use a sealer that is designed for concrete moisture protection. Most epoxy sealers will do the job.

Normally the sill plate of an exterior stud wall would sit on a sill gasket (thin foam sheet, etc.) but this is an unconditioned garage and the climate is mild so the gasket is probably not necessary. Moisture from the slab shouldn't affect the pressure treated plate. All the other sill plates are probably the same. Be sure to remove the original driveway apron where no longer needed and maintain the required minimum space between untreaded wood and the ground.

Hi Res2,

Thank you. I was just worried because we're closing the wall.

Once we remove driveway apron and walkway to the swing door, we'll have flowerbeds. How low does dirt have to be below where sill plate it? He said he'd flash on outside.

Monica

Thank you