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Hello.

I have a 2.5 car garage where I am converting the back single bay into a room (11X17). I have a plan, proper egress etc, and am comfortable there. However I have one outstanding question.

I am installing 2X6 joists on end over the above grade slab for subfloor support and want to build in a means of moisture ventilation for the subfloor cavity. The slab is dry and 2003 construction, built by a contractor who built the house for his own use (good quality).

Nevertheless the nature of concrete is to transport moisture.

The front runner option to provide ventilation:

Since I am framing a wall to close off the room from the rest of the garage (already has three walls), I am thinking that I can just vent the subfloor into the rest of the garage. Probably just extend the sheet rock from the exterior of the wall down to an inch from the floor to allow airflow.

Thoughts?
 

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I'd need a drawing to advise on ventilation but I can add a couple of comments.
1. there are two sources of moisture. One comes from the ground through the concrete as water or moisture vapor and the other comes from humid reaching the cooler concrete. Ideally the concrete slab would have a good layer of rigid foam under that but a garage floor probably would not.
2. As far as I know you will need a fire rated barrier between the garage and any living space and that would eliminate any ventilation provision as you suggested.

If you have an approved set of plans and a permit I'm sure that would be spelled out and inspected.

Bud
 

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I don't think the AHJ will like you venting the subfloor into or from the garage space due to fire risk.
Imagine spilling a bunch of gasoline and a fire starts. It would race right into the subfloor space.

Nope. Just can't see that being allowed.
 

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I don't really see the point in venting it. Is it wet? It shouldn't be. Prevent water infiltration and prevent condensation and there won't be a need to vent it.
Have you thought about putting in some rigid foam insulation between those 2x6 sleepers? Not sure of your climate or the use for the room, but it sure is nice to not have a cold floor, and the insulation will help to prevent moisture from condensation.
 
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