How are you measuring the moisture?
Is the concrete slab sealed?
What type of floor are you planning on installing?

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This post was edited by millworkman on Mon, Nov 25, 13 at 8:52

We want to install 3/4 inch solid maple or oak hardwood flooring using heavy tar paper as a vapor barrier. The concrete slab was not sealed. I taped a 18 X 24 inch piece of thick plastic to the sub floor a few days ago and so far, no moisture vapor has shown up on the underside the plastic. There is an air space between the concrete and the sub floor equivalent to the height of the 2 X 4 "joists". I am thinking maybe this should be vented for air circulation.

You are probably okay to go. Venting is a good idea.