wood over concrete

We have back porch built on a 4 concrete slab. I would like to enclose it and make it a storage room. It has a 7 step down from the house floor. I was thinking of just covering it with plastic 6mil, putting 2x8 (and shims) across. Put some fiberglass (Upstate NY) insulation and nail sub-floor, done. Then I got wondering if I should vent it to my basement. What would be the recommended way of raising this floor and avoiding rot?
Thanks TP
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Is the purpose of the 2x8s to make the floor an equal height to the interior? I would be more concerned about the suitability of the slab to support the structure. Is there a suitable footer? If there is I would leave the step down. Put pressure treated sleepers on the slab, rigid insulation between the sleepers. I would not vent it to basement. I would make sure that the grade will take away all water.

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No wrote:

Yes the purpose is to equal height the floors. Also to fill the span of 12 feet. Footers are to code. Water is not a problem. I like the idea of rigid insulation. Expensive alternative to pink. So just seal it up tight...
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TP wrote:

Has anyone used this airkrete stuff? http://www.airkrete.com/index.html I remember seeing DIY gallons you get pour together and make this foam. maybe that would seal up this floor space. Any opinions?
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I just watched a program in Canada called Holmes on Homes and he repairs messes contractors have made. The one I just watched was a kitchen hardwood floor that was on it's 3rd incarnation. The problem? Plastic on concrete below the wood floor. The plastic and the cold below caused sweat to build up under the wood floor and ruined it. Just a heads up.
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