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How to pour concrete over existing concrete slab if existing wood framed wall structure is there? And where to place the new vapor barrier?(I have an existing garage that I would like to convert into an Accessory Dwelling Unit (ADU). The slab is on the ground, so I would like to pour over new concrete to provide the appropriate height difference between ground and top of slab.)

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    The first question in most climates is whether the existing slab is insulated (often not in a garage) and how that will impact the change in use as conditioned space. You generally want some insulation between the floor and the old, uninsulated slab. Pouring concrete directly on top is thus often a poor solution. – Ecnerwal May 27 '17 at 14:34
  • First of all, where are you? Climate makes a big difference. Second, I'm not sure why you're trying to raise the floor. There's nothing wrong with having the floor at ground level (as long as there's proper drainage outside). – Mike Baranczak May 27 '17 at 16:30
  • Hello, its in California. – user70152 May 29 '17 at 23:31
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To address the vapor barrier aspect:

With some materials you have to be careful about sandwiching them between two vapor barriers which can trap moisture - but concrete is not one of them. Adding a vapor barrier before new concrete is a good precaution if you're unsure if one exists already.

Closed-cell foam insulation can itself act as a vapor barrier (I recommend taping the seams and / or using spray foam to seal it all up).

So whether you use plastic sheeting (polyethylene I believe) or insulation, you can lay it on the existing concrete and then pour over that.

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