Optimal placement of vapor barrier under slab?
We’re building a passive-solar home (SIPs on a monolithic slab for thermal mass). I’m wondering about the placement of the vapor barrier underneath the slab. Should the order be (from bottom up): gravel fill layer, vapor barrier, XPS, then concrete? Or can it go: vapor barrier, gravel fill, XPS, then concrete? Are there any moisture issues associated with the latter method?
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