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Slab-On-Grade Vapor Barrier (One More Time)

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Okay, I know this horse has been beaten into dogfood, but I'm slow (and don't
face this that often).

Please remind me.

"Old" practice was to put e.g. 6-mil "Visquine" or equal directly under the
slab. Later it was discovered this was NOT a good idea, as the hydrostatic
pressure in the slab through-thickness weakened the cement (and caused the slab
to cure more slowly at the bottom). This led to cracking and a not-so-great
slab.

LATER practice (this is where I'm fuzzy) was to put SAND directly under the
slab, THEN the Visquine. That way any water would bleed out of the slab into the
sand, yet "new" water vapor wouldn't come up from the soil below.

Have I got that right, or has the wheel turned yet again?

Thanks in advance for your responses.

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