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RE: Vapor Barrier

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I have been following the discussion of the vapor barrier and the sand
or gravel layer under a slab.  Can anyone help me with a question?  " Is
this a detail you would specify for a commercial slab on grade design, a
residential slab design, or any ground slab design depending on the
loading conditions?"

Thanks for your opinions.

Don Carr

Dear Don:

Vapor barriers have nothing to do with loading conditions.  Their
purpose is to prevent moisture from seeping upward into and through the
floor slab.  Vapor barriers are not necessary beneath certain floors,
depending on floor finishes and building occupancy.  For example, there
is little justification for using vapor barriers beneath plain concrete
slabs in warehouses without HVAC.  The purpose of a sand cushion above
the vapor barrier is to allow the slab to cure more uniformly, with less
curling.  Also, the sand cushion provides a bit of rupture protection
for the vapor barrier during construction.

I recommend the sand cushion above the vapor barrier for all floors in
commercial buildings with vapor barriers.  However, I have yet to find a
contractor that doesn't complain about this.

In residential slabs, I recommend putting the vapor barrier on top,
immediately beneath the concrete.  This will help the builder remember
that the rebars are supposed to reinforce the slab, and not the subgrade
below it !;^>

Hope this helps,

Stan R. Caldwell, P.E.
Dallas, Texas

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The trouble with doing something
right the first time is that nobody
appreciates how difficult it was.
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