On 2008-02-29, JoeSpareBedroom wrote:

In a heating climate . . . the vapor barrier goes on the warm side
of the surface.

Not on the soil underneath, assuming there was a way to seal the
edges where the barrier met the cinder blocks?

Well, a few comments:

A vapor barrier in the floor system is to keep warm moist air from
migrating into the crawl space, condensing there, and causing the
usual problems of unwanted water. This should definitely be on the
warm side of the insulation; otherwise water vapor will travel through
the insulation, cooling as it does, and condense in the insulation,
where it would be trapped by the vapor barrier underneath.

A vapor barrier on the soil is to keep moisture in the earth from
rising into the crawl space. So it serves a different purpose. If
you have a vapor barrier in the floor system, I'm not clear on whether
having one on the soil would cause any problems. I'm just wondering
how any vapor in the crawl space would actually escape? I guess if
the crawl space walls have no vapor barrier, it can escape through
that.

Lastly, I believe that a vapor barrier, unlike an air barrier, doesn't
need to be perfect to be effective. I believe that have a 95% vapor
barrier will give you 95% of the benefit. With an air barrier, any
wind would drive the air infiltration through the remaining 5%, giving
you less than 95% of the benefit. Hopefully someone else can verify
that my recollection in this regard is correct. :-)

Cheers, Wayne