Re: [GBlist] Re: Big Green: Radiant Floor Heating

["Mike Rogers" <rogers.mike@verizon.net>]

A few clarifications:

Ralph Bicknese wrote:
"The poly under the slab is to keep moisture from the damp ground from
migrating into the concrete slab."

The primary benefit here is to slow the water vapor transport through the
slab, or to slow wicking if a capillary break, e.g. gravel with no fines, is
not used.  Use a capillary break!  An additional benefit of the poly, is
that is retards other soil gases, including radon.  (If your in a high risk
radon area, or even if you're not, consider a passive sil-gas reduction
system in new construction.)
Foil underneath the slab will not improve the thermal performance of the
slab.  To function as an effective radiant barrier an airspace is needed
between the foil and an adjacent surface.  Foil is a very effective vapor
barrier, much more effective than poly, although for almost all cases poly
is sufficient for construction purposes.  That is the reason foil is so
effective as a wrap to keep food fresh, much more effective than plastic
"Saran Wrap".

"There is absolutely no need to punch holes in the poly below a slab.  This
is counterproductive to the benefit of the poly (which to keep moisture from
the damp ground from migrating into the concrete slab)."

I agree there is no reason to  punch holes in the poly if the concrete with
mixed with the proper amount of water (i.e., a low-slump mix).  However, if
a proper capillary break is used, again something like 3/4" gravel with no
fines, the primary function of the poly will be to retard to flow of water
vapor, not bulk water.  (A secondary function of the poly, or a rigid
insulation is to prevent the concrete from filling the voids in the
capillary break material, thus defeating it.)   In this case, holes should
have very little effect.  Likewise the poly does not have to be taped, a
mere overlap will do.  This does NOT prevent bulk groundwater intrusion.  In
fact if you have bulk water intrusion, the poly may very well retard the
drying of the slab.  Moral:  Prevent groundwater intrusion!

Cheers,
Mike


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