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

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

Good question, Norbert.

As usual in cases like this, the I think the confusion arises from over
simplification to get the basic point across.

You are perfectly correct, heat flows from hot to cold, and vapor moves from
high concentration to low concentration.  In a purely technical sense, we
can't talk about moisture moving from warm to cold by diffusion.  However,
keep in mind that concentration refers to absolute humidity (NOT relative
humidity), or the ratio of water vapor mass to dry air mass.  Warm air can
hold much more water than cold air, and the partial pressure of water vapor
in the warm air is greater than the partial pressure of water vapor in the
cold air.  The difference in vapor pressure tends to drive water vapor from
warm to cold in homes.

You're certainly familiar with this in Quebec in the winter when indoor RH
levels may only be 25% and outdoors levels may be 100%.  But because the
difference in moisture holding capacity between the warm air and the cold
air, the vapor pressures promote water vapor diffusion from the inside to
the outside.  The same idea applies to the relationship between the indoor
air and the subslab air, although the temperature difference is less,
especially with an uninsulated slab when the air film just above the surface
is very close in temperature to the air below the slab.  By increasing the
insulation under the slab, we raise the temperature of the slab and help
drive vapor to the colder subslab area.  (As an aside, this is exaggerated
with in-slab radiant heat as we increase the temperature difference.)

Anyone wanting a whole lot more discussion of this should consider
referencing the excellent "Moisture Control Handbook" by Lstiburek and
Carmody available from Oikos ( http://oikos.com/ ) and other places.

Hope this helps clarify.

Cheers,
Mike

----- Original Message -----
From: Norbert Senf <mheat@mha-net.org>
To: Greenbuilding <greenbuilding@crest.org>
Sent: Sunday, October 28, 2001 9:08 AM
Subject: Re: [GBlist] Re: Big Green: Radiant Floor Heating


> At 01:08 PM 2001-10-26 -0400, Mike Rogers wrote:
> >(snip).  The physics says moisture wants to flow from hot to cold, so
> >moisture in the slab will what to flow from the warm slab through the
> >semipermeable insulation to the cold ground.  Poly would prevent this
> >flow. (snip)
>
> and
>
> At 03:23 PM 2001-10-26 -0400, Frazer Dougherty wrote:
> >(snip)
> >  and since moisture vapor flows from warm to cold, isn't poly, possibly
> > as Lstiburek says, 'counterproductive' ??
>
> Heat flows from hot to cold. Vapor (gas) diffuses from regions of high
> concentration to regions of low concentration. Vapor can also be
> transported by gas flow.
>
> Why does moisture flow from warm to cold?
>
> Curious ........ Norbert



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