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Back to basics

Posted by JT on June 12th, 1998 09:59 PM
In reply to Hmmm ... by Jay J on June 12th, 1998 09:39 AM [Go to top of thread]

2 of 2 people found this post helpful

I agree with Jay for the most part, but I get the
feeling that a little clarification on how moist
air behaves might help.

For those who may not know, reletive humidity (RH)
is the ratio of how much water air is holding to
how much it can hold at a given temperature (and
pressure, but we can ignore that). As air temperature
increases, it's ability to hold water increases and
vice versa. As the air contacts the concrete, it's
temperature falls and it's RH rises until it reaches
100% and you get condensation. From this it should be
apparent that there are only two things that can be done:
lower the RH of the air, or raise the temperature
of the concrete; neither of which is practical without
serious effort. Enclosing the porch as Jay suggests
and dehumidfying is a possibility, assuming you want
a room rather than a porch. If the slab had had insulation
placed under it, there probably wouldn't be a problem
(real helpful huh?). Since raising the slab temp. isn't
practical, covering it with some sort of air tight
insualting material and new flooring is about the only
thing left. Good luck with it. I would be tempted
to investigate the insulating properties of something
like Nobleseal (on which you can set tile). I doubt
that it's very high, but it might be just enough.

FWIW, the condensation process is highly predictable.
If you mesaure the slab temp. you can predict at what
RH condensation will occur (and vice versa) with the
aid of a psychrometric chart. A chart and a small
manual on how to use it is available for a few dollars
at if anyone is intersted.

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