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Subject: Storage space with concrete floors

Storage space with concrete floors

From: George W. Adams <cunningham-adams>
Date: Thursday, June 27, 1996
Mark Ast <imagarts [at] warwick__net> writes:

>A colleague just informed me that the concrete floors will tend to
>absorb large amounts of moisture, and that nothing sensitive should
>be placed/stored *directly* on the floor. This remark raised a
>concern that the moisture might even affect the items stored up on
>the shelving units--especially books and works on paper.

Several mechanisms exist by which concrete floors can be a major
moisture source.

If the concrete is not thermally insulated from the ground, and if
the ground is cold, moisture in the building interior air can
condense on the floor. This is a common source of moisture above a
well-constructed slab. Several palliatives are available.  By
dehumidification of the interior air, the dew point of that air can
be kept below the concrete temperature.  (In fact, you also want to
avoid any regions that will have unacceptably high humidities, even
if condensation does not occur.) Alternatively, the interior surface
of the slab can be thermally insulated, with a vapor barrier above
the insulation.

Depending on the nature of the concrete, water in the earth around
the slab can wick inward through the concrete.  More likely, if
ground water exists at the slab, it will percolate through cracks.
If the concrete is in good condition, both of these mechanisms can
be controlled by treating the interior of the slab with a
waterproofing paint, which properly applied is effective.

If the structure is recent, drawings should be available that will
show such construction details as insulation, concrete type, and
provisions for drainage beneath the slab.  The condensation problem
can also be checked by simply measuring the slab temperature.

In any case, if, as Ast implies, the concrete is directly in contact
with the ground, it would be wise to ensure that adequate
ventilation exists between the floor and any materials that are
sensitive to high humidities so that the materials themselves do not
become chilled; and to ensure that no wicking paths (such as wooden
shelving or even dirty metal shelving) exist between the materials
and the slab.

George W. Adams, PE

                  Conservation DistList Instance 10:4
                   Distributed: Friday, June 28, 1996
                        Message Id: cdl-10-4-006
Received on Thursday, 27 June, 1996

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