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Q:  What
can I do about moisture problems in a new concrete building?
We’ve moved into a concrete building in late spring
and have been experiencing air quality problems. We’ve got mold
and mildew growing on some of the furniture and in some of the closets.
Air quality people tell us moisture from the concrete is the source
of our problem. We’ve had the air conditioning units running all
summer but the problem hasn’t improved and we’re still experiencing
mold and mildew problems. Should we turn down the thermostat to
further reduce interior temperature? What other steps can be taken
to solve the problem?
A: Martha VanGeem,
Principal Engineer at Construction Technology Laboratories, Inc.
says moisture problems can occur in tight concrete buildings without
adequate ventilation that haven't gone through one or two winter
heating seasons to allow water to evaporate from the concrete. Unfortunately,
air conditioning isn't a very efficient way to remove the moisture
because cool air won't hold as much moisture as warmer air. Thus
turning down the thermostat to cool the building even more isn't
likely to help. It can even make the problem worse by increasing
condensation on the cooler surfaces. The air conditioner may also
be oversized which prevents it from cycling on frequently enough
to remove the moisture.
VanGeem says two steps will help to solve the problem.
Use dehumidifiers with air conditioning set at moderate levels (75°F
or so) during the first two summers of operation to drop the indoor
relative humidity below 80 percent. Dehumidifiers are a more efficient
way of removing moisture than reducing air temperature through air
conditioning.
Secondly, ventilate the building as much as possible
when heating and air conditioning aren't being used. The tighter
the building, the harder it is to get rid of excess moisture. Install
an air exchanger to provide fresh air when the heating and cooling
systems are not operating. Also, use bath and kitchen exhaust fans
to remove moisture produced by occupants. Many builders now install
ventilation ducts in walk in closets to increase ventilation and
decrease moisture problems. If not, keep the closets open while
you are experiencing problems.
Another solution is to turn on the heating system
after the building has been completed for two weeks or so, regardless
of the season. During this period ventilation should be also be
kept high. In a house with forced air, the fan should be on continuously.
While it's true that external heat drives some moisture further
into the concrete (toward the cold side), the heat also reduces
relative humidity in the building and allows water near the interior
concrete surfaces to evaporate. The obvious downside to this approach
is an uncomfortably high temperature in the building while finish
work is being done, or delayed use of the building if heating is
turned on after completion but before occupancy.
Condensation problems are often made worse when
vinyl or other impermeable wall coverings are placed over the concrete
at an early age. Evidence of moisture problems will be visible under
the wall covering. Always discourage the use of impermeable wall
coverings in hot and humid climates (such as on the Gulf coast).
To help prevent this problem in moderate climates, delay applying
the wall covering until the building has gone through two winters
with the heating on and two summers with indoor relative humidity
less than 80 percent.
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