Controlling Indoor
Moisture During Winter
BY W.S. BLAZOWSKI,
M.A. GOETZ, AND E.R. GOETZ
W
ater introduced into indoor air by two people in a typical
condominium unit can approach two gallons per day.
During Florida’s winter months, when the unit is usually
unventilated and the air conditioner is off, indoor moisture
can accumulate and result in significant indoor humidity
levels. Use of a dehumidifier removes this moisture and helps hold indoor
humidity at a comfortable level.
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BACKGROUND
Summertime outdoor humidity levels in Florida are high, but condensation during air conditioner operation provides substantial water
removal and effective humidity control for indoor air. Owners who are
absent for the summer months need to be sure that their air conditioners
operate frequently enough to hold down humidity and that someone periodically checks to be sure the system is functioning during their absence.
This article focuses on indoor humidity during the winter months.
When the winter outdoor temperature is in the 40–60 degree Fahrenheit range, there is usually no ventilation through open doors, open
windows, or air conditioner operation (ACs operating in the heat pump
mode do not remove indoor moisture). Resident activity continuously
adds water to indoor air, producing high humidity levels. Evidence
of high indoor humidity includes condensation on exterior walls—
especially windows and sliding glass doors—damp shower/bath
areas, and an uncomfortable, stuffy environment. These conditions
can also encourage mold growth on walls and bath areas.
MOISTURE SOURCES
Human activity contributes to high indoor humidity. Breathing and
perspiration are major contributors to indoor humidity. The moisture
content in exhaled air is higher than in the air we inhale. Estimates are
that the typical person loses one quart of water per day by respiration.
Through perspiration, water is continually lost through the skin, typically
about 0.5 quarts per day. Under more strenuous activity (for example,
exercising on a treadmill), an individual’s rate of water loss can be much
higher. Other contributors are showering, cooking, dishwashing, clothes
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washing, and evaporation
from sink and bath area traps.
The graph on page 63
shows the amount of water
(in ounces per day) from each
of these contributors for a
typical living area occupied
by two people. This data is
based on some simple experiments and estimates and
would vary with level of activity (are the occupants home all
day or not) and the occupant’s
behavior (do they take long
or short showers, etc.), as
well as the size and construction specifics of the building.
Breathing and perspiration
account for more than half
the water introduced into
the living space. Showering,
cooking, and dish washing
are also significant, and in
some cases controllable. For
example, if the shower walls
are squeegeed after a shower,
the contribution from the
shower can be reduced. The
use of lids during heating
food can also reduce the
contribution from cooking.
These reductions are shown
as green portions in the graph.
Overall, opportunities to
reduce indoor moisture are
relatively minor compared to
contributions from all sources.
Even when residents do all they
can to control moisture, their
efforts reduce moisture introduction by only 20 percent.
WHAT TO DO
Equilibrium between
water in the air and the contents of the residence results
in substantial water being
“stored” in rugs, beds, furniture, and anything else that
absorbs moisture.
Ventilation and the use of
a dehumidifier are two ways
to remove moisture. The
amount of water in outdoor
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air during winter months is
usually low enough that ventilation can reduce indoor humidity levels. Briefly ventilating
the living space will result in a
short-term reduction in humidity, followed by an increase as
some absorbed water is released
from the contents of the residence and equilibrium is
restored. Ventilation would
have to be longer term or
repetitive to make a significant
reduction in indoor humidity.
But residents are unlikely
to ventilate their unit when
outdoor temperatures are
lower than the desired indoor
temperature. Further, deciding when (outdoor conditions
that can have the most impact)
and how (which doors/windows to open and for how
long) to ventilate is complex.
The good news is that a
dehumidifier can continuously
extract substantial quantities of water from the indoor air. Dehumidifiers are rated at pints of water removal per day—most have moisture
removal ratings above 50 pints per day (more than four gallons). The
ratings are determined at optimal conditions for dehumidification
(high humidity and temperature). It is, therefore, likely that removal
rates will be lower than this. Experience indicates that two or three
gallons per day is a more realistic expectation. Because this amount
is greater than the two gallons of water typically introduced by two
people, the dehumidifier is an effective way to reduce indoor humidity.
The dehumidifier should be positioned where air is circulating
through the living space, not in a remote corner. And it is helpful to
keep the air handler fan in the “on” position so air continually flows
past the dehumidifier. The dehumidifier’s energy use is not lost during
wintertime use. Heat from the dehumidifier enters the residence and
reduces the energy that would otherwise be required for residence
heating. Disadvantages are that water must be emptied from the
dehumidifier at least once per day, and some residents may find
noise from the dehumidifier to be unpleasant.
Most information on dehumidifiers concerns their use during the
summer, especially in areas like basements where cold walls can result
in condensation. But dehumidifiers clearly have a role to play in controlling indoor moisture during winters in Florida. It is important that
residents be aware of this option.
Bill Blazowski is an Engineer and current Board Member and Chair of
the Facilities Committee at Bacopa Bay in Saint Petersburg. Margaret and
Elizabeth Goetz are students in Tennessee and participated in the analysis. ■
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