Environmental Engineering Reference
In-Depth Information
cases, water may seep into supply air ducts in building slabs and then be
vaporized and distributed into building spaces by warm/hot forced air.
Water vapor is often drawn into building interiors through leaky return air
ducts in houses constructed on crawlspaces.
1. Wet building sites
Wet building sites affect both the absolute humidity (actual amount of water
vapor a volume of air contains, i.e., mass per unit volume, or vapor pressure)
and relative humidity (percent water in air compared to the maximum it can
hold at a given temperature) in buildings constructed atop them. In many
cases the relative humidity may be high though the absolute humidity is
moderate. At a given absolute humidity, relative humidity increases as tem-
perature decreases. As a consequence, buildings maintained at relatively low
temperatures (e.g., vacation homes) and materials in unheated basements
(commonly around 55°F, 13°C) are potentially subject to mold infestation.
Significant mold growth may also occur on soil in crawlspaces because of
wet surfaces and/or high relative humidity. Mold spore dispersal from
crawlspace infestation can result in elevated indoor airborne mold concen-
trations when spores are entrained in leaky return air ducts. Similarly, ele-
vated airborne mold concentrations can occur in living spaces from moldy
basements which serve as utility areas for forced air heating and/or cooling
systems. Mold spores become entrained in leaky return air ducts and/or the
leaky housing of the furnace or air conditioning unit.
2. Occupant practices
Elevated relative humidity may also occur in residential buildings as a result
of occupant behavior and practices. When thermostats are set back during
sleeping hours, an increase in relative humidity occurs, corresponding to the
temperature decrease. Thermostat setbacks, used to reduce energy costs, may
result in mold and mildew growth on cool surfaces. Occupants (particularly
“empty nesters”) often close doors and supply air registers in bedrooms as
well as other rooms to reduce heating costs. Since room temperatures and
air circulation are reduced, this practice often has the unintended conse-
quence of causing mold infestation.
In many countries where winter temperatures are cool but not cold, res-
idential structures may not be heated at night. Interior temperatures in such
houses may drop to 35 to 40°F (3 to 5°C). On wet to moderately wet sites,
such houses may experience nighttime relative humidity
70%. Consequently,
mold infestation of building materials and furnishings such as draperies,
mattresses, and carpeting may be common. In a study conducted by the author
and colleagues in a typically damp region of southeastern Australia, 19 mold-
infested mattresses were identified in a population of 40 houses.
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3. Water vapor-generating sources
Relative humidity in buildings can be influenced by a variety of factors.
These include the use of vapor-generating sources such as swimming pools,
