Environmental Engineering Reference
In-Depth Information
spas, steam baths, large fish tanks, and decorative recirculating fountains,
pools, waterfalls and streams. The higher the surface area and temperature
of vapor-generating sources, the higher the absolute and relative humidities.
High relative humidity may also occur in localized areas such as bathrooms.
In the absence of local exhaust ventilation, it occurs episodically with exten-
sive bath and shower use. Consequently, mold growth in showers and on
bathroom walls is common.
4. Poor air circulation
Inadequate air circulation within building structures may increase the risk
of mold infestation. This is the case in localized areas such as closets and
walls obstructed by furniture. Poor air circulation is only a problem if com-
bined with nutrient sources (such as shoes in a closet), elevated relative
humidity, and/or a cold surface. When furniture obstructs walls, it prevents
heat transfer to wall surfaces; wall cooling causes localized high humidity
with subsequent infestation of wall and adjacent furniture surfaces. Poor air
circulation occurs in homes that are not heated by forced air systems, in
closed-off rooms, and where furniture or other large objects are placed too
close to walls.
B. Cold floors
In the absence of adequate insulation, floor surfaces in residences constructed
on slabs or crawlspaces are often colder than room air above them. These
cold floors may be uncomfortable to occupants as a result of thermal asym-
metries; they may also result in high localized relative humidity, which may
cause, in the worst case, condensation, or more commonly, an optimum
environment for the infestation of jute-backed carpeting or soiled all-syn-
thetic carpeting. Mold infestation of carpeting is common in some countries
and climatic regions where interior moisture levels are high and floor sur-
faces are cool or cold. Mold infestation of carpeting in basements is common
because floor surface temperature is typically the same as that of the ground
with which the basement slab is in contact (55°F, 13°C).
C. Condensation
When air becomes saturated it reaches its maximum potential to hold water
at a given temperature; it then has a relative humidity of 100% and conden-
sation will occur. The temperature at which condensation occurs (i.e., dew
point) differs for air that contains different moisture (vapor) levels or abso-
lute humidities. The higher the absolute humidity, the higher the tempera-
ture at which condensation takes place. In the psychometric chart in
Figure
condense water at approximately 66°F (18.5°C) if the absolute humidity is
0.013 lbs H
2
O vapor/lb dry air; condensation will occur at approximately
60°F (15.5°C) if the absolute humidity is 0.011 lbs H
2
O vapor/lb dry air.
