Why is mold a problem, and how should I get rid of it?
Mold growth is one of the leading causes of indoor air quality problems. Mold
spores are potential allergens, meaning that they can trigger allergic responses
much like animal dander or pollen. In addition, some types of mold produce
toxins that can be harmful to humans. All visible mold growth should be removed
from the indoor environment as quickly as possible. Mold sampling should only be
performed to determine a specific hypothesis. If mold growth is already visible,
sampling can be a waste of time and money, as all mold growth is cleaned in the
same manner.
Light mold growth may be cleaned by the homeowner or building manager (while
using the proper personal protective equipment, including eye protection,
gloves, and dust masks). Cleaning methods should include an antifungal detergent
and light abrasion to remove all fungal growth. If the area of mold growth is
greater than ten square feet, or if the growth has invaded porous building
materials such as drywall, a professional mold remediation contractor should be
consulted.
What is toxic black mold?
The term “toxic black mold” actually refers to some species of the genus
Stachybotrys. This particular genus of mold can produce harmful mycotoxins that
may be toxic to humans. While no definitive link between Stachybotrys and
specific human health effects exists, anecdotal evidence suggests that this type
of mold can cause a wide variety of symptoms. Other types of mold have also been
shown to produce mycotoxins that may be harmful to humans, including Aspergillus
and Penicillium.
“Toxic black mold” is actually a misnomer, as many varieties of mold can appear
black. In fact, Stachybotrys can appear in a variety of different colors,
including pink. All black mold growth should not be assumed to be “toxic black
mold.” However, any mold growth in the home needs to be removed as soon as
possible to prevent allergic reactions or other adverse health effects.
What is relative humidity, and how does it affect mold
growth?
The amount of water that can dissolve in a given volume of air is dependent on
the temperature. Absolutely dry air has a relative humidity of 0%; completely
saturated air has a relative humidity of 100%. When a constant volume of air and
dissolved water is cooled, the relative humidity goes up. When the temperature
has dropped to a point where the relative humidity is 100%, water will condense
to a liquid in the air itself or onto surfaces with a temperature below the dew
point (defined as the temperature at which a given volume of air reaches 100%
relative humidity). Ideally, the relative humidity indoors should be between 35
and 55%. If the relative humidity is too low, your skin will dry out and static
electricity will increase. If the relative humidity is too high, condensation
will occur on cool surfaces (such as windows, air conditioning vents, and on
floor joists and beams in crawlspaces). This condensation provides the moisture
necessary for mold growth.
How can site grading and drainage outside my home affect
indoor air quality?
Site grading and drainage plays a huge role in indoor air quality. If surface
water is directed towards or trapped against the foundation wall, moisture may
enter the basement or crawlspace. This moisture will raise the relative humidity
of the home, encouraging mold growth. To keep surface water from entering the
home, the homeowner or building manager should take a number of actions. These
include the installation of gutters (provided they are well-maintained and that
the downspouts discharge at least six feet from the foundation wall), re-grading
the soil near the foundation to ensure the proper slope to direct water away,
and eliminating swales or gullies that carry water towards the home.
Accumulations of mulch or organic debris against the foundation walls or on the
roof (pine needles, leaves, etc.) that might trap and hold moisture should also
be eliminated.
Should I be concerned about mold and
moisture in my crawlspace?
Crawlspaces are not intended to be part of the breathing environment; however, because of negative building pressure and unsealed penetrations between the crawlspace and the living area, mold, moisture, and even radon are being sucked from the crawlspace into the home above. For these reasons, it is important to ensure that crawlspace air cannot travel into the living area of the home or building. All plumbing insertions, ductwork, or other penetrations should be properly sealed to eliminate air movement between the crawlspace and the breathing environment.
Vapor barriers should be installed in the crawlspace to minimize the introduction of soil moisture into the crawlspace air. Many vapor barriers are constructed of 6-mil polyethylene, which must be replaced every three to four years. Ideally, a sturdy 20-mil vapor barrier should be used. All soil should be covered and all joints should be taped or otherwise sealed. By extending the vapor barriers up the foundation walls and over the crawlspace vents, soil moisture is allowed to escape and the crawlspace itself is kept dry. Encapsulating a crawlspace in this manner also can reduce energy bills by up to 30%.
Crawlspace vents have long been installed with the intent of allowing moisture to escape. However, outside air (especially during the summer) is often much more humid than air in the crawlspace. As outside air moves through the crawlspace and cools, it loses some of its moisture. Over time, enough moisture can accumulate to encourage mold growth and water damage. This phenomenon is especially true in homes with un-insulated pipes or ductwork in the crawlspace. These exposed metal surfaces are often well below the dew point for outside air and are therefore prime locations for condensation. Floors should also be insulated (even in warmer climates, where the floor insulation actually prevents condensation on floor surfaces cooled by HVAC systems).
We recommend that vented crawlspaces be fully encapsulated to prevent this moisture problem. Crawlspaces should be inspected regularly for signs of moisture or other problems.
What is negative building
pressure?
Negative building pressure results when an HVAC system inside a building draws
in more air from the return ducts than it expels through the supply ducts. A
building can be wholly under negative pressure, or exhibit zones of negative
pressure in certain areas (e.g. a room contains a return air grille and no
supply grille). Given the current configuration of most HVAC systems, most
buildings operate under some degree of negative pressure.
When buildings operate under negative pressure, some air must be introduced from
outside to balance the pressure differential. Ideally, this air would enter the
HVAC unit through a valve or duct and be dehumidified before being introduced to
the indoor environment. Unfortunately, however, this is rarely the case. HVAC
units often have no allowance for outside air, meaning the air must be brought
in from different areas. This often results in air being sucked in around doors
and windows, through cracks in the foundation, or through other penetrations in
the building skin. When this occurs, the introduced air is not dehumidified
before entering the indoor environment. Over time, the indoor relative humidity
will increase, potentially causing mold growth on cool interior surfaces or
inside wall cavities. In addition, negative building pressure can introduce soil
gases into the home, including radon.
To eliminate negative building pressure, the HVAC system must be reconfigured so
that any outside air that comes into the home passes over the condenser coils.
This process requires careful calculation and precision, so it should only be
attempted by a professional.
What about my heating,
ventilation, and air conditioning (HVAC) system?
Proper operation of the HVAC system is the backbone of any indoor air quality
plan. The HVAC system should be regularly inspected by qualified personnel to
ensure proper operation. In addition, regular examination of the ductwork is
necessary to identify and repair all leaks, damage, or other problems.
The thermostat inside the home should be set to the proper room temperature
(~72°F) to ensure both regular system cycling (which provides dehumidification)
and also to prevent abnormally cool surfaces that might encourage condensation.
In addition, the fan should always be set to the “AUTO” position. Setting the
fan to “ON” will cause the fan to continue running even when the thermostat set
point temperature has been satisfied. Areas of negative building pressure will
develop, causing humid outdoor air to enter the home at an increased rate. This
air will be circulated throughout the home by the HVAC system (which no longer
dehumidifies the air when the compressor is not operating). Over time, the
indoor relative humidity will increase and mold growth will begin.
Microbial growth (including bacteria, mold, and algae) often occurs in the
condensate drain pan, since this area receives a constant supply of moisture
during the cooling season. This microbial growth can clog the drain line,
resulting in an overflow that can damage underlying building materials. The
drain pan should be regularly inspected to prevent this microbial growth. The
regular addition of chlorine bleach or an algicide tablet will discourage
microbial growth and prevent clogging. The discharge point of the condensate
drain line should be located outside the home or building, at a minimum of six
feet away from the foundation wall.
