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Why Do I Have Mold In My Crawl Space?
The never-before-told truth!
By Barry C. Westbrook, CIH Home inspectors have become very picky about
moldy crawl spaces. The reason is simple – LIABILITY. Considering the publicity
surrounding the perils of “toxic mold”, the last thing any home inspector wants
is a law suit. Fair enough. However, once discovered, the problems really begin
when realtors, property owners, and mortgage bankers try to determine what to do
about the moldy conditions.
Here is an actual excerpt from a home inspector’s report:
"Mold growth is visible on many of the floor joists and sub floor in
the crawl space. The level of mold growth is low to moderate. The presence
of certain mold and mold spores can result in mild to severe health effects
in humans and pets and can deteriorate the structure of the dwelling. Health
effects include, but are not limited to: sinus congestion, blurry vision,
sore throat, dry cough, aches and pains, skin irritation, bleeding of the
lungs, headaches, memory loss and fever."
This type of report creates adversity between the buyer and the seller of the
property. Considering all the nasty health effects described above, is a
moderate exposure acceptable? Even if I were willing to risk it, should I expose
my children to moderate levels of mold?
The home inspector is doing the prudent thing by stating the known health
effects of mold exposure and trying to establish a relative description of the
magnitude. In effect, the inspector is passing the risk assessment on to someone
else: either the buyer of the property or another professional with more
experience in assessing health exposure. The report goes on as follows:
"As a practical matter, there are several things that can be done to
minimize
mold growth: (1) Keep the crawl space well ventilated (2) Divert downspouts
away from the foundation wall and keep gutters clean."
So now you have a plan, or do you? While getting storm water away from the
foundation and keeping the gutters clean are certainly good things to do, they
are not likely to prevent mold from growing in the typical crawl space. The
reason for this relates to two of the biggest myths in building construction:
1. Standing water is the main cause of mold in the crawl space.
2. Mold can be prevented by keeping a crawl space well ventilated.
Let’s see why these statements are not true.
While different types of mold need varying amounts of moisture, let’s stick
to the basics: mold can not grow without moisture. For mold to grow on floor
joists and sub floors, water vapor must condense out of the air onto the surface
of the wood. Where does this water come from? The prevailing wisdom is that this
moisture comes from the ground, but reflecting on some basic science principles
will show why this is not the case.
It takes a lot of energy in the form of heat to get water to go from a liquid
to a gas. If you were a colony of mold looking down from the sub floor of your
home, you would see an occasional molecule of water evaporating from the ground
and becoming mixed with the air in the crawl space. Meanwhile, you would also
see millions and millions of water molecules coming into the crawl space through
the foundation vents already mixed in the outside air. The actual amount of
moisture in the outside air varies directly with the temperature, but in the hot
summertime, air can hold a tremendous amount of moisture. Take a look at the
chart below prepared by the non-profit Advanced Energy Corporation and you will
see that the relative humidity in a vented crawl space is almost totally a
function of the outside relative humidity. When the outside humidity is high,
the humidity in the crawl space will also be high. In absolute terms, the amount
of moisture entering the crawl space from the outside is many, many times the
amount of moisture evaporating from the ground surface.
Dew point is the other scientific principle necessary to understand the mold
phenomenon. For water vapor in the air to condense onto a surface, the
temperature of that surface must be below the dew point. At that point, the air
is 100% saturated and can no longer hold all the moisture. The air begins to
drop some of the water onto the surface as condensation. The colder the surface,
the more condensation will form. That is why there is usually more mold
underneath bathrooms and kitchens. These floors are not insulated as well as
carpeted floors, and become very cool in the summertime. Moisture forms on the
cool surface in the crawl space and mold proliferates. We also see cupping and
buckling of wood floors for the same reason.
So is mold in the crawl space a problem? The answer depends on your
expectations. One of the main problems with moldy crawl spaces is represented on
the chart below, also provided by the Advanced Energy Corporation. You will see
that much of the air we breathe in our “naturally ventilated” homes comes from
our crawl spaces through penetrations in the floor. The flow of air from the
crawl space is driven by the difference in building pressure between the first
floor and the outside.
If you wouldn’t want to continuously breathe the air in your moldy crawl
space, then you will probably be concerned to know that much of the air that you
breathe in your house came from there. Mold not only produces airborne spores,
but it can also generate noxious volatile organic compounds (gases), which cause
a variety of symptoms including fatigue, nausea, headaches, respiratory
irritation, eye irritation, inability to concentrate, and dermatitis.
What Can Be Done?
In any discussion of indoor environmental quality it is important to
acknowledge that there is a wide spectrum comprising the extremes from unsafe to
pristine. There is also large variability in human sensitivities to mold spores
and fungal particulates depending on sex, age, genetic makeup, and overall
health. With these considerations in mind, the environmental condition of most
homes should not be considered unsafe. However, air quality can definitely be
improved once you
understand a few building basics.
1. Keeping Crawl Spaces Dry
Building codes require a minimum amount of crawlspace ventilation based
on the length of the foundation perimeter. The intent is not to prevent
moisture from entering the crawlspace but to allow any excess moisture under
the building to evaporate. If the outside air is sufficiently dry and the
amount of excess moisture is not too great, this process is adequate.
Unfortunately, the outside air is usually much too wet to dry out the
crawlspace. Particularly during the summer months, vented foundations
transport moisture from the outside to the inside rather than the other way
around.
Condensation in the crawlspace can best be prevented by sealing it off
from the ground and from the outside using a continuous liner such as the
one shown below.
2. Building Pressure
Negative building pressure causes a continuous flow of air from the crawl
space / basement into the first floor. Depending on the outside humidity
levels, this influx of outside air can bring in large amounts of excess
moisture and mold spores. This is a common cause of high humidity in
basements. We developed the Proof Positive™ System which we believe to be
the best and most economical method of correcting negative building
pressure.
The most important aspect of the ProofPositive™ System is its ability to
effectively prevent the infiltration of outside air and at the same time
bring in ventilation air that is filtered to the highest level of purity.
The filtration standard is high efficiency particulate air (HEPA) which
means that our system must remove > 99.97% of particles with a diameter of
0.3 microns. Mold spores have a diameter ten times greater than this, so
practically all of them are removed by this system.
The installation is performed in three steps:
1. We first identify major building penetrations using a blower door. The
blower door (see photo below) increases the relative pressure between the
house and the outside so that we can identify major points of infiltration.
2. Secondly, we seal the previously identified penetrations using caulk,
mastic, expanding foam, or weather-stripping, as appropriate.
3. Third, we install an outside air duct into the return air side of your
HVAC system in order to place the house under slight positive pressure with
respect to the outside. The unit will be outfitted with a HEPA filter to
remove particulates from the outside air supply. After this phase is
completed, unconditioned outside air along with dust,
pollen, radon, moisture, and mold spores will not be able to infiltrate the
house when the HVAC fan is operating. After you install this system you will
notice a dramatic reduction in dust on indoor surfaces.
 
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