How to Control Moisture in a Ventilated Crawl
By: Mark J. Donovan
For many, a home’s crawl space is rarely
accessed or used. However, an out-of-sight / out-of-mind mentality on a
crawl space that is not properly moisture controlled could cost you a lot of
If a crawl space is not moisture controlled
properly, your home is susceptible to rot, mold and insect damage. In
addition, if an air conditioning or heating system is located in the crawl
space it may also be damaged by high moisture content. Finally, a non
moisture controlled crawl space is energy inefficient which translates into
higher heating and air conditioning bills.
Crawl spaces can be found with and without
operable ventilation systems. Though venting a crawl space may seem to make
sense to reduce moisture levels, it actually can cause more harm than good in
hot, humid climates. A vented crawl space in a hot, humid climate actually will
increase the humidity level within the crawl space and make it more susceptible
to moisture damage. The moist hot air reacts with the cooler air in the crawl
space and causes condensation. The condensing moisture attaches itself to floor
framing surfaces where mold can begin to form.
Though the rules can vary depending upon your local climate
conditions, there are several basic steps you should do to minimize the moisture
levels in your crawl space.
First, when installing the crawl space
foundation, make sure you put in a French drain (perimeter drain) around the
foundation walls. The Perimeter drain should be designed so that water runs away
from the outside foundation walls.
Second, make sure you seal the outside of the
foundation walls to finished grade level with a waterproof sealer.
Third, when backfilling in around the
foundation, make sure the finished grade gently slopes away from the foundation
Fourth, the floor of the crawl space should be
covered with a 4-6 mil layer of polyethylene vinyl (plastic) to reduce moisture
transfer from the ground into the crawl space area. The seams should overlap by
1-2 feet and should be taped. The plastic should go up the sides of the wall of
the crawl space 6-12 inches. Finally, add 2 inches of sand over the plastic to
minimize the risk of breakage.
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Lastly, install gutters around the eves of the house
or addition to control the flow of rain water run off. Downspouts should be
positioned to direct the water away from the foundation walls.
During hot summer months seal off the vents in
the crawl space to minimize the influx of hot moist air that would otherwise
condense in the cool crawl space area. Likewise, in the winter months seal off
and insulate the vents of the crawl space to reduce the threat of cold air that
could freeze pipes in the crawl space.
During cooler/moderate temperature seasons
(spring and fall), open the vents of the crawl space to enable moisture within
the crawl space to be dissipated.
Note: The recommend ratio of ventilation area to
crawl space is 1:150.
In order to minimize the affects of the colder
drier air in the crawl space you will want to insulate the “roof” of the crawl
space. Basically you will want to install rolled insulation between the floor
joists. The rolled insulation will not require a moisture barrier surface, as
the plywood floors above act as a moisture barrier. However, after you have
installed the rolled insulation you may want to cover and staple it with a layer
of polyethylene vinyl to create a vapor barrier between the bottom side of the
insulation and the crawl space airspace. Make sure when installing the rolled
insulation you use insulation hangers to secure the insulation in between the
floor joists. Do not rely on the friction between the floor joists and the rolled
insulation to hold the insulation in place.
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