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How to Control Moisture in a Ventilated Crawl Space

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 money.


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.

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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 walls.


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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- How to Control Moisture in a Ventilated Crawl Space -

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