Crawlspace odors are typically caused by two factors: lack of ventilation and pest infiltration. Moisture, caused by poor air circulation under the home, can lead to mold and mildew in the enclosed space. Unfortunately, there is no magic-wand cure for the elimination of any odors. By following these steps, though, you can create an area under the home that is well ventilated, dry and free of roaming pests and insects.
Remove all debris from the crawlspace. This includes any fallen insulation, animal nests and waste material.
Look for and stop any water infiltration. The source can be leaking and sweating pipes or bad joints in the drain lines. Cracks in foundation walls can allow seepage from groundwater.
Replace any affected insulation. This can include fallen or stained fiberglass. The insulating blankets can harbor rodents and insects.
Place high-quality, 20-mil-thick plastic over any exposed dirt or soil. Moisture and insects can migrate through the soil, creating the first signs of odors.
Remove any vegetation in front of the crawlspace vents. This should include any bushes and plants within a 2- to 3-foot area of the vents' entrance. Vegetation that touches the house may harbor pests that can find their way into the crawlspace.
Install an approved style of crawlspace ventilation in the foundation walls. These vents will have adjustable metal louvers and a fine-mesh screen to keep out insects. Typically, two vents are installed in every wall. Vents should be placed 8 to 10 feet from every corner of the crawlspace.
Seal any cracks with silicone caulk. Close any large openings with an approved method. This may include the use of treated lumber if the wood material comes in contact with the soil.
Operate a crawlspace dehumidifier once the space is sealed with the proper ventilation. Even with adequate ventilation, crawlspaces in some areas of the country may still require a dehumidifier.
Consult a professional if the odor persists. Certain biocides or fungicides may have to be employed. Only certified professionals can apply them, and there may be some environmental concerns in using these types of products.