If your crawl space has open vents, it is allowing humid air from the outside to enter the space. As that humid air cools in the crawl space, relative humidity levels rise creating the perfect conditions for mold to grow. If the RH levels go above 100%, condensation will occur in every surface, soaking your floor joists, beams, and other structures. With a vented crawl space, the question is not whether it has any mold, but how much damage is being caused by it.

Depending on how big the mold problem and how much damage it caused, solutions can range from fungicide application to removal and replacement of moldy structures. This is not a DIY job, so please leave it to professionals. Now, the most important thing is to address the moisture that caused the problem in the first place so that it will not keep coming back. Sealing and encapsulating the crawl space to keep groundwater and humidity from the outside from getting into the crawl space is a good start. Running a good crawl space dehumidifier to keep RH levels below 60% so mold will not have chance to grow again is the safest and most definitive solution for mold problems in crawl spaces.

Standing water in crawl spaces can happen for several reasons. From groundwater intrusion to condensation around pipes and ducts dripping onto the floor, to undetected plumbing leaks. The best way to address this type of problem, in a place that homeowners almost never inspect, is to provide adequate drainage through drainage channels, floor drains, discharge lines, and a good sump pump to pump any water out and away from the crawl.

Bouncy, sagging, and creaking floors above a crawl space are a sign that the floor structures are being weakened or compromised by excessive moisture in the crawl space. Untreated, this moisture can lead to mold and dry rot, and compromises the structural integrity of your home.

There are several sources of bad smells in the typical vented crawl space. The most common source is mold, but bacteria growing in standing water, animal urine, and feces are also quite common. The best way to get rid of these foul odors is to keep your crawl space dry, free from mold and critters. All of which can be accomplished with proper encapsulation, sealing, and dehumidification.

While only a few mold species, such as black mold, release dangerous mycotoxins, all mold releases spores in the air. Because the air in your home is constantly moving upwards, at least 1/3 of the air you breathe indoors is coming from your crawl space, along with mold spores, dust mite droppings, and other pollutants. Indoors, the concentration of these harmful particles in the air can trigger mild to severe allergy symptoms in sensitive people.

Vented crawl spaces are notorious as a source of energy waste year-round, especially if you have HVAC ducts running through them. During the summer, the cool air running through ducts housed in a hot crawl space starts to gain temperature before it reaches the rooms. During the winter, the hot air in the ducts will cool down in the freezing crawl space. Your HVAC will be working harder than it should all year, just to keep your home comfortable.

Freezing air from the outside is constantly being sucked through the vents and then moving upwards through the floors, making them cold to the touch, and often causing cold drafts that make the entire indoor space uncomfortable and hard to heat.

Crawl spaces are very prone to moisture-related problems. For that reason, they should be encapsulated and sealed prior to insulation. As for the best insulation materials, choose those that are impervious to moisture and mold, such as rigid foam board or closed-cell spray foam. Fiberglass is the worst possible choice.

While there is not a sure-fire way, besides periodic inspection, to make sure your crawl space is free of pests and critters, crawl space encapsulation is a powerful deterrent. Not only does it provides a physical barrier, but a clean, dry crawl space offers none of the elements that tend to attract insects, strays, and wildlife to your crawl space.

Modern building science findings state that the crawl space vents, once thought to help keep the crawl space dry, are a major source of moisture problems by allowing humid air from the outside into the space. Crawl space vent fans, by sucking even more outside air in, will significantly amplify that effect. Crawl space vents should be closed and the crawl space sealed, encapsulated, and dehumidified instead.

The crawl space liner creates a physical barrier against radon intrusion, but does not replace an actual radon mitigation system.

If your home tested high for radon levels, the best course of action is to combine crawl space encapsulation with a radon mitigation system. They work together. One is not effective against radon without the other.