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A vapor barrier keeps moisture out of the crawl space, while insulation reduces the amount of unconditioned air—and sometimes moisture—that enters the space. Coupling a vapor barrier with insulation can help prevent humidity and related moisture damage in a crawl space while also making your home more energy-efficient. Just be sure to use insulation that won’t get damaged from moisture, like rigid foam board or expanding spray foam insulation.

Underfloor insulation alone will not cause moisture and mold, but improper installation can. Although the purpose is to seal any drafts, your subfloor still needs ventilation. If you seal the area too tightly and prevent all airflow, this can cause moisture to get trapped. This moisture eventually leads to mold and—if your floors are wood—rotting.

When doing any type of project involving mold, it's important to wear the right protective gear. You'll want an N-95 or KN-95 mask to keep yourself from inhaling mold spores, which can be irritating or trigger an allergic reaction in people with weakened immune systems. You'll also want to wear protective eyewear and gloves to limit your contact with mold.

Ductwork sweats due to a temperature difference between the air inside the ducts and the surrounding air, similar to how a cold drink develops condensation in the heat. This sweating is a sign that your HVAC system is not operating at peak efficiency. The best way to keep ductwork from sweating is to wrap it in proper insulation, which also improves your system's energy efficiency. Another option is to close off your crawl space and install a dehumidifier to dry the air and lower its dew point.