Basements are some of the most common places for mold to grow. This is an area where there’s a lot of dampness, old plumbing, leaking, wood that can rot after time, and a buildup of humidity from things like outdoor conditions coming in through windows and hot water heaters. That musty smell often attributed to basements is a sign of mold and mildew, which needs to be taken care of before it extends to the rest of your home.
First, you need good cross ventilation under the house so air will circulate freely. Stagnant air is holding moisture, creating condensation, mold, and potentially serious wood rot problems. Check that all of your vents in the basement are unobstructed. Repair any torn vent screens so animals don't move into the crawl space around the basement. If you don't have sufficient vents, you should add more.
Second, cover the ground of the basement with black plastic sheeting if it’s not being used as a living space. This will trap and hold the ground moisture under it instead of allowing it to rise to the floor joists. Tape or weight all overlapped joints down, including along the foundation, so no gaps exist.
Finally, remove any and all sources of humidity in the basement or crawl space. You need to lower the humidity level below 60 percent. By getting rid of any source of water leakages, whether it’s in the structural areas above the basement or in the windows, sealing areas in the floors and walls, and removing any water puddles, you can significantly lower the humidity to get rid of mold and mildew. A dehumidifier is also a good investment in this situation to lower the humidity in the basement, especially if your hot water heater is located in the room.
Mold-resistant coatings are effective sometimes to keep mold off of areas, but you should not depend on this or coat the entire basement. It’s better to work to ventilation up and humidity down rather than depend on one source to do the entire job.