There are many factors that can lead to mildew or moldy smells in a basement. The first and most common is the presence of water either as moisture or excessive humidity. The next is a lack of air movement. If these two are combined there will either be a mold build-up or the likelihood of one.
Most commonly mildewy or moldy basement smells are due to wet foundation walls and the presence of finishing materials such as carpet and drywall. The building materials are a constant food supply and will aid mold growth if given enough water or dampness.
There are two basic ways to remove or prevent a moldy smell.
1. The prevention of the water or moisture present. This is the most desired cure and the best starting point to eliminate or prevent mold growth and, of course, the moldy smell. Water elimination may involve plumbing repairs, the waterproofing of either the building’s interior or exterior below-grade areas, or the balancing of the HVAC system, among others.
2. The removal of building materials. While this is a less desirable approach, removal of the food that is aiding the mold growth (if the water problem cannot be solved) will slow or prevent further mold growth even if the cause of the moisture problem is not addressed.