Why Many Basements Smell
And what you can do about it.
Most people prefer a basement home over a crawlspace home. The only downside is that a basement often has a musty smell. Most often that smell is identified as Aspergillius/Penicillium mold. The mold spores become elevated because basements more often than not have elevated humidity (above 55%) or water and/or moisture intrusions.
Why would the relative humidity be elevated in a basement.
Basements are naturally cool. Basements are built below grade and the earth will keep the basement wall about 65 degrees. This often means that the heating and air conditioning unit does not need to be used or used very little for cooling. When an HVAC unit is cooling it is removing water from the air by condensating at the ac coil, then the condensation drain drips to the outside. This actually is your home dehumidifier. If the home does not need or needs very little ac, often the relative humidity will become elevated above 55% and with elevated humidity comes mold growth (usually Aspergillius/Penicillium) and odors.
Problem with building below grade
Many basement walls were never correctly water proofed during construction. When rain water is absorbed into the basement walls these walls may stay wet for weeks on end because they cannot dry out to open air.
Many times, rather than filling around the exterior walls a contractor will take shortcuts and fill in with soil rather than gravel. Our soil is heavy in red clay which takes longer than other soils to dry. Gravel is preferable because it allows water to flow to drain pipes, whereas red clay doesn’t drain well and also clogs pipes.
Basement floors are made using concrete. When carpet is laid over concrete often the carpet will trap moisture from the concrete. This creates mold and mold odors. A concrete floor should have a moisture barrier under it when it was poured. Many older poured concrete floors did not have moisture barrier put in place under them. Concrete is actually porous, concrete will absorb moisture and then breathe the moisture into a home. If the home has carpet, the carpet will work as an odor absorbent trap and then the basement may never smell good again until the carpet is removed. The bottom side of the carpet is the part that can’t be cleaned.
Basements can be unused for extended periods of time. Any space that is not used and the air is left dormant will become stagnant and begin to smell. Ventilating a basement on dry days (less than 55%) can help relieve stagnant air. A basement that is rarely used can smell just from lack of use.