Why is my basement musty?

Aug 30, 2019

The team at Indoor Science performs hundreds of indoor air quality assessments per year, from commercial and residential to industrial. After doing a number of assessments, we begin to notice patterns and common questions. One common question being “why is my basement musty?”. This may seem like a simple question but there are many factors that could be causing a musty odor in a basement. This blog post will highlight common issues in the basement that could cause a musty odor and provide helpful information on keeping your basement odor-free.

Musty Basement Odor

In the past, we have posted blogs that discuss odors and how we go about testing to try to identify the sources. In this post, we are discussing a specific type of odor in the basement of a home. This odor is usually characterized as a musty, damp, or earthy smell. The odor may not be perceptible in other parts of the home besides the basement. A musty odor is typically associated with dampness and mold growth. Therefore, to eliminate or minimize this odor, damp and moldy sources need to be addressed.


A very common problem in basements is too much moisture. Because basements are below-grade, water from the surrounding soil can infiltrate the space. Water infiltration can happen from a crack in the foundation wall or floor.  To prevent excess water from infiltrating the basement, ensure that cracks in the foundation are sealed. These issues can be easily identified when the basement is unfinished because everything is exposed. Identifying water infiltration issues gets more challenging when the basement is finished. Indoor Science can use an infrared camera and moisture meter to help identify damp areas in a finished basement.

A crack in the foundation that was repaired.

General Seepage

Water seepage into the basement can also occur when the soil outside of the basement is saturated. Water doesn’t exclusively travel through cracks; it can also slowly seep its way through a poured concrete or cinderblock wall. Many foundations do not have a vapor barrier on the outside so water can move through via capillary action and diffusion. This problem is exasperated when the ground around the foundation isn’t drained properly. A working sump pump is key in this situation. A sump pump draws excess water away from the surrounding soil into a sump pit. The sump pump then drains the water away from the foundation. The sump pump needs electricity to operate which is why it is critical that it has a battery back up in case the power goes out. The battery back up can save your basement from a flood due to a power outage.

Basement Plumbing Leaks

In addition to water infiltration and seepage, plumbing leaks can add moisture to the basement.  Evidence of leaks can be seen around plumbing penetrations on the subfloor of the 1st floor (or basement ceiling). Look for discoloration, water stains, or rotted wood around plumbing penetrations to determine if there is an active leak or one from the past.


If you see a white, powder-like substance coming from masonry don’t freak out. That is a material called efflorescence. Efflorescence is composed of salts and minerals and is wicked through the masonry when water passes through. While the efflorescence itself is not hazardous to health it is an indication that you have a seepage problem and water is getting through.

Image of efflorescence.

How do I get rid of the musty odor in my basement?

Moisture, no matter the source, fuels the growth of mold and bacteria that give off microbial volatile organic compounds (mVOCs), which are the ultimate cause of the musty or earthy odors. You may not even see visible mold, but it could be growing on dust at a microscopic level. If you get rid of the moisture, the biochemical processes in the microorganisms will stop, and they will no longer produce the mVOCs and musty odors.  

In our humid midwestern climate, it is recommended to operate a dehumidifier in the basement to keep the humidity below 60%. I’d recommend a model that can be connected directly to a drain. This can save multiple trips to the basement to empty a full dehumidifier water bucket. If there is no nearby floor drain, look for a model with a built-in pump that can lift the water up to a nearby laundry sink or other drain.

If you’re looking for someone to inspect and repair issues consider accessing the Basement Health Association portal.


High humidity, water sources from leaks, and seepage add moisture to the basement which can lead to mold growth. Mold growth and moisture can definitely cause a musty or damp odor. Ensuring that there is no moisture and using a dehumidifier in the basement can help combat mold and unpleasant odors.