Why does my basement smell musty?

A musty basement smell almost always means moisture. Mold and mildew grow on damp materials, and the smell is the sign that something in the space is staying wet or the air is humid enough to keep it that way. The fix is to find where the water or humidity is coming from, not to cover the odor.

What is usually behind the smell

Humid air. Basements are cool, so warm summer air that drifts in condenses on cold walls, slabs and pipes. EPA advises that if possible you should keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity.^[1] ENERGY STAR's dehumidifier sizing chart puts a space that feels damp and has musty odor that may be intermittent at 50-75% relative humidity, and one that consistently feels and smells damp, with damp spots on walls and floors, at 75-90% relative humidity.^[2] A musty smell that comes and goes with the weather points to humidity; one that never leaves points to a wet source.

Water getting in. Gutters that overflow, downspouts that empty beside the foundation, soil that slopes toward the house, a cracked wall or a failing sump pump all put liquid water where it should not be. The Northern Virginia Soil and Water Conservation District puts it this way: Wetness or erosion near your foundation walls and drip line can be very damaging.^[3] Its list of causes includes:

• Gutters that need to be cleaned or are undersized^[3]
• Downspouts and sump pump pipes^[3]
• Poor grading around foundation walls^[3]

EPA's list of prevention steps includes: Make sure the ground slopes away from the building foundation, so that water does not enter or collect around the foundation.^[1]

A leak or a past flood. A slow pipe leak, a water heater that wept for a season or an old flood can leave wet drywall, carpet or stored boxes that keep growing mold long after the water is gone. EPA says: It is important to dry water-damaged areas and items within 24-48 hours to prevent mold growth.^[1] Anything that stayed wet longer is a candidate for the smell.

Condensation on pipes and ducts. Cold water pipes and ducts sweat in humid weather. To prevent condensation, EPA's advice is to "Cover cold surfaces, such as cold water pipes, with insulation."^[1]

Appliances. A clothes dryer, stove or kerosene heater that is not vented outside adds water vapor. EPA's list says: Vent appliances that produce moisture, such as clothes dryers, stoves, and kerosene heaters to the outside where possible.^[1]

Hidden mold

If the room smells moldy and you see nothing, the cause may be behind a surface. EPA says you may suspect hidden mold if a building smells moldy, but you cannot see the source.^[1] It lists likely hiding places as the back side of dry wall, wallpaper, or paneling, the top side of ceiling tiles, the underside of carpets and pads, and areas inside walls around leaking or condensing pipes.^[1]

This is where a basement owner should stop and bring in help. EPA warns that investigating hidden mold requires caution when it involves disturbing possible sites. If you believe that you may have a hidden mold problem, consider hiring an experienced professional.^[1]

What helps while you work out the cause

A small humidity meter will tell you more than your nose. EPA describes a moisture or humidity meter as a small, inexpensive ($10-$50) instrument available at many hardware stores.^[1] Readings that sit well above the range above, especially in summer, mean the space needs dehumidifying whatever else you find. ENERGY STAR says portable room dehumidifiers are typically used to dehumidify a single room or space. Whole-home dehumidifiers are typically installed to use your home's air ducts, and the drain should be routed during installation by an HVAC professional.^[2]

A dehumidifier lowers humidity but does not remove mold that has already grown. Dead mold may still cause allergic reactions in some people, so it is not enough to simply kill the mold, it must also be removed.^[1] Bleach and scented sprays are not a fix.

Air sampling is rarely the next step either. In most cases, if visible mold growth is present, sampling is unnecessary, EPA says.^[1]

Why it matters before you finish the basement

If you plan to finish the space, a musty smell is a warning. New drywall, insulation and flooring over a damp slab or a wet wall give mold more to grow on and hide the problem. Water and humidity should be corrected first, which is the order described in what finishing a basement involves. Moisture work also changes the price, as explained in how much it costs to finish a basement in Virginia. Permit questions for the finish itself are covered in do you need a permit to finish a basement in Virginia. To keep a basement from turning musty again, see how to keep a finished basement dry.

When to call a licensed pro

Call one when the smell is strong, when it returns after you dry the space, when you see staining or growth on walls or ceilings, when a leak or flood was left for more than a day or two, or when you cannot find the source. A moisture or mold specialist can trace the water, and a plumber, drainage contractor or waterproofing contractor can fix the cause.

In Virginia, the contractor classes are set by job size: Class C covers single jobs over $1,000 but under $30,000, Class B covers $30,000 up to but not including $150,000, and Class A covers $150,000 or more.^[4] The Board for Contractors requires a master tradesman licence as a condition of licensing electrical, plumbing, and heating, ventilation and air conditioning contractors.^[4] DPOR recommends getting at least three written bids on identical plans and specifications, so you can compare both the prices and the contractors.^[5] Ask each bidder to say what causes the moisture and how the fix addresses it, not only how the mold will be removed.

More background is in our remodeling answers and the remodeling services directory.

To have a pro look at the smell, describe the problem once and local contractors will respond.