A musty smell means damp material somewhere in the building is supporting growth. It is a moisture finding rather than a cleaning problem, and it usually arrives before anything is visible, because the material is behind a finish, under a floor covering or inside a cavity.
That makes the smell useful. It is the earliest and cheapest signal a house gives, and the only mistake that costs real money is covering it up.
Narrowing it down without spending anything
- Walk the house and score each room from one to five for the smell, at the same time of day, twice: once in the morning and once in the evening.
- Note whether it changes when the heating or cooling runs. A smell that arrives with the system is pointing at the system or at something near the return.
- Note whether it changes after rain. A smell that follows weather is pointing at the roof, the flashing, the grading or ground water.
- Get low. Smell at floor level along exterior walls, at the base of a shower, under sinks and behind anything plumbed, because the source is usually near the floor.
- Put a hygrometer in the worst room and read it morning and evening for a week. The EPA advises keeping indoor humidity below 60 percent relative humidity, ideally between 30 and 50 percent.
A week of that produces a room, a time of day and a trigger, which is most of a diagnosis and costs the price of a hygrometer.
The usual sources, in the order they turn up
- A basement that runs humid through the summer, with contents and a rug on a cool slab. No leak, just air that never gets below the guidance.
- A slow leak under a sink or behind a vanity, wetting the back of a cabinet and the base of a stud bay.
- A shower or bath joint that has opened, letting water behind the surface into the framing.
- Ground water entering a basement through a wall or a slab joint, which is the everyday condition along the Passaic River corridor in Passaic County and in the border towns of Essex County and Bergen County.
- A roof or flashing failure, which appears as a ceiling stain feet from where the water got in.
- Carpet or a rug that got wet once and was dried by air, never to a measured standard.
- The heating and cooling system itself: a wet coil, a blocked condensate line, or affected material near the return.
The last one is why a smell can seem to be everywhere. Air is being moved through the house, so the strongest smell is not necessarily nearest the source.
What the smell is telling you about time
Growth needs moisture that has stayed. The EPA advises drying water damaged areas and items within 24 to 48 hours to prevent mold growth, so a smell means the window was missed somewhere, whether that was in a leak last month or in a spill that dried on top and stayed wet underneath.
What not to do
Do not fog, perfume or ozone the room. Ozone is a lung irritant and nothing is ozoned in an occupied home here. A fragrance removes the signal, which makes the source harder to find later, and it buys weeks and no fix.
Do not use bleach on wet building materials. The EPA does not recommend a chemical or biocide that kills organisms, chlorine bleach for example, as a routine practice during mold cleanup, and it warns never to mix a chlorine bleach solution with cleaners containing ammonia because toxic fumes can be produced.
What a homeowner can fix
- Run a dehumidifier where the readings sit above 60 percent, draining to a pump or a drain instead of a bucket.
- Lift rugs off cool slabs and move stored contents off the floor and away from exterior walls.
- Clear the gutters and extend the downspouts so they discharge away from the foundation.
- Run the bathroom exhaust fan during and after a shower, and check that it vents outside.
- Clean an area of visible growth smaller than about 10 square feet, roughly a 3 foot by 3 foot patch, which is the line the EPA gives for what a homeowner can handle in most cases.
When to call instead of working on it yourself
Call when the smell survives cleaning, when it came after a water event, when it is in or near the heating and cooling system, when the visible area is larger than that patch, when the water was sewage or floodwater, or when the same spot has come back after being cleaned once. Those are set out in full in When to get a mold inspection, and the signs that mean now.
What you buy at that point is a written finding: where the growth is and how large, the moisture readings and where they were taken, the water source, and a scope that another contractor could quote from the same document.
How the work ends
The affected porous material comes out, the surfaces around it are HEPA vacuumed and wiped, the assembly is dried with readings taken at set points each day, and the job closes with a written post remediation verification. Under ANSI/IICRC S520 the target is condition 1, a normal fungal ecology, which is what that verification is checked against.
If a smell survives a completed job, something is still wet or something was missed, and the moisture readings are where that gets settled. The method is on Mold Remediation, the drying side is on Water Damage Restoration, and the estimate is free and in writing before any work begins.
This article is about Mold Remediation.