Visible growth is easy. This is about the other kind — inside a wall, above a ceiling, under a floor — and how to know it is there before it announces itself.
The smell, and what it means
The musty smell is real and it is diagnostic. It comes from microbial volatile organic compounds released by fungi as they grow, and there is no other common household source that produces the same thing.
A persistent musty smell with nothing visible almost always means growth somewhere you cannot see.
What is worth noticing about it:
When it is strongest. Worse when the air conditioning starts and fading as it runs points to the coil, the drain pan or the ductwork. Present continuously and worse in one room points to that room. Strongest in a closet or a corner points to an exterior wall condensation problem.
Whether you stop noticing it. People acclimatise to a smell in their own house within days. If visitors mention it and you cannot smell it, believe the visitors.
Where it comes up from. In a pier and beam house, a musty smell throughout usually originates in the crawl space and is carried up by the stack effect.
The six places it hides in a house here
1. Behind and under bathroom fittings
Under the sink at the back corner, behind the toilet at the flange, and inside the wall behind the shower valve. All are wet, dark and out of sight.
2. Inside the air handler
The evaporator coil and drain pan are cold and wet by design for most of the year here. A coil that has never been cleaned is the most likely source of a whole-house musty smell.
3. In the attic on the north slope
Bath fans discharging into the attic, and soffit vents blocked by insulation. Growth appears on the underside of the roof deck, usually worst on the slope that gets least sun.
4. Under the floor
Crawl spaces in older Bryan and rural property, and the belly wrap under manufactured homes, which holds leaked water against the floor decking indefinitely.
5. Behind furniture on exterior walls
A sofa or wardrobe pushed tight against an exterior wall stops air moving there. The wall surface runs colder, and in a humid house it condenses. Closets on exterior walls are the same problem with a door on it.
6. Under flooring on a slab
Where a slab was poured without a vapour retarder, moisture rises through the concrete continuously. Under vinyl, laminate or carpet it has nowhere to go.
Physical signs worth acting on
- Paint that bubbles or peels in a specific area, particularly low on a wall or on a ceiling
- Drywall that feels soft when pressed, or a wall that sounds different when tapped
- Baseboard pulling away from the wall, or trim that has swelled at the bottom
- Tiles that sound hollow, meaning they have debonded from a substrate that got wet
- Grout that stays dark long after the shower is dry
- Flooring that cups, lifts or feels spongy in one area
- A stain that reappears within weeks of being painted over
- Condensation on the inside of windows in winter, which indicates high indoor humidity generally
The measurement anyone can take
Buy a hygrometer. They cost very little and they answer a question nothing else does.
Put it in a hallway, away from a bathroom or kitchen, and read it over a week.
Consistently above 60% relative humidity means conditions throughout the house support growth on almost any surface, with no leak needed anywhere. That is a mechanical problem — usually an oversized air conditioner that short cycles and never dehumidifies — and it explains growth in several rooms at once far better than any hunt for a leak will.
Below 50% and you are looking for a specific water source rather than a general condition.
Symptoms that track the building
Worth taking seriously as a signal, without over-interpreting it.
If somebody in the household is consistently worse at home and better after a few days away, and returns to symptoms on coming back, that pattern is informative. It does not diagnose anything, and it is a reason to look at the building.
Conversely, symptoms that persist regardless of location point somewhere other than the house.
What to do before calling anyone
- Put a hygrometer in the hallway and read it for a week
- Open the cabinet under every sink and look at the back corner with a torch
- Look at the air handler drain pan for standing water
- Go into the attic and look at the underside of the roof deck, especially the shaded slope
- Pull furniture away from exterior walls and look behind it
- Note where and when the smell is strongest
That costs an hour and it very often locates the problem, or at least narrows it enough that an assessment goes straight to the right place.
When to get it assessed
A persistent smell with nothing visible after you have looked. Symptoms that track the building. A stain that keeps coming back. Any of it in a property you are buying, selling, or renting out.
And where you can see growth but cannot find the water — because remediation without the water source is temporary.
See mold inspection for what an assessment includes and mold testing for when sampling is worth its cost.
Get a free assessment
Tell us what you are seeing and where. We come out, find the moisture source, and put the scope and the price in writing before anything is torn out.