Something smells musty and you cannot see why
Find the water first. The smell is a symptom of moisture, not a problem in itself.
Is this an emergency?
Not an emergency, but don’t leave it a season.
You’re probably seeing
- A persistent musty or earthy smell with nothing visible
- The smell is stronger in one room, in a cupboard, or near the floor
- It gets worse in humid weather or when the HVAC runs
- Visible dark spotting on walls, ceilings, grout or window frames
- Symptoms that ease when people leave the house for a while
What it probably is
A slow, hidden plumbing leak
The most common hidden causeStrongest under a sink, behind a toilet, or in the cupboard backing onto a bathroom. Often accompanied by soft or discoloured cabinet base.
High humidity in a basement or crawl space
Very common, and a ventilation problem rather than a leakWhole-house or lower-level smell that tracks the weather. No single leak - the air itself is carrying too much moisture.
Poor bathroom ventilation
Very common and often cheaply fixedStrongest in or near a bathroom, worse after showers. Extractor fan absent, weak, or vented into the attic rather than outside.
A past leak that was never properly dried
Common, and the reason drying deadlines matterThe smell dates from a specific event - a burst pipe, a roof leak, an overflowing bath - that was mopped up rather than dried out.
Condensation on cold surfaces
Common in cooler monthsAround windows, on exterior walls, or along uninsulated ductwork. Seasonal, and worse where insulation is thin.
A dirty or wet HVAC system
Common in cooling seasonThe smell arrives with the air when the system starts and fades when it stops. A damp evaporator coil or a blocked condensate drain.
What to do before anyone arrives
Follow the smell to its strongest point
Go room by room, then cupboard by cupboard, then low to the floor. Musty smells are heavier near the source and near the ground. Open cabinets under sinks and check the base for soft or stained wood.
Look everywhere water is supposed to be, and everywhere it has been before
Under every sink, behind toilets, around the water heater, at window frames, in the basement or crawl space, and at any spot that has leaked in the past. A torch and ten minutes cover most of it.
Check whether the smell tracks the HVAC
If it arrives when the system starts and fades when it stops, the problem is in the system - typically a damp coil or a blocked condensate drain - rather than in the room.
Measure or at least judge the humidity
An inexpensive hygrometer settles a lot of arguments. Persistently high indoor humidity is a ventilation and dehumidification problem, and no amount of cleaning fixes it while the air stays damp.
Judge the size of any visible growth
The EPA guidance is that a mouldy area under about 10 square feet - roughly a three-foot square - is in most cases a job a homeowner can handle. Beyond that, it points towards professional remediation.
It also advises an N-95 respirator at minimum plus goggles and gloves, and says people with asthma or weakened immune systems, children and pregnant people should not take part in cleanup at all.
Fix the moisture before you clean anything
Cleaning growth without removing the water that caused it guarantees it returns, usually in the same place. If you cannot find the moisture source, that is the thing to pay someone to find.
This is the end of the homeowner sequence for anything beyond a small patch, anything inside a wall or duct, and anything where you cannot identify the water source.
What it costs to fix
| What it is | Whose job is it? | What decides your bill |
|---|---|---|
| Finding the moisture source | Yours first, then a pro if hidden | Whether it is visible or inside a wall |
| Cleaning under ~10 sq ft of growth | Yours, per EPA guidance | Cleaning materials and protective equipment |
| Fixing a leak | Plumber, roofer or HVAC | Access, and which trade it turns out to be |
| Improving ventilation or adding a dehumidifier | Electrician or HVAC | Often the cheapest permanent fix for a damp basement |
| Professional remediation | Restoration | Area, containment, and how much porous material is removed |
| Mould testing | Lab or inspector | Usually unnecessary - see the FAQ. Spend it on finding the water instead |
Thresholds and protective-equipment guidance from the EPA homeowner mould guide; cost drivers described qualitatively.
The most useful thing on this table is the last row. the EPA states that where visible mould growth is present, sampling is in most cases unnecessary - and that because no federal limits exist for mould or spores, a sample cannot be checked against any standard. Money spent identifying what kind of mould it is would almost always be better spent finding and stopping the water.
Can I fix this myself?
The EPA advises an N-95 respirator at minimum, plus goggles and gloves, for any mould cleanup, and states that people with asthma or weakened immune systems, along with children and pregnant people, should not take part in cleanup work at all. Do not mix bleach with ammonia or any other cleaner. And do not dry-brush or sand growth - it aerosolises exactly what you are trying to remove.
Reasonable to DIY
- Tracking the smell to its strongest point
- Checking under sinks, around toilets and at window frames
- Measuring indoor humidity
- Cleaning a patch under roughly 10 square feet, with protection
- Improving bathroom ventilation habits
Call a pro
- Any visible growth beyond about 10 square feet
- Anything inside a wall, duct or subfloor
- A smell with no findable moisture source
- Drying out a structure that is still wet
- Any situation where occupants have symptoms
This is the page where the honest answer is often that you do not need to buy anything yet. A small visible patch with an obvious cause - a bathroom with a weak fan, a cupboard under a dripping trap - is within EPA guidance for a homeowner to handle, and the permanent fix is the ventilation or the washer rather than the cleaning. Call someone when the patch is large, when it is inside something, or when you genuinely cannot find the water. That last one is the most common good reason.
Sources
Sources last checked
- Homeowner's and Renter's Guide to Mold Cleanup after DisastersU.S. Environmental Protection Agency (with HUD, FEMA, NIH and CDC)Retrieved 2026-09-14
- Is Sampling/Testing for Mold Necessary?U.S. Environmental Protection AgencyRetrieved 2026-09-14
- Mold Cleanup in Your HomeU.S. Environmental Protection AgencyRetrieved 2026-09-14
- Mold Course Chapter 2: Why and Where Mold GrowsU.S. Environmental Protection AgencyRetrieved 2026-09-14
Sources for the figures and the legal requirements cited on this page.
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