Musty Smell in the House: Find the Source by When It Smells

That damp-cardboard smell is almost always something growing on material that stays wet, and the pattern of when you notice it tells you where to look. A smell that follows rain points at water getting in from outside, one that shows up when the air conditioner runs points at the AC system, and one that hits you when you come home from a trip points at humidity building in a closed house. Find the moisture and you have found the source.

The existing guides cover the basement version and the smell that lingers after a known leak. This page is for the whole house, when you do not yet know which room or wall is the problem.

Why the Smell Comes and Goes

The odor comes from compounds that mold and bacteria give off while they are actively growing. They grow fastest when the material they sit on is wet and the air is warm. So a smell that comes and goes is not a sign the problem is small. It usually means the moisture feeding it comes and goes, and your nose is tracking the wet spells.

That is useful, because it turns a vague complaint into a clue.

The AC pattern deserves its own warning. If the smell arrives with the first blast of air from the vents, the system is moving air across something damp. It may be a clogged condensate line letting the pan overflow, or mold on the coil or inside the ducts. The air conditioner mold guide covers what a homeowner can clean and what needs a technician.

Musty Smell but No Mold You Can See

This is the version most people are searching for, and it is common. Mold does not need to be on a surface you can see. The places it grows first are the ones you do not look at: the back side of drywall behind a leaking supply line, the underside of carpet pad, the inside of a closet wall that shares a cold exterior corner, the insulation in the ceiling below a bathroom.

Not every musty smell is mold, though. A few other sources smell similar:

• A dry drain trap. A guest bath or floor drain that is never used lets sewer gas in. That smell is sharper and more sulfurous; run water into every drain for a minute and see if it fades.
• Mildewed fabric. Towels left damp, a gym bag, a load of laundry forgotten in the washer, or the washer's own door gasket.
• Old books, boxes and upholstered furniture stored somewhere humid.

If you have ruled those out and the smell keeps returning, assume moisture somewhere in the structure until a meter says otherwise. The mold smell guide describes the differences between mold, mildew and sewer odors in more detail.

How to Track It Down

Four steps, in this order, find the source in most houses without opening a single wall.

1. 1Note when the smell shows up. Keep a short log for a week or two: the time, the weather, whether the AC was running, whether someone had just showered. Match it against the table above. Two or three entries usually show the pattern.
2. 2Follow your nose to the strongest spot. Walk the house slowly after it has been closed up for a few hours, first thing in the morning is good. Open closet and cabinet doors and put your face near the floor, because the odor is heaviest low down. The spot where it is strongest is almost always within a few feet of the source.
3. 3Measure humidity and moisture. A hygrometer tells you whether the air is the problem: above 60 percent, mold can grow on surfaces all over the house; under 50 percent, it struggles. A moisture meter tells you whether a material is the problem. Press a pinless meter flat against drywall near the strongest spot and compare it with the same wall in a room that smells fine. A reading far higher than the comparison wall, or wood framing above 16 percent moisture content, means water is getting in there. The moisture meter guide explains how to read one.
4. 4Fix the moisture, then remove what grew. If the air was the cause, a dehumidifier and better airflow fix it. If a material reads wet, the leak behind it has to be found and stopped, the material dried or replaced, and any growth removed. Killing the smell without drying the source brings it back the next wet week.

Do not try to cover it. Air fresheners, candles, ozone machines and bleach sprayed on a wall all mask the odor while the growth continues underneath.

When It Is Time to Call Someone

A few signs mean this has moved beyond airing the house out:

• The meter reads wet in a wall, ceiling or floor, and you cannot find a leak you can fix yourself.
• You see staining, bubbling paint, or soft drywall near the strongest spot. The signs in the hidden water damage guide apply here.
• The smell follows rain, which means the building envelope is letting water in.
• Someone in the house has asthma, allergies or a weakened immune system and symptoms are worse at home. The CDC and EPA both list those groups as the people most affected by indoor mold.

Wet building material starts growing mold within 24–48 hours, so a wall that reads wet today will not dry and clean itself. A water damage and mold company can find the wet area with meters and thermal imaging, dry it, and remove what grew, and a contained job the size of one wall cavity is typically $1,500–$3,500. If you want that done, (888) 840-6512 is answered 24/7 and connects you with an IICRC-certified restoration company that serves your area; the company gives you an arrival window for your address on the call, and there is no charge for the referral.

If what you have is humidity rather than a leak, you do not need a restoration company. Hold the house under 50 percent, run the bathroom fans, keep a gap between furniture and outside walls, and the smell should fade within a couple of weeks.