Why Does My Basement Smell Musty?

A musty basement smell usually means something down there is damp and mold has started to grow on it. The EPA says a moldy odor suggests mold is growing in the building and should be investigated.

The smell often shows up before you can see any mold. If you find the damp spot and dry it out, you can stop small growth early.

What causes the musty smell in a basement?

Mold gives off gases as it grows. The EPA calls them microbial volatile organic compounds, and it says they can be the source of the musty smell that comes with mold growth.

Mildew is a kind of mold too, so a mildew smell points to the same thing: moisture that has not dried out. Basements often smell first because they are the coolest, dampest rooms in most homes, and the air down there barely moves.

Can a basement smell musty with no mold in sight?

Yes. The EPA notes that you may suspect hidden mold when a building smells moldy but you cannot see the source.

In a basement, our team checks these spots first:

• The back side of drywall on finished walls, low near the floor.
• The sill plate, where the wood framing meets the concrete foundation.
• Carpet pad that got wet once and dried slowly.
• Cardboard boxes stored flat on the concrete floor.
• The underside of the floor above, between the joists.

Hidden mold is also why air fresheners and candles do not fix a musty basement. They cover the smell for a day or two while the mold behind the wall spreads.

Where does basement moisture come from?

Check these sources first:

• Water through the foundation. Wet soil against the outside wall wicks moisture through the concrete, most of all in spring.
• Condensation. Warm, humid air hits cool concrete and cold water pipes and turns back into water. The EPA recommends covering cold surfaces, such as cold water pipes, with insulation.
• Slow plumbing leaks. Water heaters, washer hoses and drain lines can drip for a long time before anyone looks behind them.
• Window wells. A well that fills during a storm lets water over the sill and into the wall below it.
• A loose dryer vent. It blows warm, wet air straight into the room instead of outside.

Why do Front Range basements get musty?

Colorado air is dry most of the year, so a musty basement surprises a lot of people here. On the Front Range, the water often starts in the soil around the house.

Going back to 1882, March is Denver’s snowiest month, with about 11 inches on average, per the National Weather Service. April comes next with close to 9 inches, and all of that melt soaks the soil right against your foundation walls.

Our team also sees a lot of older Denver and Lakewood homes graded toward the house. On those lots, every storm sends water straight at the basement wall.

In summer, the basement is the coolest part of the house. Moisture from storms, showers and swamp coolers condenses first on cool concrete and cold pipes down there.

For one example, here is a Fort Collins basement job where mold had spread across two rooms. It grew on the drywall near the concrete floor and along the carpet tack strips.

How do I find the source of a musty smell?

1. Follow your nose. The smell is strongest near the source, so check corners, closets and the space under the stairs.
2. Look low. Check the bottom few inches of drywall, the baseboards and the carpet along outside walls.
3. Move stored boxes. Look at the bottoms of the boxes and the concrete under them.
4. Walk it after a storm. The day after a heavy rain or a big melt, look for dark, damp patches on the walls and floor.
5. Measure the humidity. A basic humidity meter from a hardware store shows how wet the air is. The EPA says to aim for indoor humidity below 60 percent, ideally between 30 and 50 percent.

What should I do about a musty basement?

Fix the water first. The EPA warns that if you clean up mold but do not fix the water problem, the mold will most likely come back.

Then dry anything that got wet. The EPA says to dry water-damaged areas and items within 24 to 48 hours to prevent mold growth.

For a small patch, under about 10 square feet, the EPA says most people can handle the cleanup themselves. Call for help if you notice any of these:

• The smell comes back after the area is dry.
• The mold covers more than about 10 square feet.
• You think the mold is inside a wall, under carpet or under the floor above.
• The basement had a leak or flood that was wet for more than two days.

Some wet drywall, insulation or carpet may have to come out. The EPA notes that absorbent materials, such as ceiling tiles and carpet, may have to be replaced once they grow mold.

If the smell will not go away, send us photos of the areas that worry you. We send back a number, usually the same business day. Our basement mold removal page shows where we find mold in Front Range basements and how we quote it.

Call (720) 443-4222 in the Denver area or (970) 469-9030 in Northern Colorado.