Musty and sewer smells in the basement

Which smell is it?

If it might be gas

Natural gas has an odor added so leaks can be detected. It smells like rotten eggs, which is how many people describe sewer gas too. If you think it could be gas, PHMSA recommends that you:

• Don’t turn lights or other switches on or off, light anything, or use a phone near the smell
• Leave the area right away
• Call 911 from a safe place, away from the house
• Keep others out of the area

Telling them apart

A musty or moldy smell is more earthy. Sewer gas is stronger, more chemical, or like rotten eggs, though it doesn’t always smell exactly the same. When I smell sewer gas, I check the floor drains, traps, utility sink, sewage ejector, sump area and cleanouts. When I smell something musty, I look for moisture: behind stored things, in the carpet and drywall, at the rim joist, under the stairs and in the mechanical room. Your nose tells you something is wrong. It just may not tell you where.

Musty smells

If it smells musty and I can’t find any mold, I start moving things: boxes, furniture, the edges of the carpet. I look behind finished walls wherever there’s a hint, check for moisture around windows and exterior doors, check the HVAC system and its condensate line, and take moisture readings on the foundation walls and framing. I also ask when it smells. After it rains is one clue. Only in the summer is another. Only when the HVAC is running is another.

If the humidity is high, start with basement humidity. If you find mold, see mold in basements.

Sewer smells

What I find most often is a dry trap in a floor drain, a loose cleanout cap, a problem around the lid of a sewage ejector, or a leak at a plumbing connection. I can usually tell that it’s coming from the plumbing, but if a drain, vent, trap or sewer connection needs to be repaired, I call a plumber.

A simple test for a dry trap. If your floor drain hasn’t been used in a while, the water in the trap can evaporate and let sewer gas through. Pouring some water down the drain to refill the trap is a reasonable first check. You don’t have to pour a ton of water, just enough to refill the trap. Use it as a simple test, not as a permanent fix.

• If the smell comes back soon, it probably isn’t just evaporation. Have the trap primer, the cleanout, the venting and the rest of the drain system checked for a leak.
• Don’t pour bleach, drain cleaners or other chemicals down it to “kill the smell.”
• If the drain has a trap primer, a device that keeps water in the trap of drains that are rarely used, check that it’s working.

What doesn’t work

An air freshener won’t solve your problem. It just makes it smell different for a while. Once you’ve found and fixed the source, activated carbon and some odor absorbers can help, but I wouldn’t use them as the solution. I wouldn’t suggest an ozone machine to a homeowner to get rid of mold or a musty smell, especially in a home where people are living. I’ve been to homes that used one before calling us. You could smell the ozone, and the moisture problem was still there.

According to the EPA, no federal agency has approved ozone generators for use in occupied spaces, and ozone at levels within public health standards does not effectively remove mold or many odor-causing chemicals.

Who to call

• Possible gas leak: 911 from outside, then your gas company
• Sewer smell that doesn’t go away: a plumber
• Musty smell you can’t trace, or mold you can’t handle yourself: a moisture or mold professional (mold inspection and testing)
• Water coming in: see basement water