Three smells, three very different problems
- Musty or earthy
- Usually microbial growth on damp materials or moisture rising from soil. Slow-building, rarely urgent, but it will not leave on its own.
- Sewage or rotten eggs near a drain
- Often a dry trap, a loose cleanout cap or a failing sewer line. Unpleasant and worth fixing soon.
- Sulfur or rotten eggs near the furnace or water heater
- Possibly natural gas, which utilities treat with an added odorant. Leave and call for help from outside.
- Sharp, chemical or solvent
- Stored paint, fuel cans or cleaning products. Check storage shelves before assuming a moisture problem.
Musty odor compared with sewer gas
These two get confused more than any others in a lower level, especially in older Lakewood ranches where the laundry, floor drain and storage room share one space.
Musty or mildew
- Smells like a damp towel, wet cardboard or a closed-up cabin.
- Spread through a whole room or follows a wall, rather than one fixture.
- Usually stronger in summer and after rain, weaker in a dry January.
- Lingers in fabric: clothes stored downstairs pick it up.
Sewer gas
- Sharp, sour or rotten, sometimes with a sulfur edge.
- Strongest right at a floor drain, utility sink, unused shower or cleanout.
- Can appear suddenly, for example after a long trip or a hot dry spell.
- Often fades quickly once water is poured into the drain.
Which way to go next
Use your nose, then follow the matching path.
- IfYou smell rotten eggs near a gas appliance, or hear hissingDo not flip switches or use a phone indoors. Get everyone outside and call 911 or the gas utility's emergency line from a safe distance.
- IfThe smell is strongest at one drainPour a few quarts of water into it, wait a day, and see whether it fades. Read the drain trap guide if it comes back.
- IfThe whole lower level smells like damp cardboardStart a humidity log and walk the room-by-room source checklist before thinking about treatment.
- IfThe smell is earthy and strongest in rooms above a crawl spaceLook through the crawl space hatch for bare soil, standing water or fallen insulation.
- IfYou see sewage, or water is backing up from a drainStop using water in the house, keep people away from the area, and contact a plumber; sewage cleanup is a different job than odor work.
Why naming the smell matters
A homeowner who describes a sewer problem as musty may end up paying for deodorizing that cannot work, because the source is a plumbing gap that keeps venting. The reverse happens too: someone convinced the smell is a drain keeps pouring water down it while a carpet pad in the corner slowly grows mildew.
Write down three things before talking to anyone: where in the basement the smell is strongest, what time of year it began, and whether it changes with weather, laundry days or the furnace running. That short note does more to steer an inspection than any description of how bad the odor is.
Some basements have two smells at once. A 1960s Lakewood split-level, for instance, can have a dry trap in the laundry room and damp paneling in the family room. Treat them as separate problems, fix the plumbing first because it is usually quick, and then reassess what remains.
Questions to ask whoever inspects it
- Which smell do you think this is, and what makes you think so?
- Did you check every floor drain, cleanout and unused fixture?
- Will you measure humidity and moisture in materials, or only smell?
- If you suspect a plumbing problem, will you say so before quoting odor work?
- What would you expect to find if your first guess is wrong?
Common questions
- Can a musty basement smell like rotten eggs?
- Not usually. Mildew smells damp and stale. A rotten-egg or sulfur note points toward sewer gas or a gas leak, and the gas possibility should be ruled out first.
- Is radon the musty smell?
- No. Radon is a radioactive gas with no odor at all. It is common in Colorado, so testing is worthwhile, but it never explains a smell.
General information for Lakewood, not a quote or professional advice. In an emergency, call 911.