Short answer
Track the source first, then fix it at the cause. Musty smells usually come from moisture and microbial growth (mold/mildew) in basements, bathrooms, crawlspaces, or HVAC systems. Sewage-like odors usually come from plumbing issues—dry P-traps, a failed toilet wax ring, a blocked vent stack, or a sewer/septic problem. Verify which category you have, correct the moisture or plumbing fault, clean and sanitize, and prevent recurrence with ventilation and maintenance.
How to identify the odor source
Musty (earthy, damp)
- Common sources: wet drywall or carpet, leaky foundations, crawlspaces without vapor barriers, HVAC condensate pans/lines, and damp basements.
- Clues: odor intensifies after rain or AC use, visible spotting on walls/ceilings, elevated humidity.
Sewage-like (rotten eggs, sulfur, fecal)
- Common sources: dry P-traps in little-used drains, loose/failed toilet wax ring, clogged/blocked roof vent stack, cracked drain lines, septic backups, or bacterial growth in hot water systems.
- Clues: gurgling drains, odor strongest near specific fixtures, smell stronger with HVAC running (drawing sewer gas from a leak), sulfur odor only on hot water taps.
Step-by-step checklist
1) Quick tests (15–30 minutes)
- Rehydrate P-traps: Run water for 15–30 seconds in every sink, shower, tub, and floor drain. Add 1–2 tablespoons of mineral oil to infrequently used drains to slow evaporation.
- Cover suspect drains with plastic wrap and painter’s tape for an hour. If the room odor drops, you likely have a sewer-gas source at that drain.
- Hot vs cold water test: If sulfur smell appears only on hot water, the water heater anode may be reacting—see below.
- Sniff HVAC: If odor is strongest from supply registers, check the air handler, filter, and condensate system.
2) Moisture/musty track (2–6 hours, plus drying time)
- Find damp areas with a moisture meter and a hygrometer.
- Fix the source: improve grading and gutters, seal foundation cracks, repair leaks, and add a 6–10 mil vapor barrier in crawlspaces with sealed seams.
- Dry it out: run a dehumidifier until RH is stable.
Target indoor RH: 30–50%
Basement/crawlspace RH: ≤ 55%
- Clean mold on hard, non-porous surfaces with detergent and water. For non-porous disinfection:
Bleach solution: 1 cup household bleach to 1 gallon water
Ventilate. Do not mix with ammonia or acids.
- Remove and discard moldy porous materials (soaked carpet pad, ceiling tiles, drywall with visible growth) if affected area is >10 sq ft or material is degraded.
- HVAC: replace filter, clean evaporator drain pan, and clear condensate line with a wet/dry vac from the exterior outlet. Add pan tablets seasonally. Consider professional cleaning if there’s visible mold inside ducts.
3) Plumbing/sewer-gas track (1–4 hours)
- Dry traps: Top up traps and install trap primers for floor drains if they routinely dry out.
- Toilets: If the toilet rocks or there’s staining/odor at the base, replace the wax ring and secure the closet bolts. Leave a small gap uncaulked at the back to reveal future leaks.
- Vents: From the roof, inspect the vent stack for nests/debris. Clearing from the roof is risky—consider a pro. Indoors, listen for gurgling after flushing; it can signal blocked venting.
- Main line issues: Multiple slow drains and sewage odor suggest partial blockage—snake or hydrojet the main. A camera inspection can reveal cracks or offsets.
4) Hot-water sulfur odor fix (1–2 hours)
- Temporarily raise water heater to 140°F for several hours and flush to reduce bacterial growth. Then return to a safe setpoint.
- Replace a magnesium anode with an aluminum-zinc alloy anode if odors persist (check manufacturer guidance).
Water heater setpoints:
Normal: 120°F (safe for scald prevention)
Odor treatment/pasteurization: ~140°F (short term)
Tools and materials
- Flashlight/headlamp, inspection mirror
- Hygrometer and pin/pinless moisture meter
- N95 respirator, gloves, safety glasses
- Utility knife, pry bar (for suspect trim/drywall)
- Wet/dry vacuum (for condensate/drain clearing)
- Plumber’s test plugs, adjustable wrench, screwdriver set
- Dehumidifier (50–70 pint), fan/air mover
- Detergent, EPA-registered disinfectant or bleach, rags/brushes
- Mineral oil (for traps), bio-enzymatic drain cleaner
- Replacement toilet wax ring and closet bolts
Approximate costs: hygrometer $10–20; moisture meter $30–60; dehumidifier $200–400; wax ring $5–10; enzyme drain cleaner $10–20; main line snaking $150–400; camera inspection $300–800; plumber smoke test $200–500; anode rod $50–150.
Safety
- Wear an N95 and gloves when cleaning mold; isolate the area and ventilate.
- Never mix bleach with ammonia or acids.
- Sewer gas contains methane and hydrogen sulfide—ventilate well, avoid open flames, and exit if you feel dizzy or nauseated.
- Roof work is hazardous; use fall protection or hire a pro.
- Set water heater back to 120°F after odor treatment to reduce scald risk.
Tips for best results
- Use continuous bath fans on timers/humidity sensors; run 20–30 minutes after showers.
- Install a vapor barrier in crawlspaces and seal vents if converting to a conditioned crawlspace.
- Add a small amount of mineral oil after refilling rarely used traps (guest baths, floor drains).
- Schedule annual HVAC service to keep coils and drains clean.
Common mistakes
- Masking smells with fragrances instead of fixing the source.
- Bleaching porous materials instead of removing and replacing them.
- Forgetting floor drains and guest-bath traps—common sewer-gas culprits.
- Ignoring gurgling drains and recurring backups, which often precede bigger failures.
When to call a professional
- Visible mold area larger than about 10 sq ft, or recurring mold after fixes.
- Strong sewer odor with multiple slow drains or backups.
- Suspected cracked/offset drain lines, or if odors worsen when the HVAC runs (possible hidden leak).
- Roof vent clearing, smoke testing of plumbing vents, or camera inspections.
- Septic system alarms, wet areas over the drain field, or frequent pump-outs.
Address the source methodically—moisture control for mustiness, and trap/vent/sewer integrity for sewage odors—and the smell will go away and stay away.