How do I diagnose and fix a persistent musty or sewage smell in my home?

Short answer

A persistent musty smell usually points to hidden moisture and mold. A sewage smell typically means sewer gas is escaping due to a dry or faulty trap, vent blockage, a loose cleanout, a bad toilet wax ring, or a problem with a sump/ejector/septic system. You can often diagnose and fix the source in a weekend with basic tools: locate the source, address moisture or plumbing issues, clean and dry affected areas, and verify proper ventilation and trap seals.

What the smell is telling you

• Musty/earthy: Moisture problem causing mold or mildew, often from leaks, high humidity, wet materials, or HVAC condensate issues.
• Sewage/rotten: Sewer gas entering the home through dried P-traps, failed toilet wax rings, blocked vent stacks, loose cleanouts, or cracked lines/ejector lids.
• Sulfur/rotten egg only on hot water: Often a water heater anode reaction; different fix (see below).

Step-by-step: Track it down and fix it

1) Start with quick checks (15–30 minutes)

• Walk room to room with a flashlight and sniffer test at floor drains, toilets, under sinks, laundry standpipes, sump/ejector pits, and near HVAC returns.
• Tape a sticky note over suspect drains; if the note flutters, air is moving through the drain (possible dry trap).
• Note if odor is worse after running water, during windy days, or after heavy rain (points to venting/drain issues).

2) If it smells musty: moisture and mold path

Tools: flashlight, hygrometer, moisture meter, screwdriver, utility knife, fan/dehumidifier, PPE (gloves, N95, goggles)

• Measure humidity: Aim for 30–50% RH indoors. If >60%, run a dehumidifier.
• Inspect for leaks (30–60 min):
  • Under sinks, behind toilets, around tubs/showers, dishwasher, fridge ice line, washing machine hoses, water heater. Use a paper towel to detect damp fittings.
  • Check basements/crawlspaces for damp walls or musty carpet. Probe baseboards/drywall with a pin moisture meter.
  • Look in the attic for roof leaks and verify bath fan ducts terminate outdoors (not into the attic).
• HVAC condensate (20–40 min):
  • Clear a clogged AC/furnace condensate line with a wet/dry vac at the exterior outlet, then flush with 1 cup white vinegar.
  • Confirm the condensate trap has water (a dry trap can let air/odors in).
• Dry and clean:
  • Run fans/dehumidifier 24–48 hours. Remove and discard soaked carpet pads or badly moldy ceiling tiles.
  • Clean small mold areas (<10 sq ft) with detergent and water. Avoid straight bleach on porous materials; it doesn’t penetrate well.
• Prevent:
  • Use bath fans during showers and for 20 minutes after. Fix gutter/downspout issues and slope soil away from the foundation.

3) If it smells like sewage: plumbing/vent path

Tools: bucket, adjustable wrench, plumber’s tape, toilet wrench, closet bolts/wax ring, flashlight, ladder, drain brush/hand auger, silicone, mineral oil

• Refill all traps (10 min): Run water in every sink/tub/shower for 10–15 seconds; pour 1–2 quarts into rarely used floor drains.
  • Add ~1/2 cup mineral oil to seldom-used traps to slow evaporation.
• Toilet wobble or stain = replace wax ring (1–2 hours, $5–$15 parts):
  1. Shut off water, drain tank/bowl.
  2. Remove caps and closet bolts, lift toilet onto cardboard.
  3. Scrape off old wax, inspect flange, install new wax or wax-free ring, new bolts.
  4. Reset toilet, snug nuts evenly, reconnect supply, test, then caulk the front/sides at the base.
• Check P-traps and drains:
  • Ensure every fixture has a P-trap (no S-traps). Replace cracked or improperly sloped traps. Where code allows, use an AAV (air admittance valve) if venting is inadequate.
• Vent stack blockage (30–60 min):
  • From the roof, carefully inspect the vent opening for leaves, bird nests, frost. Clear debris with a hand auger and flush with a garden hose (avoid overfilling). Use ladder and roof safety gear.
• Cleanouts and caps:
  • Tighten or re-tape loose cleanout caps with PTFE tape. Replace missing gaskets on sewage ejector pit lids; seal penetrations with silicone.
• Septic/yard clues:
  • Wet spots or strong odor in the yard near the sewer line or tank suggests a line break or full tank—call a pro.

Targets and specs
- Indoor RH: 30–50%
- Bathroom fan sizing: ~1 CFM per sq ft (min 50 CFM)
- Trap seal depth: ~2–4 inches of water
- Drain slope: ~1/4 inch per foot

Special case: sulfur smell only on hot water

• Likely interaction between water chemistry and the water heater’s magnesium anode producing H2S. Options: replace with an aluminum-zinc anode, disinfect/flush the tank, or install treatment. Budget $40–$120 for an anode plus labor if you don’t DIY.

Tools and materials

• Flashlight/headlamp, hygrometer, moisture meter
• Adjustable wrench, pliers, screwdriver set, utility knife
• Toilet wax or wax-free ring, closet bolts, caulk
• PTFE thread tape, silicone sealant
• Drain brush/hand auger, wet/dry vacuum
• Dehumidifier, box fans
• PPE: gloves, N95, eye protection

Safety

• Wear PPE for mold cleanup; avoid mixing cleaners (never mix bleach and ammonia).
• Roof work: use a stable ladder, fall protection; avoid wet/icy roofs.
• If you suspect a natural gas leak (sulfur/rotten egg from gas line), leave the home and call your utility—don’t troubleshoot.
• Shut power at the breaker when working near wet areas or condensate pans.

Tips for best results

• Mark dates on rarely used drains and top off traps monthly.
• Add mineral oil to floor drains in guest baths or basements.
• Run bath fans on a timer for 20 minutes post-shower.
• Consider a trap primer for basement floor drains that dry out (parts $50–$150 plus installation).

Common mistakes

• Masking odors with fragrances instead of fixing the source.
• Ignoring a slight toilet wobble—often the culprit.
• Overusing harsh drain chemicals, which can damage pipes and traps.
• Leaving bath fans venting into the attic.

When to call a professional

• Strong sewage odor throughout the house, frequent gurgling drains, or recurring dry traps.
• Evidence of a broken or offset sewer line, or persistent yard wet spots.
• You can’t clear a roof vent safely.
• Large mold areas (>10 sq ft) or moisture you can’t dry within 48 hours.
• Need a smoke test or camera inspection (typically $200–$500) to locate hidden leaks/vent issues.

With a systematic check of moisture sources and plumbing traps/vents, most musty or sewer odors can be eliminated quickly and affordably.