Short answer
Persistent musty or chemical smells in older homes usually come from moisture and mold/mildew, HVAC biofilm or dirty components, sewer gas from dry or leaking traps, off‑gassing from old finishes/adhesives or mothballs, old carpet/smoke residues, dead pests, or overheating electrical components. Fixes focus on finding the source, controlling moisture (keep RH 30–50%), cleaning or removing contaminated materials, sealing/encapsulating surfaces, maintaining the HVAC system, restoring plumbing seals, and improving ventilation and filtration.
What’s causing the smell? (and how to confirm)
- Mold/mildew from damp basements, crawlspaces, or bathrooms
- Clues: Earthy smell strongest near foundation, after rain, or in closets. Visible staining, efflorescence, or high humidity.
- Confirm: Use a hygrometer (aim for 30–50% RH) and a moisture meter on wood/subfloors.
- HVAC issues ("dirty sock" odor, sour or musty air)
- Clues: Smell strongest when AC or heat runs. Dirty filter, slimy coil, or standing water in condensate pan.
- Confirm: Inspect filter, evaporator coil, and drain pan for biofilm/debris; check ducts near returns for dust buildup.
- Sewer gas (rotten egg/organic smell)
- Clues: Smell near floor drains, unused fixtures, or around toilets.
- Confirm: Dry P-traps, loose toilet (bad wax ring), cracked vent, or blocked vent stack.
- Chemical/VOC sources (oil-based finishes, adhesives, mothballs, formaldehyde in old particleboard, cigarette smoke)
- Clues: Sharp, solvent-like, sweet, or mothball odor. Stronger in closed rooms, closets, or from specific furniture/cabinets/floors.
- Confirm: Close a suspect room for a few hours; if odor spikes on opening, you’ve localized the source.
- Dead pests or nesting (rodents in walls/attic)
- Clues: Putrid, sweet odor localized to one area; often with droppings or insulation disturbance.
- Electrical overheating (fishy or hot/bakelite smell)
- Clues: Localized to outlets/switches, panels, or fixtures, worse under load.
- Confirm: Warm cover plates, discoloration. Turn off circuit and call an electrician.
Step-by-step: Track down and fix
1) Map the odor
- Walk room-to-room, sniff low/high, near returns, drains, closets. Note times it’s strongest (rain, HVAC on, mornings).
2) Check moisture first
- Basement/crawlspace: Look for water intrusion, damp soil, musty insulation.
- Bathrooms/laundry: Fan airflow test (tissue should stick to grille). Dry around washers and tub surrounds.
- Fixes:
- Add/resize gutters and extend downspouts 5–10 ft.
- Grade soil to slope away from foundation (~6 in over 10 ft).
- Install a 6–10 mil vapor barrier in crawlspace; seal seams and piers; consider encapsulation + dehumidifier.
- Run a 50–70‑pint dehumidifier to maintain RH 30–50%.
- Address leaks and replace water-damaged drywall/insulation.
Target indoor RH: 30–50%
Bathroom/kitchen ventilation: ≥ 50–100 CFM to exterior
HVAC condensate drain slope: 1/8–1/4 in per foot
Downspout extension: 5–10 ft from foundation
3) Service the HVAC
- Replace filter (MERV 11–13 if system allows).
- Clean evaporator coil and drain pan with EPA-registered coil cleaner; clear the drain and add a proper trap.
- Sanitize return plenum; seal large duct leaks with mastic (not duct tape).
- If odor persists or ducts are contaminated with debris/rodent droppings, schedule professional duct cleaning.
4) Eliminate sewer gas sources
- Fill dry traps: Run water in unused fixtures; add 1–2 tbsp mineral oil to slow evaporation.
- Re-seat toilets with a new wax/seal if wobbling or if odor around the base.
- Check and cap open/abandoned drains; ensure vent stack is unobstructed.
- If smells persist, a plumber can smoke-test the system to find hidden leaks.
5) Remove or seal odor-holding materials
- Carpets/pads: If musty or smoke-soaked, removal is usually the cure. Seal subfloor with a shellac-based odor-blocking primer.
- Smoke and old finishes: Clean with TSP substitute or alkaline cleaner; then prime walls/ceilings with shellac-based primer (excellent odor blocker) before repainting.
- Mothballs: Remove all sources (naphthalene/paradichlorobenzene), ventilate, and use activated carbon.
- Urine odors: Enzyme cleaner on subfloors; then seal with odor-blocking primer.
6) Improve ventilation and filtration
- Run continuous low-speed ventilation or schedule window airing on dry days.
- Use a portable HEPA air cleaner with activated carbon in affected rooms.
7) Pests and dead animals
- Inspect attic, crawl, and voids; remove carcasses with PPE; replace contaminated insulation; sanitize surfaces.
Tools and materials
- Hygrometer and pin/pinless moisture meter
- Headlamp, knee pads, respirator (N95/P100), nitrile gloves, safety glasses
- Wet/dry HEPA shop vac, scrub brushes, microfiber cloths
- Dehumidifier (50–70 pint), box fan/air mover
- EPA-registered coil cleaner, enzyme cleaner, TSP substitute
- Shellac-based odor-blocking primer and compatible topcoat
- MERV 11–13 HVAC filters, duct mastic, condensate tablets, PVC for trap
- 6–10 mil polyethylene vapor barrier, butyl tape, mechanical fasteners
Safety
- Wear respiratory protection and gloves for mold, rodent, or chemical cleanup.
- Don’t use bleach on porous materials; it leaves water behind and can worsen mold.
- Avoid ozone generators in occupied spaces; they can damage lungs and materials.
- Electrical “fishy” or hot smells: shut off the circuit and call an electrician.
- Mold larger than ~10 sq ft or strong sewage odors inside walls warrant a pro.
Tips for best results
- Tackle moisture and HVAC first; many odors resolve once humidity and air handling are fixed.
- Keep a smell log and humidity readings for a week to spot patterns.
- Encapsulate smelly but sound surfaces with shellac-based primer; it blocks more odors than typical stain-blockers.
- Use mineral oil in seldom-used traps seasonally.
Common mistakes
- Masking odors with fragrances instead of fixing the source.
- Running a dehumidifier in a leaky crawlspace without a vapor barrier.
- Closing foundation vents without drainage and vapor control in place.
- Skipping the condensate trap or leaving the drain un-sloped.
When to call a professional
- Persistent musty odor with visible mold >10 sq ft, or if household members have respiratory symptoms.
- Recurring sewer gas after basic trap fixes (plumber for smoke test and vent repair).
- Strong chemical smells from unknown materials, or suspected formaldehyde off-gassing in large areas (indoor air quality specialist).
- Electrical odors or warm/discolored outlets (licensed electrician).
- Suspected dead animals in inaccessible cavities (wildlife control/remediation).
Rough cost/time
- Dehumidifier: $200–350; setup 1–2 hours.
- Coil cleaning and drain service (DIY): $20–60 materials; 1–2 hours.
- Shellac-based primer + repaint: $35–60/gal; 1–2 days for a room.
- Carpet removal and subfloor sealing: $2–4/sq ft DIY; 1 day per room.
- Plumber smoke test or toilet reset: $150–400.
Address the biggest source first—usually moisture—then work down the list. With a methodical approach, most stubborn odors can be traced and resolved without guesswork.