Quick answer
Mold in an attic or crawl space is likely active if you have ongoing moisture and see soft, fuzzy, or slimy growth with a musty odor. Confirm with measurements: if wood moisture content is above ~16–20% and relative humidity is sustained above 60%, treat the mold as active. If the affected area is larger than 10 sq ft, insulation is contaminated, or you can’t correct the moisture source, call a professional remediation company.
How to tell if mold is active vs. old staining
Active mold needs moisture. Visual clues help, but measurements make the call.
- Visual signs of active growth:
- Fuzzy, velvety, or slimy texture; can smear when wiped.
- Expanding blotches over weeks, especially on the underside of roof sheathing, joists, or sill beams.
- Fresh white/gray/green growth on recently installed lumber or new insulation.
- Musty, earthy odor that’s stronger in the space than outdoors.
- Signs of old/dormant issues or non-mold staining:
- Flat, dry discoloration that doesn’t smear (e.g., soot, rust bleed, sap stain).
- “Shadowing” on sheathing from past moisture but wood is now dry.
- Old nail bleed or historic water marks without current dampness.
Measure moisture to confirm
Use affordable tools to remove the guesswork:
- Pin-type wood moisture meter (joists, sheathing, sill plates)
- Hygrometer/data logger (relative humidity and temperature)
- Infrared thermometer (find cold spots/condensation risk)
- Bright flashlight and inspection mirror
Target values:
Attic RH: ideally < 60% (short peaks in winter are common)
Crawl space RH: 45–55% maintained
Wood moisture content (MC): < 15–16% is safe; 16–20% = risk; >20% supports growth
Dew point margin: keep surface temps >3°C / 5°F above dew point
Ventilation (attic): ~1:300 NFA ratio with a continuous air barrier; 1:150 if no vapor retarder
If you measure high RH and wood MC above ~16%, assume active mold until you correct moisture and re-test dry.
Step-by-step inspection checklist (60–90 minutes)
- Safety first
- Wear an N95 or, better, a P100 respirator, goggles, nitrile gloves, long sleeves; use knee pads in crawl spaces.
- In attics, step only on joists or a secured catwalk; watch for protruding nails and live wiring.
- Visual survey
- Attic: check underside of roof sheathing near the eaves, around bath fan ducts, and below roof penetrations.
- Crawl space: inspect sill plates, joists, band/rim joists, and around plumbing lines.
- Odor and swipe test
- Musty smell supports active growth. A light white-towel swipe that smears suggests fresh growth.
- Measure
- Take 6–10 wood MC readings across suspect and control areas.
- Log RH/temperature; identify condensation risks with an IR thermometer.
- Look for moisture sources
- Attic: roof leaks, blocked soffits, bath/dryer vents dumping into attic, air leaks from house (can lights, hatches), missing baffles.
- Crawl space: bare soil without a vapor barrier, standing water, downspouts discharging at foundation, unsealed vents, plumbing leaks.
- Document
- Photos and readings today; re-check in 2–4 weeks after any fixes to confirm drying.
Fix moisture first (the real “remediation”)
- Attic:
- Air seal ceiling penetrations (caulk/foam around can lights, top plates, bath fan housings). Weatherstrip the hatch.
- Vent bath and dryer ducts outdoors with sealed, insulated ducting. Costs: $75–$250 in parts.
- Clear soffit vents and add baffles; verify continuous ridge/soffit airflow. Baffles: $2–$5 each.
- Add/repair roof underlayment and flashing if leaks exist.
- Crawl space:
- Install 6–10 mil poly vapor barrier sealed to walls and piers; tape seams (expect $0.50–$1.50/sq ft DIY materials).
- Manage drainage: extend downspouts 6–10 ft, adjust grading; sump pump if needed.
- Consider conditioning or dehumidification to maintain 45–55% RH (unit: $250–$600).
Cleaning small areas yourself (<10 sq ft)
If moisture is corrected and growth is localized:
- Tools/materials
- P100 respirator, goggles, nitrile gloves, disposable coveralls
- HEPA shop vac, soft nylon brush, pump sprayer, microfiber cloths
- EPA-registered cleaner (quaternary ammonium or botanical disinfectant) and/or 6–8% hydrogen peroxide cleaner
- Borate solution (e.g., DOT-based) for wood after cleaning (optional inhibitor)
- Steps (2–4 hours)
1. Lightly mist the area to minimize dust.
2. HEPA vacuum growth and adjacent surfaces.
3. Apply cleaner per label, dwell 10–15 minutes; agitate with brush.
4. Wipe and repeat as needed. Peroxide-based stain removers can reduce discoloration but aren’t the “fix.”
5. Allow to dry thoroughly; use fans/dehumidifier if needed.
6. Apply borate to bare, dry wood per label for future resistance.
- Tips
- Don’t use bleach on raw wood—it doesn’t penetrate well and adds moisture.
- Don’t paint over mold. If you later choose a coating, apply only after verified dryness and cleaning (many pros use pigmented shellac or perm-rated coatings on dry, cleaned wood).
Common mistakes
- Adding roof vents without air sealing the ceiling below (pulls more indoor moisture into the attic)
- Blocking soffit vents with insulation
- Venting bath/dryer into the attic or crawl space
- Using a standard vacuum (spreads spores). Use HEPA only.
- Skipping a continuous, sealed vapor barrier on crawl space soil
- Assuming color = toxicity. Only a lab can speciate; the fix is still moisture and cleanup.
When to call a professional
- Area > 10 sq ft of visible growth (EPA guideline)
- Recurring growth despite your moisture fixes
- Insulation is contaminated and needs removal/replacement
- Structural wood with MC > 20% that won’t dry quickly
- You smell strong musty odors upstairs or suspect HVAC contamination
- Anyone in the home is immunocompromised or has severe respiratory issues
Typical costs: attic remediation $1,500–$4,000; crawl space remediation $2,000–$6,000; crawl space encapsulation $3,000–$8,000. A moisture/mold assessment with a written plan runs $200–$600 and is money well spent if readings are high.
Bottom line
Active mold is less about what it looks like and more about moisture conditions. If RH and wood MC are high, treat it as active, stop the moisture, and clean appropriately. For larger or recurring problems, bring in a pro who can measure, remediate, and verify dry-out with instruments.