The Humidity Playbook: How to Keep Mold Out of Your Home

Mold prevention is moisture control. Everything else is secondary. This playbook gives you the numbers, the equipment and the habits.

The numbers

• Indoor relative humidity (RH): 30 to 50%. The EPA's guidance is to keep it below 60%, ideally 30 to 50%. Above ~50% dust mites thrive; above ~60 to 70% surfaces start supporting mold.
• Cold surfaces matter more than the room average. Air near a cold window, exterior wall corner or uninsulated duct can be 15 to 20°F cooler than the room, which pushes local surface RH far higher. That's where condensation and mold begin.
• Dew point is the temperature at which air saturates. If a surface is colder than the air's dew point, water condenses on it. In humid regions that happens on cold A/C ducts in hot attics.

Buy two things

1. A hygrometer (about $10 to $20). Put one in the main living area and one where you suspect trouble.
2. A properly sized dehumidifier if you're consistently above 50%. In humid climates a whole-home unit tied to the HVAC outperforms portable ones.

Habits that matter

• Run bath fans during showers and for ~20 minutes after, vented outdoors, not into the attic.
• Use the range hood when cooking, if it vents outside.
• Dry wet things quickly: mop up spills, hang towels, don't leave wet laundry in the washer.
• Fix leaks within 24 to 48 hours. The EPA's rule of thumb: porous materials dried within 24 to 48 hours usually don't grow mold.
• Keep A/C drain lines clear and change filters.
• Keep furniture a few inches off exterior walls so air can circulate.

Outside the house

• Grade soil away from the foundation, and keep gutters clear with downspouts discharging several feet away.
• Seal roof penetrations and windows. Look for stains after storms.
• In hot, humid climates, don't over-cool: setting A/C very low increases condensation on ducts and registers.

After a leak

1. Stop the water.
2. Remove standing water and wet porous items (carpet pad, insulation).
3. Dry with fans and a dehumidifier; confirm with a moisture meter.
4. Anything still damp after 48 hours: open it up and inspect.

When it's not DIY

Mold in walls or attics, anything over about 10 sq ft, flood-contaminated materials, or a recurring problem calls for a professional. In the Houston area, HTX Mold Removal handles remediation; see also the Houston humidity case study.