Free mold tool

Is your room a mold risk? Find out in 30 seconds.

Two sliders. One button. No signup. We show you the dew point and the exact next step.

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Move the sliders. See your risk instantly.

Indoor relative humidity 55%

You can read this from a $15 hygrometer or most smart thermostats.

Indoor temperature (F) 70 F

Room type

Healthy range

Risk score out of 100

Move the sliders to see your risk read.

Heads up: Educational guidance based on ASHRAE indoor air comfort ranges and standard psychrometric math. Not a medical, structural, or legal diagnosis. If you see active mold, odor, or anyone is symptomatic, bring in a licensed inspector.

Why humidity drives mold (the short version)+

Safe range: ASHRAE Standard 55 + EPA both target indoor humidity 30 to 50 percent. Above 60 percent, mold risk rises on porous surfaces.

Dew point: The temp at which water condenses on cold surfaces. If dew point is 60 F and window glass is 55 F, condensation forms every time. Wet surface becomes a mold farm in 24 to 48 hours.

Fix plan (5 steps): Dehumidifier at 45-55 percent RH · HVAC fan 30 min/hour · vent bath 20 min after showers · insulate cold pipes and windows · recheck in 48 hours.

FAQ

What humidity level prevents mold?

Keep indoor relative humidity between 30 and 50 percent. ASHRAE and the EPA both target this range for both comfort and mold prevention. Above 60 percent, mold growth risk rises on every porous surface.

Is 65 percent humidity too high?

Yes. Sustained indoor RH above 60 percent is the threshold where common household mold species (Aspergillus, Cladosporium, Penicillium) can colonize within days. 65 percent is firmly in the risk zone.

How do I measure indoor humidity accurately?

Use a calibrated digital hygrometer (around $15) placed away from sources of heat and cold (not next to a vent, not next to a window). Take readings at different times of day, the humidity is not constant.

What is the dew point, and why does it matter for mold?

The dew point is the temperature at which water vapor in the air condenses into liquid water. If any surface in your home drops below the dew point, condensation forms there, and mold follows within 24 to 48 hours.

Does running the AC reduce humidity?

Yes, air conditioning pulls water out of the air as it cools it. But only if it runs long enough. Short cycles cool the air without drying it. A properly sized AC or a dedicated dehumidifier works better than an oversized AC that cycles off quickly.