The Critical 60% Threshold
Mold requires moisture to grow. The EPA and ASHRAE recommend maintaining indoor relative humidity below 60% to prevent mold growth. Ideally, indoor humidity should be kept between 30-50% for both comfort and mold prevention.
The 60% Rule: Keep indoor relative humidity below 60% consistently. Above this threshold, mold can begin growing on susceptible surfaces within days.
| RH Level | Mold Risk | Comfort |
|---|---|---|
| Below 30% | Very Low | Too dry (irritation) |
| 30-50% | Low | Optimal |
| 50-60% | Moderate | Acceptable |
| Above 60% | High — mold growth likely | Uncomfortable |
Understanding Humidity Measurements
Relative Humidity (RH)
Percentage of moisture the air holds compared to its maximum capacity at current temperature. RH changes with temperature — colder air holds less moisture, so the same water vapor produces higher RH. Target: 30-50%, maximum 60%.
Dew Point
The temperature at which air becomes saturated and water vapor condenses into liquid. Any surface cooler than the dew point will “sweat.” A dew point above 60°F generally feels humid; above 70°F is oppressive.
Material Moisture Content
Percentage of water weight in building materials (drywall, wood, etc.). Measured with moisture meters. Elevated readings indicate water intrusion. Wood above 19% MC is at high risk for mold growth.
Condensation: The Hidden Problem
Even in environments where overall indoor RH is acceptable, condensation can create mold conditions on cold surfaces. This is one of the most commonly overlooked moisture problems.
- Cold windows — especially single-pane or failed double-pane (most common)
- Cold water pipes — uninsulated supply lines in walls or crawl spaces
- Attic sheathing — warm moist air from living space condensing on cold roof deck
- Exterior wall corners — thermal bridging creates cold spots where mold appears first
- Behind furniture on exterior walls — blocked airflow lets wall get cold
- HVAC supply registers — cold air creating condensation in humid summer months
Diagnostic Tip: If mold appears in corners, around windows, or on exterior walls — condensation is the likely cause, even if overall indoor humidity seems normal.
Monitoring Indoor Humidity
Hygrometers
Inexpensive digital hygrometers ($10-30) can monitor indoor relative humidity. Place in multiple locations — living spaces, basement/crawl space, near any problem areas. Consider data-logging models that track humidity over time.
Warning Signs
- RH consistently above 60%
- RH spikes during cooking/bathing without rapid return to normal
- Basement/crawl space humidity higher than living space
- Condensation on windows in winter (indicates too much indoor moisture)
- Significant RH difference between rooms
Humidity Control Strategies
Exhaust Ventilation
Bath fans and kitchen hoods remove moisture at the source. Ensure they vent outdoors (not into attic). Run bath fans during showers and 15-20 minutes after. Size kitchen hoods appropriately for range output.
Dehumidification
Standalone dehumidifiers for basements or problem areas. Whole-house dehumidifiers integrate with HVAC. Size appropriately — undersized units run constantly without achieving target RH.
Air Conditioning
A/C removes moisture while cooling (the water dripping from condensate line is indoor moisture). Properly sized units run longer cycles and dehumidify better than oversized units that short-cycle.
Oklahoma Humidity Challenges
Humid Summers
Oklahoma summers often reach 70-80% outdoor RH. Without consistent A/C operation, this drives indoor humidity above 60%. Running A/C is essential not just for comfort but for mold prevention.
Transitional Seasons
Spring and fall bring wide temperature swings — hot days followed by cool nights. This promotes condensation on building surfaces as indoor/outdoor temperature differentials change rapidly.
Crawl Spaces
Many Oklahoma homes have vented crawl spaces that draw in humid outdoor air during summer. Ground moisture evaporates into crawl air, often reaching 80%+ RH. Encapsulation and dehumidification may be needed.
Research & Citations
- EPA. “Mold Resources: Moisture Control.” EPA.gov.
- ASHRAE Standard 62.2: Ventilation and Acceptable Indoor Air Quality.
- DOE Building America. “Humidity Control Strategies.”
- Lstiburek, Joseph. “Humidity Control Needs to be Fixed.” Building Science Corporation.
- Home Energy Magazine. “Humidity, Condensation, and Mold.”