| Scenario | Area | Height | Indoor | Outdoor | Moisture sources | Output (example) |
|---|---|---|---|---|---|---|
| Basement drying after minor leak | 800 ft² | 8 ft | 75°F / 55% RH | 85°F / 55% RH | 3 people, +800 g/hr, light wet materials | CFM target, ACH, fan size, risk indicator |
| Office fit-out with curing materials | 200 m² | 2.7 m | 24°C / 55% RH | 30°C / 60% RH | 12 people, +2000 g/hr, moderate wet materials | L/s target, ACH, condensation warning if applicable |
- Humidity ratio (W): W = 0.62198 × Pv / (P − Pv), where Pv = RH × Psat(T).
- Saturation vapor pressure: Psat(T) is approximated from temperature using a standard Magnus/Tetens form.
- Ventilation moisture removal: G ≈ ρ × Q × 60 × (Win − Wout).
- Required flow: Q = G / (ρ × 60 × ΔW), then adjusted for runtime and safety factor.
- ACH: ACH = (Q × 60) / Volume.
- Dew point: computed from temperature and RH to check surface condensation risk.
- Enter area and ceiling height for the zone you need to protect.
- Set your target RH, then enter typical outdoor temperature and RH.
- Choose activity level and add extra moisture sources from the project.
- Select wet materials level if the site is drying or curing.
- Add dehumidifier capacity and existing airflow, if measured.
- Click Calculate and review airflow, ACH, and warnings.
- Download CSV/PDF for records and commissioning documentation.
Moisture balance drives required outdoor air
Ventilation is sized to remove water vapor, not just to dilute odors. This calculator estimates moisture generation from occupants and additional sources, then compares it with the drying potential of outdoor air using the humidity-ratio difference (ΔW). When ΔW is small, very high airflow would be needed to hold the target RH, so dehumidification or source control becomes the primary strategy. A baseline minimum ventilation is also estimated, so the recommendation becomes the higher of humidity-control airflow or the occupant-and-area minimum, then reduced by any measured existing ventilation credit.
Why dew point matters more than temperature
Relative humidity can look acceptable while surfaces still condense. Dew point converts indoor temperature and RH into an absolute moisture metric. If a cold surface runs near or below the indoor dew point, liquid water films can appear, accelerating microbial growth on drywall, wood, and dust layers. Entering a coldest surface temperature helps flag this hidden risk early.
Ventilation versus dehumidification tradeoff
Outdoor ventilation works only when outdoor air is effectively drier than the target indoor condition. The tool reports a warning when outdoor humidity ratio is higher than the target, because adding outdoor air can increase indoor RH. In those periods, a dehumidifier credit (pints/day) reduces the net moisture load, lowering the required ventilation and stabilizing RH.
Construction-phase risks and scheduling
Fresh concrete, wet trades, wash-downs, and water events create short-term moisture spikes. The runtime setting converts a daily moisture load into a needed flow for the hours you can operate fans or make-up air units. A safety factor adds margin for uneven mixing, infiltration uncertainty, and changing weather. Continuous operation with moderate flow often outperforms short bursts of very high flow.
Verifying performance on site
Use the recommended flow as a commissioning target, then confirm with measured airflow and logged RH/dew point. Track indoor RH at multiple locations, especially near exterior corners and basements. If RH stays above 55–60% or condensation appears, increase drying capacity, improve drainage and vapor control, and reduce internal moisture sources before relying on more outdoor air.
What RH target reduces mold risk in most projects?
Many teams aim for 50–55% RH during occupancy and drying. Keep RH lower if cold surfaces are present. If you see condensation or musty odor, treat it as a control failure and reduce moisture sources.
Why does the calculator warn that ventilation may not dry the space?
If outdoor humidity ratio is higher than the target indoor condition, outdoor air adds moisture. In that case, increase dehumidification, raise supply air temperature, or postpone high-ventilation drying until outdoor air is drier.
Should I use floor area or the whole building area?
Use the area of the zone you are trying to protect and control, such as a basement, apartment, or one floor. Mixing between zones can change results, so split large projects into separate zones when possible.
How do I estimate extra moisture sources in g/hr?
Add known sources: cooking, showers, wet cleaning, curing materials, and water on surfaces. When unsure, start with a conservative high estimate and apply a safety factor, then refine using measured RH and dew point trends.
How do dehumidifier pints/day affect the result?
Pints/day are converted to an hourly moisture removal credit and subtracted from the internal moisture load. Higher dehumidifier capacity lowers the humidity-control ventilation requirement, which is useful when outdoor air is humid.
What field checks confirm the ventilation rate is working?
Measure supply/exhaust airflow, then log RH and dew point at several points for at least 24–72 hours. Verify that RH remains at or below the target and that cold surfaces stay above dew point.