Example data table
These scenarios show how ventilation and site moisture can drive capacity needs.
| Scenario | Outdoor | Target | Outdoor air rate | Internal moisture | Estimated daily removal |
|---|---|---|---|---|---|
| Small refurbishment bay | 30°C, 65% | 24°C, 50% | 600 m³/h | 0.20 kg/h | ≈ 65 L/day |
| Concrete curing zone | 33°C, 75% | 24°C, 45% | 900 m³/h | 0.60 kg/h | ≈ 135 L/day |
| Large enclosure | 28°C, 70% | 22°C, 50% | 2000 m³/h | 1.20 kg/h | ≈ 290 L/day |
Formula used
This calculator estimates the moisture removal required to maintain a target indoor humidity.
How to use this calculator
- Enter outdoor temperature and relative humidity from site measurements.
- Set your target indoor temperature and relative humidity for drying.
- Choose a method: input outdoor air rate, or use volume and ACH.
- Add moisture sources: occupants and wet materials or processes.
- Apply a safety factor, then calculate to get capacity needs.
- Download the CSV or PDF for reporting and approvals.
Outdoor Air and Infiltration Drive Latent Load
Dehumidification demand rises when humid outdoor air enters an enclosure through doors, gaps, or pressure-driven leakage. The calculator converts outdoor temperature and relative humidity into a humidity ratio (W) and compares it to the target indoor W. The difference (ΔW) is multiplied by dry-air mass flow to estimate ventilation moisture. As a rule, doubling outdoor air from 600 to 1200 m³/h can nearly double required removal when RH exceeds 60%.
Choosing Targets for Drying and Material Protection
Targets should match drying goals and material limits. Many projects aim for 40–55% indoor RH with temperatures near 20–26°C to balance comfort and evaporation. Lower RH increases removal and helps control corrosion and mold risk, but it may require more equipment. This tool reports kg/h and L/day so you can compare setpoints against dehumidifier ratings and project acceptance criteria, and improves worker visibility.
Internal Moisture Sources in Active Construction Sites
Internal sources can dominate when air exchange is controlled. A light-activity moisture rate is often 60–90 g/h per person, so ten workers can add roughly 0.6–0.9 kg/h. Wet concrete curing, wash-down, open sumps, and exposed soil can add several kg/h depending on area, temperature, and airflow over the surface. Enter known process moisture directly, then apply a safety factor for shifts and staging.
Interpreting Results: kg/h, L/day, kW, and Btu/h
Moisture removal in kg/h is the core sizing number; one kilogram of water is about one liter. Multiplying by operating hours yields L/day and US pints/day for nameplate comparisons. The latent load in kW and Btu/h is derived from moisture rate and latent heat, supporting generator sizing and coordination with temporary cooling or heat rejection. If runtime is limited, required daily capacity increases.
Field Tips for Improving Accuracy and Reducing Capacity
Improve accuracy with measurements. Use calibrated meters, log readings at least twice daily, and note weather changes. Confirm airflow using fan curves, damper positions, or simple smoke tests, and remember that negative pressure increases infiltration. Use the room method (Volume × ACH) for early planning, then switch to measured airflow after barriers are installed. Sealing leaks and isolating wet zones often reduces capacity more than changing setpoints, and stabilizes drying schedules.
FAQs
What input should I use if I only know exhaust fan CFM?
Select the airflow method, set airflow units to CFM, and enter the delivered fan CFM. Use the actual operating point if filters, ducting, or static pressure reduce flow.
Why does ventilation moisture load sometimes show zero?
If outdoor air has a lower humidity ratio than the indoor target, bringing it in does not add moisture. The calculator clamps ΔW to zero so the ventilation term cannot become negative.
What safety factor is reasonable for construction drying?
A common starting point is 5–20%. Use the higher end when openings change often, wet trades overlap, or measurements are limited. Reduce it once airflow and moisture sources are verified.
How do I estimate process moisture from wet concrete?
If you cannot measure evaporation, enter a conservative provisional value based on wet area and conditions. Refine it after observing indoor RH trends and comparing them to collected condensate volumes.
Should I size equipment based on liters per day or kW?
Use liters per day or pints per day to match dehumidifier capacity ratings. Use kW and Btu/h for power planning, heat rejection, and coordination with HVAC or temporary cooling.
Does changing temperature units affect the calculation?
No. The calculator converts °F to °C internally before computing humidity ratio and latent load. Results are consistent; only the input and display units change.