Quick answer: In a hydroponic greenhouse, the water the crop takes from the nutrient solution comes back out as humidity: plants transpire almost all of the water they absorb. So the daily top-up of your reservoirs, after correcting for drain-to-waste and leaks, is a practical measure of the moisture load your ventilation or dehumidifiers must remove. Aquaponic systems add evaporation from fish tanks and filters. Cover open water, keep floors dry, then size ventilation or dehumidification for what remains.
Quick answer
- Crop transpiration ≈ water taken up by the crop ≈ reservoir make-up water minus drain, spills and leaks.
- Open fish tanks and filters add evaporation; covers cut it sharply.
- Size dehumidifiers for the night hours, at the greenhouse condition.
Where the water in the air comes from
| Source | Hydroponic | Aquaponic | How to reduce it |
|---|---|---|---|
| Crop transpiration | Main source | Main source | Cannot reduce without slowing growth; remove it |
| Open nutrient solution (channels, rafts, reservoirs) | Moderate | Moderate | Cover channels and reservoirs; lids on rafts |
| Fish tanks, sumps, biofilters | — | Can be large, especially aerated and warm | Covers, lids, enclose tanks in their own room |
| Wet floors, spills, cleaning | Variable | Variable | Drainage, dry cleaning methods, fix leaks |
| Outdoor air | When outside air is humid | Same | Seal the house; ventilate only when outside air is drier |
Use your water meter to estimate the load
Plants release almost all the water they absorb through their leaves, keeping only a small fraction in their tissue. In a recirculating system, that means:
crop transpiration ≈ make-up water added − solution dumped to drain − spills and leaks
Example: a 1,000 m² lettuce NFT greenhouse tops up 4,000 L a day, and dumps 400 L of solution a week (about 60 L a day). The crop transpires roughly 3,900 L a day. Most transpiration happens in daylight; if about a fifth happens in the 12 night hours, the night load is about 780 L, around 65 L per hour. That is the water that ventilation or dehumidification must remove at night to hold humidity steady.
This figure is a better sizing input than any rule based on floor area. Record it through the season; mature crops in summer transpire far more than young crops in winter.
Fish tanks and biofilters
Open, warm, aerated water evaporates continuously. A rough estimate uses the ASHRAE evaporation equation for indoor pools (see our indoor pool dehumidification guide):
W (kg/h) ≈ 0.144 × A × (p_w − p_a) × F_a
Example: 10 m² of open tank surface at 24 °C, in greenhouse air at 24 °C and 70 % RH, with a low activity factor of 0.5 for quiet water: 0.144 × 10 × (2.98 − 2.09) × 0.5 ≈ 0.64 kg/h, about 15 L per day. Aeration, splashing returns and warmer water raise this. Covering tanks removes most of it.
If tanks and plants share the same air, add the two loads. If you can, put fish tanks and filters in a separate, covered room with its own ventilation.
Holding the target
- Cover open water: reservoirs, channels, rafts, sumps and tanks.
- Keep floors dry and drain spills.
- Move air through the crop with horizontal airflow fans; aim for steady air at plant level.
- Vent and heat when outside air holds less water than inside air.
- Dehumidify when venting cannot remove the night load, outside air is humid, or the house is sealed for CO₂ or heat.
For crop targets, see lettuce and leafy greens humidity and greenhouse humidity control methods.
Choosing dehumidifiers
- Size from the measured night load (example above: about 65 L/h, roughly 1,560 L/day equivalent) at the greenhouse condition, not at the 30 °C/80 % RH catalogue rating.
- Several units spread through the house, or ducted units feeding air tubes, so every bed gets dry air.
- Corrosion protection where nutrient mist or humid fish-room air is present.
- Condensate: dehumidifier water is low in minerals; many growers test it and return it to the system or use it for make-up water.
See our greenhouse dehumidifiers and ceiling-mounted and ducted dehumidifiers.
FAQ
Frequently asked questions
What humidity is best for a hydroponic greenhouse?
It depends on the crop: about 50–70 % RH for lettuce, and a night target that keeps leaves dry for fruiting crops.
How do I calculate the moisture load in a hydroponic greenhouse?
Use the daily make-up water, minus solution dumped and leaks. That is close to the crop's transpiration, which ends up in the air.
Do fish tanks raise greenhouse humidity?
Yes. Warm, aerated, open tanks evaporate continuously. Cover them, or keep them in a separate room.
Can dehumidifier water be reused in hydroponics?
Often yes. It is condensed water with few minerals; test it and treat it as needed before adding it to the system.
Sources
- ASHRAE Handbook—HVAC Applications, Natatoriums (evaporation equation).
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- University of Kentucky Center for Crop Diversification, Hydroponic lettuce.
