Greenhouse misting, fog and humidity control
Greenhouse misting systems do three different jobs: keeping cuttings wet on a propagation bench, raising humidity to hold a VPD target, and cooling air by evaporation. Low-pressure mist suits propagation, where wet leaves are the goal; high-pressure fog suits humidification and cooling, where foliage should stay dry. Start by identifying the job.
Guides in Greenhouses
Greenhouse misting systems: a complete planning guide
How to choose, size and control a greenhouse mist or fog system: droplet size by goal, water quality, nozzle layout, humidistat and VPD control, disease risk.
Propagation mist systems: how to design and run a mist bench for cuttings
Design a propagation mist bench for cuttings: nozzle layout, mist intervals, timer vs sensor controllers, water quality, disease prevention and hardening off.
Greenhouse humidity control and VPD: how to calculate and manage it
What vapor pressure deficit is, how to calculate VPD with the Tetens formula, common target ranges by growth stage, and how misting, fog and venting move VPD.
Greenhouse cooling: high-pressure fog vs pad-and-fan
Fog vs pad-and-fan greenhouse cooling: shared physics, a worked 30 x 96 ft sizing example, uniformity, water quality, upkeep and which to choose by climate.
Greenhouse misting covers any system that sprays water into a greenhouse to control plant water loss: mist over propagation benches, fog to raise humidity, and evaporative cooling in hot weather. Unlike patio misting, where the goal is human comfort, the goal here is managing how fast plants lose water and how long their leaves stay wet. The guides in this section explain how to choose, size and control each type of system.
Which job is your system doing?
Most greenhouse misting problems come from using one kind of system for another job. Identify yours first:
| Job | Leaves should be | Typical system | Read |
|---|---|---|---|
| Rooting cuttings | Wet (thin film) | Low pressure mist bench, 40-80 psi, solenoid and timer or sensor | Propagation mist systems |
| Raising humidity for a growing crop | Dry | High-pressure fog on a humidistat or VPD controller | Humidity control and VPD |
| Cooling in hot weather | Dry | Fog with ventilation, or pad-and-fan | Fog vs pad and fan |
| General planning, all three | Depends | Any | Greenhouse misting guide |
What physics do you need to know?
Three ideas explain nearly every recommendation in this section.
- Droplet size decides wetting. Evaporation time grows with the square of droplet diameter, so a 50 micron droplet lasts about 25 times longer than a 10 micron droplet. Fog vanishes in the air; mist lands on leaves. See misting vs fogging.
- VPD drives plant water loss. Vapor pressure deficit combines temperature and humidity into the air's drying power. The same RH means very different things at different temperatures, which is why a fixed humidistat setting can be right in the morning and wrong at noon. Calculate yours with the VPD calculator.
- Ventilation sets how much water you need. In a vented house, added moisture leaves with the air. Cooling 1 kg of air by 1°C evaporates about 0.41 g of water, so the water requirement scales with airflow, not just floor area.
How to use this section
- Read the overview. The greenhouse misting guide helps you pick a pressure class, size output and choose controls, with a worked hobby greenhouse example.
- Set your target. Use humidity control and VPD to decide what conditions you are aiming for at each growth stage.
- Go to the specific job: propagation benches or cooling.
- Sort out water and hygiene. Fog needs clean, low-mineral water (see hard water and water treatment), and warm greenhouse lines need a flushing and draining routine (see water hygiene and Legionella).
What equipment is shared with other misting systems?
Greenhouse fog systems use the same high-pressure plunger pumps, unloaders, nozzles and tubing as commercial patio systems, run for more hours per year. Pumps, nozzles and filtration are covered in depth in the pumps and nozzles sections. Two greenhouse-specific points: long run hours favor stainless steel or ceramic-insert nozzles that hold their orifice size, and drip-free operation matters more than on a patio because a single dripping nozzle over a crop creates a disease hotspot.