How Humidity Affects Cooling in Polyhouses

In a polyhouse, evaporative cooling only works when the air can still absorb water. That’s why the same polyhouse cooling system can perform very differently across seasons: on dry days it cools fast, and on humid days it cools less and can increase wetness/condensation.

The good news: if your setup is automatic, you don’t need to “manually adjust every day.” You just need the right temperature + humidity logic, once—then the controller handles mist/fog output consistently.

Why humidity changes cooling performance (simple explanation)

Mist/fog cools air mainly through evaporation. Evaporation removes heat only if the air has capacity to take more moisture.

• Lower humidity → faster evaporation → stronger cooling
• Higher humidity → slower evaporation → weaker cooling + higher wetness risk

So when outside humidity rises (monsoon/cloudy days), running the same output can cause less temperature drop and more chance of leaf wetness or condensation—even with a good polyhouse fogging system.

The “set once” automation strategy that works

For an automatic polyhouse humidity control system, the most reliable control method is:

Use temperature to start cooling, and humidity to limit it

This prevents the common mistake of “more mist = more cooling” on humid days.

Recommended automatic control logic (easy to program):

• Start fog/mist when: Temperature ≥ Start Setpoint
• Stop fog/mist when: Temperature ≤ Stop Setpoint (hysteresis)
• Humidity lockout when: RH ≥ Max RH Setpoint (prevents saturation/condensation)
• Cycle control: run in short pulses, not continuous

This approach keeps cooling stable and avoids over-wetting without daily changes.

Suggested setpoints (use as a starting framework)

Exact targets depend on crop and growth stage, but this structure is what makes automation stable:

1) Temperature setpoints (start/stop)

• Start: when polyhouse heat begins stressing the crop
• Stop: a few degrees lower to avoid rapid ON/OFF

Example structure (not crop-specific):

• Start at T-high
• Stop at T-low (a small gap below T-high)

2) Humidity limit (max RH lockout)

Set a maximum RH where you pause mist/fog to avoid saturation.

• On humid days, the system will still run, but only when RH allows
• This is what keeps automated cooling from turning into wet floors or wet canopy

3) Pulse timing (ON/OFF)

Even in automation, pulsing is your best control tool:

• Short ON bursts + longer OFF gaps allow evaporation and mixing
• If you see wet leaves or condensation → reduce ON time and increase OFF time

One control table you can use in your controller

This is a simple “if/then” style setup many growers use:

This keeps your polyhouse cooling system predictable across dry and humid periods.

How to keep the system “automatic” across seasons

Instead of re-tuning constantly, use two saved profiles in your controller:

Profile A: Dry season / hot afternoons

• Earlier start setpoint
• Normal RH limit
• Slightly stronger pulse pattern

Profile B: Humid / monsoon

• Same temperature start (or slightly higher)
• Stricter RH limit
• Gentler pulses (short ON, longer OFF)

Switching profiles is a one-click seasonal change, not daily adjustments.

If cooling feels weak, don’t increase mist blindly

When humidity is high, extra water often adds wetness without meaningful temperature drop. If the temperature isn’t falling:

• Check if the system is hitting the RH lockout (that’s expected on humid days)
• Improve airflow/venting so mist disperses and evaporates better
• Keep pulsing conservative so you don’t push the polyhouse toward saturation

Quick troubleshooting for automatic setups

• Cooling drops suddenly vs last week: humidity likely increased → controller will hit RH limit more often
• Leaves feel wet after cycles: ON time too long, nozzles too close, or airflow weak
• Condensation on film/roof: RH limit too high or night/early morning schedule needs a lockout
• Mist looks heavy and falls: high RH + low mixing → increase OFF time and airflow