Greenhouse cooling: high-pressure fog vs pad-and-fan

Pad-and-fan cooling draws outside air through a wall of wetted cellulose or similar media at one end of a greenhouse and exhausts it with fans at the other. Fog cooling sprays water at high pressure (typically 800-1,500 psi) through fine nozzles inside the greenhouse, producing droplets under about 10-20 microns that evaporate in the air. Both are evaporative cooling. They differ in where evaporation happens, how evenly the cooling spreads, and what they demand of your water and maintenance.

Do fog and pad-and-fan cool by the same amount?

They share the same limit and the same water cost. Both cool air by evaporating water into it, and neither can cool below the wet-bulb temperature of the incoming air. Wetted pads commonly reach about 70-90% of the wet-bulb depression (their saturation effectiveness). A well-designed fog system can achieve comparable cooling, but results depend strongly on nozzle layout, droplet size, air movement and control, so manufacturer and site results vary.

The water cost is fixed by physics: evaporating one gallon absorbs about 8,700 BTU, and cooling 1 kg of air by 1°C requires about 0.41 g of water evaporated. So cooling a given airflow by a given amount evaporates the same water whichever method does it. The choice between them is about uniformity, water quality, control and cost, not water efficiency. Check your climate's wet-bulb depression with the cooling potential calculator.

How do fog and pad-and-fan compare?

Fog vs pad-and-fan greenhouse cooling

Factor	Pad-and-fan	High-pressure fog
Where evaporation happens	In the pad, at the inlet wall	Throughout the house air
Temperature uniformity	Gradient: coolest at pad, warmest at fans	More uniform when nozzles are evenly spaced
Ventilation	Mechanical: exhaust fans sized for the house	Works with natural vents or modest fan ventilation
Humidity	High near pad, falling toward fans	Raised evenly; useful for propagation and VPD control
Water quality	Tolerates ordinary water with bleed-off; scale and algae shorten pad life	Needs RO or very low-mineral water; 1-5 micron filtration
Risk of wetting crops	Low (water stays in the pad)	Moderate if droplets are too large, nozzles drip or control overshoots
Maintenance	Pads, pump, distribution pipe, sump, bleed-off, algae control, fan belts	High-pressure pump, nozzles, filters, RO unit, lines
Energy	Large exhaust fans run continuously in hot weather	High-pressure pump; fewer or smaller fans if natural ventilation works
Structure	Needs a pad wall and fan wall; house length limited by gradient	No inlet wall; suits naturally ventilated and retrofitted houses
Freeze season	Pads and sump must be drained and covered	Lines must be drained and winterized

Worked example: cooling a 30 x 96 ft greenhouse

Nozzle flows here are planning midpoints; published charts disagree by up to about 40% for the same nominal orifice, so verify with a timed bucket test. Pump sizing follows the same rules as any high-pressure system; see how to size a misting pump.

How does climate change the choice?

Climate affects both methods almost equally, because both are capped by the wet-bulb temperature.

Evaporative cooling potential by climate (80% effectiveness, planning estimate)

Design condition	Wet-bulb	Depression	Cooled air at 80%
Phoenix, 108°F / 12% RH	69°F	39°F	About 77°F
Sacramento, 100°F / 20% RH	70°F	30°F	About 76°F
Dallas, 98°F / 40% RH	79°F	19°F	About 83°F
Houston, 95°F / 55% RH	82°F	13°F	About 85°F

In humid climates, both methods deliver modest cooling at the cost of very high humidity, which also raises disease risk and lowers VPD toward the level where transpiration slows (see humidity control and VPD). There, shade, ventilation and airflow carry more of the load, and fog is often run for VPD control rather than for maximum cooling. The same logic applies to people: see misting in humid climates.

Which should you choose?

• Choose pad-and-fan if the house is fan-ventilated or being built new, runs east-west with a clear pad wall, your water is hard and RO is impractical, and you want the most established, lowest-risk cooling for a large range.
• Choose fog if the house is naturally ventilated (roof and side vents), you need even conditions end to end, you also need humidity for propagation or VPD control, or there is no practical place for a pad wall.
• Use both if a long pad-and-fan house has a hot fan end, or one section needs propagation humidity.
• For hobby greenhouses: a portable evaporative cooler (a small pad-and-fan) or a small fog kit on a humidistat both work. Pick the cooler for hard water and simplicity, fog for humidity-loving plants. The trade-offs mirror those in misting fan vs evaporative cooler.
• Do not rely on either alone if shading is missing. Shade cloth or whitewash cuts the heat load before it enters, reducing the water and airflow needed for the rest.

In what order should cooling stages come on?

Stage cooling from cheapest and driest to most water-intensive, so evaporative cooling only runs when ventilation and shade are not enough. A typical sequence as house temperature rises:

1. Vents open. Roof and side vents (or the first exhaust fan) replace hot inside air with outside air. This costs nothing in water and keeps humidity from building.
2. Shade deploys. A shade curtain or seasonal shade cloth cuts solar gain before it becomes heat in the house.
3. More ventilation. Additional fan stages in a fan-ventilated house.
4. Evaporative cooling starts. Pad pump or fog pump starts once ventilation alone cannot hold the setpoint.
5. Humidity limit applies. Fog pauses or pad water shuts off if RH exceeds a set limit (or VPD falls below a set floor), so the crop is not held in saturated air.

The humidity limit in step 5 is where fog and pad differ in practice. Fog can be pulsed in seconds, so a controller can hold a VPD target closely. A pad takes minutes to wet and dry, so pad cooling is closer to on or off, and humidity near the pad stays high while it runs. The targets that step 5 enforces are covered in humidity control and VPD.

How should a fog cooling system be laid out?

Spread the evaporation evenly through the air that is actually moving through the house. Practical rules:

• Lines at truss or gutter height, running the length of the house, typically several parallel lines in a wide house so no bay is left without fog.
• Denser fog near the inlet side of a naturally ventilated house, where incoming air is hottest and driest and evaporation is fastest.
• Horizontal airflow fans to mix fog into the air and stop it settling as droplets directly under nozzles.
• Zones by bay with separate solenoids, so the pump can stage output instead of switching all nozzles at once, and so propagation areas can run wetter than finishing areas.
• Anti-drip nozzles and a pressure release at shutoff, since fog pulses on and off many times an hour and each stop is a chance to drip on the crop.

Droplet size is the critical variable: below about 600-700 psi, a high-pressure system's droplets grow and begin to fall on plants rather than evaporate. Watch the pressure gauge as closely as the thermometer. Pump types and pressure control are covered in the misting pump guide.

What should you check on a pad-and-fan design?

Most pad-and-fan shortfalls trace to air, not water. Check these before blaming the pads:

• All incoming air passes through the pad. Open doors, gaps around the pad frame and leaky side vents let hot air bypass the pad, and the fans pull it in happily.
• Fans move their rated air. Slipping belts, dirty shutters and blocked fan guards reduce airflow, which raises the temperature rise across the house.
• Pads are evenly wet top to bottom, with no dry streaks from blocked distribution holes.
• Pads dry out daily. Letting pads dry at night limits algae growth.
• Bleed-off is running, so minerals left by evaporation do not concentrate in the sump and scale the media.
• Pad-to-fan distance is reasonable. Design guides commonly keep it within roughly 100-200 ft; longer houses see a larger gradient.

What goes wrong with each method?

Common failure modes and their causes

Symptom	Pad-and-fan cause	Fog cause
Cooling weaker than expected	Dry streaks in pad, scaled or algae-clogged media, air leaking around pads or through open doors	Clogged or worn nozzles, low pump pressure (below about 600-700 psi droplets grow), vents too closed
Wet crops or benches	Rare; water blown off wet pads at high face velocity	Oversized droplets, dripping nozzles, fogging into saturated air
White residue on leaves or glazing	Rare	Hard or untreated water
Hot end of house	House too long for airflow, or undersized fans	Uneven nozzle spacing, too few nozzles at one end
Algae, odor, biofilm	Sump and pad kept wet continuously, low bleed-off	Stagnant water in warm lines between uses

Both systems hold warm water that can grow Legionella, and fog aerosolizes it directly. Flush lines, drain when idle, keep sumps clean and follow a disinfection routine; see water hygiene and Legionella. For fog system layout and control, see the greenhouse misting guide, and for more topics, the greenhouses hub.