Evaporative Cooling for Greenhouses: Fan-and-Pad Systems, Fog and Sizing Math

Most commercial and serious hobby greenhouses are cooled with a fan-and-pad system: a wall of wet cellulose pads on one end and exhaust fans on the other. The fans pull outdoor air through the pads, where evaporation cools it, then draw that air down the length of the house and out. In dry climates the air entering the house can be 20 to 30 F cooler than outdoors; even in humid regions, a few degrees plus vigorous ventilation keeps plants out of heat stress. Smaller houses can use a standard evaporative cooler blowing in, and propagation houses often use high-pressure fog.

This guide covers how each system works, how to size fans and pads, water use, and the mistakes that most often hurt performance.

Why greenhouses need evaporative cooling

A greenhouse is a solar collector. On a clear summer day, full sun delivers roughly 300 BTU per hour per square foot of glazing at peak. Without cooling, inside temperatures can run 20 to 40 F above outdoor air. Ventilation alone cannot bring the house below outdoor temperature; it can only limit the rise. To get below ambient, you need evaporation.

Evaporative cooling also suits plants in another way: it raises humidity, which reduces transpiration stress on hot, dry days. That benefit has a limit, because humidity that stays very high, especially into cool nights, favors fungal disease. Good systems cool hard in the afternoon and dry the house out in the evening.

Fan-and-pad systems: how they work

The layout is simple:

• Pads mounted in the end wall (or a side wall) facing the prevailing summer wind. Most use corrugated cellulose media in 4 in or 6 in thickness, the same family of rigid media found in whole-house coolers. Munters CELdek is a well-known example. See types of evaporative cooler pads.
• A water distribution pipe along the top of the pads, a gutter at the bottom, and a sump with a recirculating pump. A float valve maintains the sump level, and a bleed-off line discharges some water continuously to control mineral buildup.
• Exhaust fans in the opposite wall, typically 36 to 54 in diameter belt-drive or direct-drive agricultural fans with gravity or motorized shutters.
• Controls that stage fans on temperature, then start the pad pump once fans alone cannot hold the setpoint.

Because the fans exhaust, the house runs under slight negative pressure. Air enters wherever it can, so every door, gap and side vent must be closed when the pads are running, or air short-circuits around the pads and you lose cooling.

Temperature gradient

Air warms as it travels from pad to fans, absorbing solar heat. A well-designed system holds this rise to about 7 to 10 F. That is why pad-to-fan distance is limited: guidance commonly puts the practical maximum around 150 to 200 ft, with shorter distances giving a more even house. Plants near the pads will always be cooler than plants near the fans; place heat-sensitive crops accordingly.

Sizing fans: the CFM per square foot method

The standard starting point, found in ASABE EP406 and the widely used Greenhouse Engineering handbook, is about 8 CFM per square foot of floor area for a greenhouse at low elevation under high summer light. That works out to roughly one air change per minute in a house with an average height near 8 ft. The base rate is then adjusted:

Elevation

Thinner air carries less heat per cubic foot. At about 5,000 ft, air density is roughly 83% of sea level, so airflow increases by about 1.2x.

Light intensity

Brighter sites or less shading need more air; heavy shade cloth allows less.

Allowable temperature rise

Designing for a smaller pad-to-fan rise (say 5 F instead of 7 F) requires more airflow.

Fans should be rated at the static pressure they will actually run against, typically about 0.05 to 0.10 in of water with pads in place. Agricultural fan test data (CFM at a given static pressure, and CFM per watt) is far more useful than free-air ratings. Our page on CFM explained covers static pressure in more detail.

Sizing pads, water flow and sump

Pad area depends on how fast air passes through the face of the pad. Too fast and efficiency drops while water can be pulled off the pad; too slow and you are paying for pad you do not need.

The sump should hold enough water to fill the pads and pipes when the pump starts without running dry; a common rule is roughly 0.75 to 1 gallon per square foot of pad. Saturation efficiency for 4 in cellulose is commonly around 75-80% at design velocity, and 6 in pads reach the mid to upper 80s.

Smaller greenhouses: positive pressure coolers

For hobby houses under roughly 500 sq ft, a full pad wall with exhaust fans is often more than needed. A standard evaporative cooler can push air in, with a louvered vent or small exhaust fan at the far end to let it out. This is called positive pressure cooling, and it has one advantage: air enters only through the pads, so leaky doors do not short-circuit the system.

• Size for roughly one air change per minute. A 10 ft x 12 ft house with an average height of 8 ft has 960 cubic feet, so about 1,000 CFM. Add 20-30% for full sun and no shade cloth.
• A side-draft cooler with 8 in rigid media gives higher efficiency than a portable with thin pads. Our types of swamp coolers page compares options.
• Provide outlet area of roughly 1 to 2 sq ft per 1,000 CFM on the far wall, as with any cooler; see venting and relief air.
• Run a thermostat or humidistat control, and allow fan-only operation in the evening to dry the house.

Fog and mist systems

Fog systems cool by spraying very fine droplets directly into the greenhouse air, where they evaporate before reaching leaves. They are common in propagation houses, where they cool and also hold the high humidity cuttings need.

Fog needs ventilation to work. If humid air is not removed, the house quickly approaches saturation and evaporation stops. Hard water is the main enemy: minerals clog the tiny nozzle orifices and leave white residue on leaves. Most high-pressure systems use cartridge filtration, and many growers use reverse osmosis or softened water.

Common mistakes and what growers check first

1. Air leaks. Open doors, torn poly, and side vents left open let air bypass the pads. Walk the house with the fans running and feel for drafts.
2. Dry streaks in the pads. Clogged holes in the distribution pipe leave vertical dry bands that pass hot air. Flush the pipe and check that water sheets evenly down the full pad face.
3. Scale and algae. Mineral crust and algae on the pad face reduce airflow and efficiency. Keep bleed-off running, shade the pads from direct sun if possible, and let pads dry daily. See bleed-off and water quality.
4. Running pads at night. Shut off the pad pump while fans continue, so pads dry out and the house is not humid all night.
5. Fan maintenance. Loose belts, dirty shutters and corroded blades can cut agricultural fan output significantly. Check belts and clean shutters at the start of each season; our belt and pulley adjustment guide applies.
6. Exhaust recirculation. Fans blowing toward a neighboring house's pads, or toward a wall a few feet away, reduce performance. Leave open space downstream of fans.

Climate fit

Performance tracks outdoor wet bulb, exactly as in homes. In Arizona, Nevada, Utah and inland California, fan-and-pad houses achieve large temperature drops. In the humid Southeast, drops are smaller but still valuable. Use our wet bulb explainer, the cooling temperature calculator, and the climate index to estimate what your site can achieve. For large commercial installations, a local agricultural engineer or university extension greenhouse specialist can review fan, pad and control design against your crop and site.