Summer cooling has a strict hierarchy, and the most common expensive mistake is buying the last item on the list to solve a problem the first item would have fixed for a tenth of the money.
The physical reason for the hierarchy is worth stating once. Shade reduces the energy arriving. Ventilation removes energy that has arrived, but it can only ever bring the inside toward the outside, never past it. Evaporation converts sensible heat into latent heat, which is the only mechanism available that takes the air below ambient. Each stage has a hard ceiling, and the next stage exists to break it.
What each stage is actually worth
| Method | Typical effect | Cost | Hard limit |
|---|---|---|---|
| Shade cloth, 50 percent | 10 to 15 degF below unshaded peak | Low | Removes light, so it must match the crop and come off for winter |
| Passive venting, sized properly | Brings the house toward outside temperature | Low | Cannot go below outside air temperature |
| Exhaust fan, sized properly | Brings the house close to outside temperature | Moderate, plus a circuit | Same ceiling as passive venting |
| Damping down the floor | Several degrees, plus humidity relief | Free | Wears off through the day, and must not be done in the evening |
| Evaporative cooler or misting | 15 to 20 degF below outside in dry air | Moderate to high, plus water and maintenance | Almost nothing in humid air |
The row that surprises people is the fan. An exhaust fan is a substantial purchase with a wiring requirement, and its ceiling is outside air temperature. On a 95 degree afternoon a perfectly sized fan leaves the house above 95. Shade cloth on the same afternoon reduces the heat arriving in the first place, which is why it beats the fan on peak temperature despite costing far less.
How evaporative cooling actually works
Water evaporating absorbs roughly 970 BTU per pound as it changes state, and that energy comes from the air around it. Air passing through a wetted pad or a fine mist therefore gives up sensible heat, measured as temperature, to supply the latent heat of evaporation, and arrives cooler and more humid.
The limit is how much more water the air can hold. Dry air has enormous capacity, so the cooling is dramatic. Air already close to saturation has almost none, so nothing much happens except that everything gets damp.
The practical rule is that evaporative cooling is worth installing where summer afternoon relative humidity is routinely below about 50 percent, is marginal between 50 and 70, and is not worth it above 70. In a humid climate it makes the disease situation worse while barely moving the temperature, which is the worst of both.
The two forms it takes at hobby scale
A portable evaporative cooler such as the Hessaire DC18 Mobile Evaporative Cooler ($159.00) draws air through a wetted pad and blows it into the house. It is self-contained, needs a power supply and a water reservoir, and works best positioned to blow along the length of the house with an exhaust opening at the far end. A 10-Gallon Portable Evaporative Air Cooler ($164.70) is the same idea at a smaller scale.
A misting system such as the MistKing Starter Misting System, 5th Generation ($199.99) sprays a very fine mist that evaporates before reaching surfaces. This is the more elegant approach because it distributes the cooling across the whole house rather than from one point, and because the same system can be used to raise humidity for propagation. It needs a water supply, decent pressure and filtration, and Adjustable Brass High-Pressure Misting Nozzles ($12.99) clog quickly on hard or unfiltered water.
Both require the same thing to work: air moving through the house. A cooler in a sealed greenhouse saturates the air in an hour and then does nothing. There must be an exhaust path, which means the cooler is an addition to the ventilation system rather than a replacement for it.
Damping down, which is the free version
Wetting a gravel or soil floor in the morning is evaporative cooling without the equipment. A few gallons spread across the floor evaporates through the day, absorbing heat as it goes, and raises humidity at exactly the driest hottest hours.
It is meaningfully effective, it costs nothing, and it is one of the strongest arguments for a gravel or soil floor over a sealed slab, as set out in greenhouse flooring and paths. Do it in the morning so the floor is dry by evening, because a floor still wet after dark holds humidity up all night.
Anyone considering a cooler should try a season of consistent morning damping down first. In a temperate climate it frequently closes the gap.
Sizing, if you do install one
Evaporative coolers are rated in CFM like fans, and the sizing logic is the same: the house needs roughly one air change per minute, which is about 8 CFM per square foot of floor before corrections. A cooler rated below that is moving too little air to cool the space regardless of how cold the air leaving it is.
Work out the requirement in the ventilation CFM calculator, and note that shading the house first reduces that requirement, because the light-load correction falls. Shade therefore makes every downstream purchase smaller as well as less necessary.
Set the shade percentage from the crop rather than from the weather using the shade cloth calculator, because over-shading costs yield in a way that is much harder to notice than over-heating.
Cooling equipment in order
The full four-stage sequence, including what to do on the handful of days that exceed everything, is in cooling a greenhouse in summer.