An evaporative cooler can drop incoming air by 15 to 25 degrees F in a dry climate and achieves very little in a humid one, because it converts sensible heat to latent heat and its effectiveness depends entirely on how dry the incoming air is. The Hessaire DC18 at roughly $190 is the pick where the climate suits it. In humid regions, spend the same money on shade cloth and exhaust capacity instead.
Works best below
40 percent RH
Typical drop, dry air
15 to 25 F
Typical drop, humid air
3 to 6 F
Coolers compared
3
Researched from published specifications and verified owner reviews · Updated 2026-08-16
Evaporative cooling is the only greenhouse cooling method whose usefulness is decided almost entirely by your climate rather than by your equipment. In arid regions it is transformative and cheap to run. In humid ones it adds moisture to a structure that already has too much and achieves very little. Getting this decision right matters more than getting the product right.
Where the climate suits it, the pick is the Hessaire DC18 at around $190, which moves enough air to serve a hobby greenhouse rather than a patio corner.
An evaporative cooler with enough airflow to serve a hobby greenhouse, which is the specification most units in this price range fail. Patio coolers move air for a person sitting in front of them; a greenhouse needs the whole volume treated. The rigid media and the tank capacity here suit a run of several hours unattended, which is what a greenhouse afternoon requires. Understand before buying that in a humid climate this unit, or any other, will disappoint.
10-Gallon Portable Evaporative Air Cooler ($164.70). A smaller and cheaper unit suited to a compact house or a propagation area, where the treated volume is small enough for a modest cooler to make a measurable difference.
The physics, which decides whether this works for you
Evaporative cooling works by converting sensible heat, the heat you can measure with a thermometer, into latent heat, the energy absorbed when liquid water becomes vapour. Air passing through a wet medium gives up heat to evaporate that water. It leaves cooler and wetter, carrying the same total energy in a different form.
That single fact explains everything about the method. The cooling available is bounded by how much more water the air can absorb, which is bounded by how dry it already is. Air at 20 percent relative humidity has enormous capacity and cools dramatically. Air at 80 percent has very little capacity and barely cools at all. No product design changes this, because it is a property of the air rather than the machine.
Figure 1. Evaporative cooling available by incoming air condition·6 rows
Figure 1. Evaporative cooling available by incoming air condition
Outside air
Relative humidity
Realistic temperature drop
Verdict for a greenhouse
95 F
15%
20 to 25 F
Excellent, transformative
95 F
30%
15 to 20 F
Very effective
95 F
45%
10 to 14 F
Worthwhile
90 F
60%
6 to 9 F
Marginal, adds humidity problems
90 F
75%
3 to 5 F
Not worth it
85 F
85%
1 to 3 F
Actively counterproductive
Researched approximate figures based on published psychrometric relationships and typical evaporative media efficiency of 70 to 85 percent. Real performance also depends on airflow rate and media condition. The point of the table is the trend rather than any individual row: below roughly 40 percent incoming relative humidity the method is strong, and above roughly 60 percent it stops being a sensible purchase.
The practical threshold for a greenhouse is around 40 to 50 percent relative humidity in the outside air during the hot part of the day. Below that, evaporative cooling is the most cost-effective cooling available. Above it, the same money buys far more cooling as shade cloth and extraction capacity.
The three tiers
Evaporative cooling in three tiers
3 tiers · priced at the time of writing
Beginner
A compact house or a single bench area in a dry climate, testing whether the method suits the site.
A small unit is enough to demonstrate whether evaporative cooling works in your climate before committing to a larger one, and in a genuinely small house it may be all that is needed. Running one for a season tells you far more than any calculation, because local humidity during the hot hours is what decides the outcome and that varies more than regional averages suggest.
The trade-off:
Limited airflow, so it treats a small volume. In anything above roughly 60 square feet it will produce a cool zone near the unit and leave the rest of the house unaffected.
Enough airflow to treat a whole hobby house rather than a corner of it, with a tank capacity that supports several hours of unattended running. In a dry climate this is the difference between a house that must be abandoned in high summer and one that stays productive, and the running cost is modest because the energy is doing evaporation rather than refrigeration.
The trade-off:
It consumes water continuously, which matters in exactly the arid regions where it works best. It also raises humidity, so it needs to be paired with sufficient extraction, otherwise the house becomes cool and damp, which trades a heat problem for a disease one.
A high-pressure misting system evaporates water directly into the air stream of the house rather than in one appliance, which distributes the cooling evenly and takes no floor space. Paired with exhaust extraction it becomes a fan-and-pad arrangement in miniature, which is how commercial growers do this. It also serves propagation humidity, so it does two jobs.
The trade-off:
It requires plumbing, a pump and nozzle maintenance, and misting in a house that is already humid is the wrong intervention. Nozzles block with hard water, which is one more argument for the rainwater capture in the larger builds.
Evaporative cooling is the fourth stage of greenhouse cooling, not the first, and buying it before the earlier stages is the most common error in this category.
First, shade.Shade cloth on the outside of the glazing removes solar energy before it enters the structure, which nothing inside can do. It has no running cost, no water use and no moving parts. For fruiting crops 40 to 50 percent shade costs almost nothing in yield because photosynthesis saturates well below full summer sun. See best shade cloth.
Second, passive venting. Roof vents with automatic openers plus a low-level louvered intake create a stack effect using no power at all. Most hobby houses have roughly a third of the vent area horticultural practice calls for, and adding vent area is cheap.
Third, forced extraction. A shutter fan sized for one air change per minute handles the hours passive flow cannot. This has a running cost but a small one, and it also removes humidity rather than adding it.
Fourth, evaporative cooling, and only if the climate supports it. By the time the first three are done, many growers in humid regions find they do not need a fourth stage at all. The full sequence is in cooling a greenhouse in summer.
Watch out
Evaporative cooling and a closed house do not mix
An evaporative cooler must be paired with an exhaust path. It works by adding moisture to air, and if that air has nowhere to go, the house simply becomes saturated and the cooler stops cooling once the air can hold no more water. The correct arrangement is cooler at one end, exhaust at the other, with continuous through-flow. Run in a closed house it produces a cool, damp, still environment, which is close to ideal conditions for botrytis on a fruiting crop.
Water, maintenance and the things that go wrong
Three maintenance issues account for most disappointment with evaporative coolers in greenhouse use, and all three are manageable.
Media scaling. Evaporation leaves behind everything dissolved in the water. In a hard water area the cooling media scales up within a season, which reduces both airflow and evaporative efficiency, usually gradually enough that the decline is not noticed. Media should be inspected annually and replaced when it is stiff and white rather than flexible. Using captured rainwater, which is soft, extends media life considerably.
Biological growth. A tank of standing water in a warm greenhouse is a good environment for algae and bacteria. Tanks should be drained rather than left standing between uses, and the unit run dry for a few minutes before shutdown so the media dries out. A permanently damp cooler in a greenhouse is a source of exactly the spores you are trying to manage.
Water consumption. A cooler serving a hobby greenhouse through a hot afternoon consumes a meaningful volume of water, and it does so in the arid climates where water is most constrained. Working this against the irrigation demand for the same house is worth doing before committing. The water use calculator covers the irrigation side, and rain barrel capacity the supply side.
Humidifiers and dehumidifiers, which are different tools
Two adjacent product categories get bought for greenhouse use and are worth distinguishing.
A humidifier adds moisture without the cooling being the point. Its legitimate greenhouse use is a propagation area, where high humidity around cuttings and seedlings reduces transpiration stress while roots establish. Used on a whole house it creates disease conditions. A fogger kit serves the same propagation purpose at lower cost.
A dehumidifier is the one appliance in this area that is usually the wrong purchase for a hobby greenhouse, and the reasoning is worth stating because it is counterintuitive. A greenhouse leaks. A dehumidifier in a leaky structure is drying the outdoors, and it will run continuously without ever satisfying. Venting on mild afternoons, heating slightly, improving circulation and watering the soil rather than the foliage all address winter humidity far more effectively and more cheaply. A dehumidifier makes sense only in a genuinely sealed space, such as a small propagation enclosure.
How we chose
We did not measure the temperature drop these units achieve, and any site claiming psychrometric testing of three evaporative coolers is telling you a story. We compared published airflow and tank capacity, media type and replaceability, pump specification, and verified owner reviews weighted toward multi-season use rather than first impressions.
The category-defining consideration, though, was not product quality at all. It was whether the method suits the buyer, which is why so much of this page is about climate rather than equipment. A well-made evaporative cooler in a humid region will produce three degrees of cooling and a mildew problem, and its reviews will reflect that even though the product did exactly what it was designed to do.
We therefore weighted airflow relative to house volume above every other product property, because the most common equipment error within this category is buying a personal or patio cooler for a structure it cannot treat. A unit that cools a person sitting in front of it will not cool 250 cubic feet of greenhouse air.
What to buy alongside, or instead
The cooling stages that come first
For most growers, especially in humid regions, these three deliver more cooling per dollar than an evaporative cooler does, and they should be in place before one is considered.
External shade removes solar energy before it enters the glazing. No running cost, no water, no moving parts, and it reduces every other cooling requirement downstream.
Logging outside humidity during the hot hours tells you whether evaporative cooling will work on your site before you buy it. Regional averages are a poor guide.
In a dry climate they work very well, dropping incoming air by 15 to 25 degrees F. In a humid one they achieve very little, typically 3 to 6 degrees, while adding moisture to a structure where humidity is already the main disease driver. The threshold is roughly 40 to 50 percent outside relative humidity during the hot hours. Below it, evaporative cooling is the best value cooling available; above it, spend the money on shade and extraction.
+- How much can an evaporative cooler cool a greenhouse?
It depends on the incoming air rather than the machine. At 95 degrees F and 15 percent relative humidity, a drop of 20 to 25 degrees F is realistic. At the same temperature and 60 percent humidity it falls to 6 to 9 degrees. This is a property of the air rather than the product, because the cooling comes from converting sensible heat into latent heat as water evaporates, and saturated air cannot absorb more water.
+- Do I need to vent a greenhouse when using an evaporative cooler?
Yes, and it is not optional. An evaporative cooler adds moisture to the air, and if that air cannot leave the house it saturates and the cooler stops cooling. The correct arrangement is cooler at one end, exhaust at the other, with continuous through-flow. Run in a closed house it produces a cool, damp, still environment that is close to ideal for botrytis on fruiting crops.
+- Is a dehumidifier worth it in a greenhouse?
Usually not, and the reason is that a greenhouse leaks. A dehumidifier in a structure that exchanges air with the outdoors is effectively drying the outdoors, and it will run continuously without satisfying. Venting on mild afternoons, heating slightly, improving internal circulation and watering the soil rather than the foliage all address winter humidity far more effectively. A dehumidifier makes sense only in a genuinely sealed propagation enclosure.
+- What is the cheapest way to cool a greenhouse?
External shade cloth, by a wide margin. It removes solar energy before it passes through the glazing, which nothing inside the house can do, and it has no running cost or moving parts. For fruiting crops 40 to 50 percent shade costs almost nothing in yield, because photosynthesis saturates well below full summer sun. After shade, passive venting with automatic openers costs nothing to run either.
+- How much water does a greenhouse evaporative cooler use?
Enough to matter, and it is consumed in exactly the arid climates where water is most constrained, so it is worth working the figure against your irrigation demand before committing. Using captured rainwater instead of hard tap water also substantially extends the life of the cooling media, since evaporation leaves behind everything dissolved in the water and scaled media loses both airflow and efficiency.
Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the
paid version of these pages: 8 printable worksheets you fill in with your own
numbers, plus the full PDF, $29.
