Cooling a greenhouse

A greenhouse is the one structure on this site where the usual advice inverts, and it is worth being precise about why.

Everywhere else on this site, added humidity is a cost. It is why humid heat feels worse than dry heat, why a laundry room feels awful, and why an evaporative cooler is the wrong machine in most homes.

The reason is that people cool themselves by evaporating sweat. Humid air has less spare capacity to absorb that moisture, so raising humidity directly degrades your own cooling mechanism.

Plants do not have that problem. A greenhouse is not being cooled for a human’s comfort, and moderate humidity is generally something growers manage toward rather than away from.

So the trade that makes evaporative cooling a bad deal indoors — cooler air in exchange for wetter air — is not a trade here. You get the temperature drop without paying the usual price. That is why evaporative cooling is standard in commercial glasshouses and rare in homes.

The numbers that follow from that: an evaporative cooler moving 3,100 CFM on 250 W, extraction at up to 750 CFM from the highest-airflow certified in-line fan we hold data for, and shading — the only lever in the whole exercise with no running cost at all.

Solar gain that never enters the structure costs nothing to remove. Every watt you block outside the glass is a watt no fan has to shift.

• External shading beats internal. Once light is through the glass its energy is inside, whatever you hang under it. Shade cloth or paint on the outside stops it arriving.
• Shade the roof, not just the sides. Summer sun is high, and the roof is the largest aperture facing it.
• Shading is a growing decision, not just a cooling one. How much light you can block depends entirely on what you are growing. That belongs to a horticultural source, not this site.
• Do the shading before sizing anything. The ventilation rate you need after shading is much lower than the rate you need without it.

The same principle applies indoors, where it is the cheapest cooling intervention there is.

A greenhouse has essentially no thermal mass and no insulation. Heat arrives continuously while the sun is up, and the only practical way out is with the air.

That makes this a high-airflow problem rather than a high-efficiency one. It is the opposite of the calculation on most of this site, where a certified bathroom fan wins on 16.7 CFM per watt and airflow is secondary.

• Roof vents work without power. Hot air rises and leaves; cooler air enters low. Automatic openers are common and need no electricity at all.
• Give the intake as much area as the extract. An extract fan with no low intake path pulls a vacuum and moves a fraction of its rating.
• Extract high, intake low. The same rule as a closet full of equipment, for the same reason.
• Circulate internally as well. Ventilation moves air through the structure; circulation stops it stratifying inside.

• Plumb the cooler in. A 10.3 gallon tank at 3,100 CFM does not last a hot day, and hand-refilling is why these get abandoned.
• Budget for pads. They are a consumable, and a degraded pad quietly costs you most of the cooling.
• Drain and clean on a schedule. Standing water in a warm structure is a maintenance problem before it is anything else — the symptoms and the fixes.
• Check the climate question honestly. Evaporative cooling delivers very little where the air is already humid, and a greenhouse in a humid region is no exception.
• Winterise it. Water left in a cooler through a freeze is how they die — the shutdown routine.