Greenhouse Ventilation: How Much You Need and How to Get It

More greenhouse plants die of overheating than of frost, and it happens fastest in spring when nobody expects it. A sealed glazed structure on a clear March day can climb thirty or forty degrees above the outside air within an hour or two, while the ambient temperature outside still feels cool enough to leave the door shut.

Above roughly 90F most plants stop photosynthesising efficiently and start losing water faster than the roots can replace it. Above 100F pollen on tomatoes and peppers becomes non-viable, so you get flowers and no fruit. Sustained heat also stresses the plant into bolting and invites spider mites, which thrive in exactly the hot dry conditions a poorly vented greenhouse creates.

Ventilation is not a comfort feature. It is the single system that decides whether the greenhouse is usable from April onward.

There are only two mechanisms, and a good greenhouse uses both.

Passive, driven by the stack effect. Hot air rises and leaves through an opening high up; cooler air is drawn in through an opening low down to replace it. This works with no power at all, but only if you give it both openings. A roof vent on its own is far less effective than a roof vent plus a low intake, because there is nothing to replace the air leaving. This is why an open door alone disappoints - it is one opening at mid height doing both jobs badly.

Mechanical, driven by a fan. An exhaust fan at one end pulling through an intake shutter at the other gives you controlled air exchange regardless of wind or sun angle. It costs power and it costs money, but it is the only option that guarantees a specific number of air changes on a still, hot day.

Roof vents. These do the heavy lifting because they sit where the hottest air collects. Spread them along the ridge rather than clustering them at one end, and open them on the leeward side in windy weather so a gust cannot lever a panel.

Low side or louvre vents. These are the intake half of the system. Sited low on the opposite side from the prevailing wind, they pull cool air across the bench line at plant level, which is exactly where you want it.

Dutch doors. A door split horizontally lets you open the top half alone. That gives you a large, adjustable, mid-height vent that does not let a dog, a rabbit or a low gust straight into the beds, and it is the fastest way to dump heat in a hurry. Every greenhouse we sell has two of them, one at each end, which also means you can cross-ventilate end to end simply by opening both tops.

Rolled-up side walls. Common on high tunnels, rare on rigid-panel greenhouses. Excellent airflow, no thermal control at all.

An automatic vent opener is a sealed cylinder of wax that expands as it warms, pushing a piston that levers the vent open, and contracts as it cools to pull it shut. It uses no electricity, has nothing to fail electronically, and typically starts opening somewhere around 68 to 77F with the trigger point adjustable on the threaded rod.

Per dollar, it is the highest-value accessory in the whole category. The reason is simple: the days your greenhouse most needs venting are often days you are at work. A manual vent that nobody opens at 10am is not a vent. Fit one on every roof vent you have, check the rating against the weight of the vent it is lifting, and close them by hand in high wind - wax openers are strong but they are not designed to fight a gust.

A circulation fan stirs the air inside without exchanging it. It runs year-round, costs very little, and does two things: it breaks up the still, humid boundary layer around leaves where botrytis and powdery mildew establish, and it evens out the temperature difference between the ridge and the floor. Size it at roughly 2 to 3 CFM per square foot of floor - so a 130 sq ft house wants something in the 300 to 400 CFM range. Hang it high, aimed along the length of the house, and let the air circle back.

An exhaust fan exchanges the air with outside. The conventional target is one full air change a minute in summer, so calculate the internal volume and buy a fan rated at least that in CFM. A 10 ft x 19.5 ft house with an average internal height of about 6 ft holds roughly 1,170 cubic feet, so a fan in the 1,200 CFM class is the right ballpark. Pair it with an intake shutter of adequate free area at the far end and low down, and put it on a thermostat.

Ventilation can only bring the inside down towards the outside air temperature. On a 95F afternoon, no amount of air exchange gets you to 80F. At that point you need to stop the heat arriving in the first place, or remove it by evaporation.

Shade cloth is the simplest answer, rated by the percentage of light it blocks. Thirty to forty percent suits most temperate summers; fifty percent and up is normal at high altitude, in the desert southwest, and at low latitudes. Fit it on the outside - shade cloth hung inside the glazing has already let the heat in.

Damping down, meaning wetting the floor and benches in the morning, uses evaporation to pull real heat out of the air and raises humidity at the same time, which reduces plant water stress. It is free and it works.

Evaporative cooling - a wet pad at the intake with an exhaust fan at the far end - is the serious option and is genuinely effective in dry climates. In humid ones it does much less.

Twinwall polycarbonate helps here in a way people rarely credit. It diffuses light rather than transmitting a hard beam, which spreads energy evenly through the canopy instead of scorching whatever the sun happens to be pointing at.

A sealed greenhouse in winter is a humid greenhouse, and humidity plus still air plus cool nights is precisely the recipe for botrytis, damping off in seedlings, and mildew on overwintering plants.

The habit worth building: crack a vent for an hour around midday on any day that is above freezing, when the air is at its warmest and the exchange costs you least. Keep the circulation fan running year-round on a low setting. Water in the morning rather than the evening so foliage dries before nightfall. And leave gaps between pots on the bench so air can move between plants rather than around the block of them.

A ventilation setup that works, in build order

1. Fit automatic openers to every roof vent before the first warm week of spring.
2. Add low-level intake on the side away from the prevailing wind, roughly matching the roof vent area.
3. Hang a circulation fan high, pointed down the length of the house, and leave it running year-round.
4. Put a max-min thermometer at plant height, not at the door, and read it for a week before buying anything else.
5. Add exterior shade cloth for high summer if the max readings stay above the low 90s F with everything open.
6. Only then consider a thermostatically controlled exhaust fan, sized to one air change a minute.