The short version: give a greenhouse roof-vent area of roughly 15–20% of its floor space, then add a second opening low down or at the far end so fresh air crosses the whole house. Good ventilation steadies temperature, drops humidity, keeps grey mould off the leaves and lets pollen move. On most days it matters more than heating.
Greenhouse ventilation rarely gets photographed and routinely gets underestimated. A closed glasshouse on a bright April morning can climb past a comfortable growing range in under an hour, and the harm that follows seldom comes from the heat itself. It comes from the still, wet air the heat leaves behind. Get the airflow right and a modest greenhouse out-grows an expensive one that stays shut.
Written by the Waldenhaus team. Last updated 16 July 2026.
Why greenhouse ventilation decides more than heat
Heat shows up first, so it gets the attention. Managing temperature counts as a job in its own right, and we cover the shading and cooling side of it in how to keep a greenhouse cool in the UK. Ventilation handles the quieter half of the same task, and it pays off in three ways a thermometer will never show you.
Start with humidity. Warm air carries a lot of water, and when it cools overnight that water lands on cold glazing and colder leaves as condensation. Grey mould (Botrytis cinerea) runs on two dependable triggers, free water on the leaf and high humidity in stagnant air, which explains why Michigan State University Extension names water and relative humidity as its key drivers. Moving air lowers the humidity and dries the leaf surface, so a well-vented house hands the spores far less to work with. The RHS puts it plainly, that ventilation helps keep down fungal diseases like the grey moulds.
Then comes pollination. Tomato, pepper and aubergine flowers self-fertilise, yet the pollen still has to shake loose and travel from anther to stigma. Around 70% humidity that happens easily. In very humid, still air the pollen turns damp and sticky and stays put, which the University of Alaska Fairbanks Extension flags as a common cause of poor set. A gentle through-draught does the shaking for you. Our guide to why greenhouse fruit is not setting goes deeper on airflow and fruit.
Then the pest climate. Red spider mite thrives in hot, dry, static air. Grey mould thrives in cool, wet, static air. The shared word, static, does the damage. Air that keeps moving suits neither, so a house you can open at both ends gives both a harder time.
The ventilation hardware, and what each part does
A greenhouse breathes through its openings, and each opening does a different job depending on where it sits. Hot air rises, so the highest opening does most of the work. A roof or ridge vent lets the hottest air lift straight out at the apex, and the RHS rule of thumb puts its area at one square metre of roof vent for every five square metres of floor, roughly 20% of the floor area, enough for about one complete air change every couple of minutes. University of Massachusetts Amherst Extension lands in the same place, suggesting ridge vents of 15–20% of floor space.
Side vents and louvres sit low down and feed cool air in underneath, supplying the flow the roof vent pulls out. On their own they work less well than roof vents, as the RHS notes, but paired with a ridge vent they close a loop, cool air in low and hot air out high. A door adds a large, quick opening for a fast drop while you work inside. The final piece, cross-ventilation, means an opening at the far end opposite the door, so air crosses the length of the house instead of stalling in a dead corner.
Wind carries more of this load than most growers expect. A light breeze of two to three miles an hour, passing over the roof, creates a slight suction that draws warm air out of an open ridge vent and supplies as much as 80% of a greenhouse's air changes on a breezy day, again per the UMass Extension guidance. On a hot, dead-still afternoon that help vanishes, exactly when you want every opening wide open.
| Opening type | What it does | When it matters most |
|---|---|---|
| Roof / ridge vent | Releases the hottest air at the apex; the most effective single opening | Every sunny day from spring to autumn |
| Side vent / louvre | Draws cool air in low down to feed the cross-flow | Hot, still days when a roof vent alone falls short |
| Door | Large, fast opening for a quick temperature drop | Peak heat, and while you work inside |
| Rear window opposite the door | Completes cross-ventilation so air crosses the whole house | Whenever one opening leaves a stagnant far end |
| Automatic vent opener | Opens and closes the vent on temperature while you are away | Working owners, allotments, heat-sensitive crops |
Automatic vent openers: the wax-cylinder trick
The clever part of an automatic opener needs no wiring, no battery, nothing to program. Inside the arm sits a sealed cylinder of mineral wax. As the greenhouse warms, the wax expands and pushes a piston that lifts the vent; as the air cools, the wax contracts and a spring pulls the vent shut. The RHS describes exactly that, wax expanding in a cylinder to open the vents, and adds a caveat worth keeping: the wax reacts slowly, so hold a door or side vent in reserve for a sudden spike.
For anyone out of the house on a bright weekday, an opener separates a scorched tray of seedlings from a healthy one. It cracks the vent mid-morning when the sun first hits, long before you would get home to do it, then closes it by evening. For allotment holders who cannot visit daily, or heat-sensitive crops in a small house that heats fast, it ranks among the more sensible upgrades going.
Our own version, the SmartVent Auto-Vent Opener (optional accessory, sold separately, £99), uses a wax-piston actuator that starts to open around 18 °C and sits fully open by roughly 25 °C, all without mains power. It swaps in for a standard roof panel (930 × 540 mm) on a NORDIC and fits in about half an hour with two people and a screwdriver. It suits the timber range; the steel arch house takes the SteelRoot Auto-Vent Opener instead, also an optional add-on. Keep one habit: an opener helps only the vent you fit it to, so leave a manual opening as your backup rather than leaning on the wax arm alone.
How our two greenhouses ventilate
Both houses open front and back, because cross-ventilation does the most for the least outlay. On the timber side, every NORDIC greenhouse arrives with cross-ventilation as standard: a stable door and a rear opening window directly opposite it, so air travels the full length of the house.
Roof vents sit on top of that, one on the two smaller sizes and two on the three larger, and 4 mm twin-wall polycarbonate glazes the lot, with a 6 mm option for colder plots. Sizes run from 8 × 6 ft upward. To make the vent open itself, the SmartVent opener above fits as an optional accessory.
The galvanised steel arch house follows the same logic. The SteelRoot greenhouse, 3.14 m across the arch, includes gable-door cross-ventilation as standard, with roof auto-vents on the options list for growers who want temperature-triggered opening. Sizes there start at 10 × 14 ft and stretch to full allotment lengths, so on the longer arches a gable door at each end already sets up a clean draught straight down the central path. Both houses keep proving the point the physics keeps making: two openings at opposite ends beat one big opening in the middle, every time.
Ventilating through the UK seasons
UK greenhouse ventilation splits into three jobs across the year, and the calendar keeps shifting under us. The Met Office reports the UK warming at roughly 0.25 °C a decade, and the summer of 2025 arrived as the warmest on record, averaging 16.10 °C against a 1991–2020 mean of 14.59 °C. Longer, hotter summers push the ventilation season earlier in spring and later into autumn than the old gardening books assume.
From late spring to early autumn the danger reads as overheating, and the RHS advice runs simple: open everything, doors and vents alike, on sunny days, and leave them open overnight while the air stays warm. A greenhouse can bake by mid-morning in April as readily as in July, so err towards opening early. Shading and damping down belong to that same warm-weather job, covered separately in the cooling guide rather than here.
The shoulder seasons, roughly March and again in October, cause the most head-scratching. A day can turn warm enough to need venting by lunchtime and cold enough to need closing by four, precisely the window where an automatic opener earns its keep. Winter flips the task to damp control. You cool nothing; you shift the moisture that condenses on cold nights. A short opening on a mild, dry winter afternoon flushes out humid air and keeps grey mould off overwintering plants, which counts for far more in an unheated house than the odd degree you lose.
Common ventilation mistakes
The mistakes stay consistent, and most cost nothing to fix once you spot them:
- Opening too late in the day. By the time a shut greenhouse feels hot to you, it has already spiked. Open on the clock or on the forecast, rather than on how the air feels when you arrive.
- Relying on one opening. A lone door or vent lets air stall at the far end. Two openings at opposite ends turn a stagnant box into a house that breathes.
- Sealing everything on a warm, humid night. That habit invites condensation and grey mould. A slightly open vent overnight in mild weather usually stays the healthier choice.
- Shutting the house tight in strong wind. A sealed greenhouse catches more lift than one with a vent cracked to relieve the pressure, which counts on exposed plots. Our guide to greenhouses for windy, exposed sites covers that in detail.
- Trusting an auto-opener as your only vent. The wax lags a fast temperature rise, so a manual opening beside it covers the gap.
Frequently asked questions
How much ventilation does a greenhouse need?
Aim for roof-vent area of about 15–20% of the floor area, tracking the RHS figure of one square metre of vent per five square metres of floor. Then add a low or far-end opening so air crosses the house instead of lifting straight out of a single hole. Small houses heat fastest and gain most from a generous roof vent plus a second opening.
Should I leave the greenhouse door open or closed?
Open it on warm, sunny days, ideally paired with a rear vent or window for a through-draught, and close it on cold nights and in hard frost. Through a mild, humid spell, a vent left slightly open overnight often beats a shut house, because it stops condensation even with the door closed. Judge it on temperature and humidity, rather than on habit.
What temperature should a greenhouse be, and when do I ventilate?
Most crops stay comfortable across a wide band, very roughly 10–27 °C, and struggle once the house runs hot and airless. Start venting when the air feels warm on a sunny morning. The detail on holding temperature down, shading and damping down sits in the cooling guide above.
Do I need an automatic vent opener?
If you spend weekdays away from home, or you raise heat-sensitive seedlings that scorch fast, an automatic opener pays for itself: a wax-piston arm opens the vent around 18 °C and closes it as the air cools, all without mains power. If you stay home to work the vents by hand, it stays a convenience rather than a must. Either way, keep at least one manual opening.
Will opening the vents let pests in?
A few, yes, though moving air suits red spider mite and grey mould far less than a sealed, static house does, so the trade usually favours ventilation. If one pest keeps causing trouble, fine insect mesh over the vents and door keeps most of them out while still letting air through.
Sources & further reading
- RHS: ventilation and shading in greenhouses, the roof-vent area rule of thumb, why louvres work less well than roof vents, and how wax-cylinder openers operate.
- University of Massachusetts Amherst Extension: natural ventilation in greenhouses, vent sizing as a share of floor area and how wind and rising warm air move the air.
- Met Office: summer 2025, the warmest on record for the UK, the warming trend behind longer, hotter greenhouse summers.
- AHDB: control of grey mould (Botrytis) in unheated greenhouse crops, cultural control of grey mould through hygiene and environment.
- Michigan State University Extension: managing grey mould in greenhouse crops, why free water and high humidity drive the disease.
- University of Alaska Fairbanks Extension: pollination and fruit development in tomatoes, how humidity and air movement affect pollen transfer and fruit set.
