How much ventilation does a greenhouse need?
Why an open door on its own does almost nothing, the roof-vent area a greenhouse should have, and the low-plus-high pairing that makes air actually move through the house.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 4 min
Short answer
You need a low opening and a high opening, both open at once. Warm air leaves through the roof vent and draws cool air in through a low louvre — that is the stack effect. Roof vent area should be roughly a fifth to a sixth of the floor area, which is far more than most amateur greenhouses come with.
Most greenhouses are sold with one roof vent and no louvre, and most owners ventilate by propping the door open. The diagram shows why that barely works: air eddies at the doorway, the hot layer stays trapped under the ridge, and the temperature at plant level hardly changes. Adding a second roof vent and a side louvre is usually the single most valuable modification you can make.
Step by step
- Understand the stack effect.Sun heats the air inside; warm air rises and leaves through a vent at the ridge; as it goes it pulls cool air in through a low opening in the side wall. That incoming air sweeps across the floor and up through the plants. Both openings are required — one alone stalls the loop.
- Check what vent area you have against the floor area.The conventional design rule is total roof vent area of roughly a fifth to a sixth of the floor area, plus side or louvre vents in addition. Measure your vents. Most 6ft by 8ft greenhouses arrive with well under half of that.
- Add a second roof vent if there is only one.Manufacturers sell retrofit roof vents for their own glazing bar profiles, and fitting one is a straightforward job. It is the highest-value upgrade available and costs a fraction of a heater.
- Add a louvre vent low in the side wall.Louvres replace a pane in the lower side and provide the low-level inlet the stack effect needs. Fit one at the opposite end of the house from the door so air is drawn along the length rather than in and out at the same point.
- Open both, and open them early.On a sunny spring morning a closed greenhouse can go from cold to scorching within an hour. Open before the sun is on the glass rather than after, and close in the late afternoon so some warmth is retained overnight.
- Add air movement when the stack effect stalls.In a still, very hot spell the buoyancy loop weakens. A small circulating fan, or simply opening the door as well as the vents, restores airflow. Air movement also directly suppresses botrytis and mildew.
- Keep ventilating in winter.Cracking a vent and a louvre open on a mild winter day changes the air and dries the foliage, which is what prevents grey mould. A sealed, lined greenhouse in January loses more plants to botrytis than the cold ever takes.
- Fit automatic openers if you are out during the day.Nobody is at home to open vents at nine in the morning and close them at five. Wax-cylinder openers need no power and cost little, and they are what makes a greenhouse survivable for a working household.
Tips
- Insect mesh over louvres and vents keeps whitefly and butterflies out but reduces airflow substantially. If you fit it, add more vent area to compensate.
- Vents that bind in their frames are effectively closed. Clean and lubricate the hinges and stays each spring — a stiff vent also stalls an automatic opener completely.
- In an exposed site, fit vent stays or restraints. An open roof vent in a gale is a large sail and it will take the glazing with it.
Common mistakes
- Ventilating by leaving the door open — It provides a single opening at one height. Air eddies at the doorway and the hot layer under the ridge is untouched. The door helps, but only alongside proper high and low vents.
- Opening the roof vent and leaving the louvre shut — The stack effect needs an inlet as well as an outlet. With only a high opening, air leaves slowly and is not replaced, and the flow through the plants never establishes.
- Shutting everything up for the winter — Still, damp air is what grey mould needs. Ventilate on every mild day, even in December.
Questions people ask
How many roof vents should a 6ft by 8ft greenhouse have?
By the fifth-to-a-sixth-of-floor-area rule, more than the one it is usually supplied with. Two roof vents plus at least one louvre is a realistic target for that size and transforms how it performs in summer.
Should I leave vents open at night?
In summer, yes, on warm nights — it keeps humidity down and reduces disease. In spring and autumn, close them in the late afternoon to keep the day's warmth. In winter, closed except for brief airing on mild days.