Greenhouse Ventilation: Roof Vents and Door Opening

Illustration of staging and potted crops inside a greenhouse

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A greenhouse can overheat quickly on a sunny day, and the two most accessible openings – roof vents and the door – are not interchangeable. Roof vents release the hottest air that collects at the ridge, while the door admits cooler outside air at low level. Balancing the two is about using each for what it does best, and adjusting the balance as the day warms and cools.

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Why roof vents and doors behave differently

Hot air rises, so roof vents are the most effective way to remove excess heat from a greenhouse. RHS guidance states that roof vents are the most useful form of ventilation and should ideally be on both sides of the ridge, equivalent to 15–20 per cent of the floor area. For larger glasshouses, one square metre of ridge ventilation for each five square metres of floor area is a general ventilation-design guide; actual air exchange depends on temperature, wind and opening geometry.

The door, by contrast, is a low-level opening. It lets cooler outside air in, but on its own it does not remove the hot air trapped at the top of the structure. Air movement over leaf surfaces has a cooling effect, so a door can help cool plants near the doorway, but it is not a substitute for roof ventilation. Side vents, if present, are less effective than roof vents.

A practical order for opening up

Ventilate early on a sunny morning, using the roof vents and low-level openings together as needed. Do not wait for the greenhouse to become hot before opening the door if the plants and weather permit it. RHS advice is to open all doors and vents on sunny days, and they can be left open at night if the temperature remains high.

Use a maximum-minimum thermometer to monitor conditions. If temperatures are allowed to build up – typically above 27°C – plant damage can occur. In changeable weather, vents and doors often have to be left partially open to limit sudden increases in temperature. This partial opening is a balance: enough airflow to prevent overheating, but not so much that a cold gust chills plants or the greenhouse loses too much warmth on a cool, breezy day.

Small greenhouses: the door’s compensating role

Smaller greenhouses have a higher glass-to-floor-area ratio, so they should ideally have an even higher percentage of ridge ventilation than larger ones. In practice, many amateur greenhouses do not, which makes them vulnerable to overheating. The door offers an alternative means of ventilation that partially compensates for lack of roof ventilation. This means in a small greenhouse, the door is not just an access point; it is a significant part of the ventilation system.

However, because the door is at low level, it works best when combined with at least one roof vent. Opening the door alone may cool the lower part of the greenhouse but leave hot air trapped at the ridge. If your greenhouse has no roof vents or very few, consider adding side louvres or fitting an automatic opener to an existing roof vent, but do not rely on the door alone on hot days.

Automatic vent openers and the door

Automatic openers that work by wax expansion are useful because they open roof vents even when you are not around. But they are slow to respond: it takes time for the wax to expand. RHS advice is to provide plenty of alternative ventilation – doors and side vents – to prevent damaging temperatures occurring before the openers respond. Provide enough alternative ventilation for the plants and weather. This may mean opening the door in the morning; do not assume a single automatic vent can handle a hot sunny day.

A practical routine is to open the door and any manual vents when you first check the greenhouse, then let the automatic opener fine-tune the roof vent as the day warms. In changeable weather, leave the door partially open so that a sudden sunny spell does not cause a damaging spike before the wax opener reacts.

Wind, draughts and night-time balance

Wind direction and strength affect how well the door and roof vents work together. On a still, hot day, the door may need to be wide open to encourage air movement, but on a breezy day, a partially open door can create a strong draught at plant level. Because the door admits air low down, it can cool leaves and pots near the doorway more than those further in. If you notice sun-flag, leaf scorch or desiccation of tender young plants, check temperature, root moisture and airflow. These symptoms have several possible causes and do not by themselves show that the door is the problem.

At night, if the temperature remains high, doors and vents can be left open, but in cooler weather close the door while leaving a roof vent slightly open if you want some air exchange without chilling the floor area. This is a judgement based on your local conditions and the plants you are growing.

Different crops need different conditions

Warmth-loving crops may benefit from higher humidity and less draught, but they can still overheat. Use crop-specific guidance, thermometer readings and suitable shading to adjust ventilation. Do not keep every vent closed during a heatwave simply because cucumbers or okra are present. Partitioning a greenhouse changes airflow and also needs monitoring.

Decisive checks before relying on the door

Check that the door can be secured partially open without slamming in wind.
Check that opening the door does not create a direct draught over seed trays or young plants.
Check that roof vents actually open fully and are not obstructed by shading or plants.
Check that automatic openers are fitted to the manufacturer’s instructions and that any manual adjustment or disengagement follows the maker’s instructions.
Check the maximum-minimum thermometer daily to see whether your current balance is keeping temperatures within the range suitable for your plants; around 27°C is a useful prompt to check overheating risk, not a universal damage threshold. If not, increase roof vent opening or add shading rather than simply opening the door wider, because the door cannot remove ridge heat as effectively.

Balancing roof vents and door opening is not a fixed setting; it changes with the weather, the crop and the time of day. The roof vent does the heavy lifting, the door provides the backup and low-level air movement, and a thermometer tells you whether the balance is working.

Useful products for this job

The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.

Automatic roof vent opener: Can open a compatible roof vent as temperature rises; check whether the full ventilation provision is adequate. Check the opener's maximum vent opening distance and weight capacity against your vent size and frame material; confirm its adjustment and disengagement instructions and that it is suitable for your greenhouse glazing type. Search Amazon for Automatic roof vent opener.
Maximum-minimum greenhouse thermometer: Records temperatures so ventilation and shading can be adjusted to the crop and weather. Choose a model that records daily maximum and minimum temperatures and can be placed at plant height away from direct sun; check battery type and whether it is suitable for humid greenhouse conditions. Search Amazon for Maximum-minimum greenhouse thermometer.
Greenhouse side louvre vent: Side louvres provide additional low-level ventilation that can work with the door and roof vents, though they are less effective than roof vents. Check the louvre dimensions against your greenhouse frame and glazing; ensure it can be closed and draught-proofed in winter, and that installation does not compromise the structure. Search Amazon for Greenhouse side louvre vent.

Related garden guides

Sources and further guidance

Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.