Let warm air escape high up
Open roof vents as the greenhouse warms and use the door or lower openings, where available, to let fresh air in. Keep plants and shelving clear of the openings so air can move through the space.
Watch the temperature at plant height
Place a thermometer near the plants, out of direct sun. Check it at different times on a warm day to learn how quickly your greenhouse heats up. The temperature outside alone won’t tell you what your plants experience inside.
Consider automatic vents and shade
A compatible automatic vent opener can help if you’re away during the day. Follow the opener and greenhouse instructions. If the greenhouse still gets too warm, consider shade cloth that suits the crops you grow.
Build a path for air through the greenhouse
Roof vents let warm air escape. An open door or side vent can provide an inlet when weather and wind conditions allow. Place a thermometer at plant height out of direct sunlight and compare temperatures in the morning and afternoon to see whether the current routine is enough.
Combine ventilation with shade
An automatic window opener can help when you are away. Confirm compatibility with your roof vent and check that the window opens freely. If the greenhouse still heats up quickly, shade the most exposed area and leave the vents clear. Check container moisture more often during hot spells.
Measure the heat before changing your setup
Leave a thermometer at plant height in a shaded position where air can circulate around it. A reading taken in direct sun can exaggerate the air temperature and make it hard to compare one afternoon with another. If you have a maximum and minimum thermometer, reset it in the morning and check it in the evening. Note which vents were open and whether the door was closed. After a few sunny days, you will have a clearer picture of when the greenhouse heats up and whether it cools again before nightfall.
Use roof vents and lower openings together
Warm air collects high in a greenhouse, so roof windows are a useful place for it to escape. A door or low vent can let outside air enter to replace it. If the door is the only low opening, check whether it can be safely secured in its open position. Follow the greenhouse instructions in windy weather. A vent that works well in calm conditions can be vulnerable to sudden gusts.
Make automatic vent openers part of a routine
If you are away when the sun comes out, an automatic greenhouse window opener can start opening a compatible roof vent as the temperature rises. Before relying on it, watch a complete opening and closing cycle. Check that the mechanism moves freely and that a hanging basket, plant support or shade screen does not obstruct the window. An automatic opener responds to temperature around the mechanism, so compare what it does with the readings near your crops.
When does a greenhouse need shade?
If the temperature keeps climbing while the vents are already open, observe where direct sunlight falls at the hottest time. Start by shading that area. Select shading suitable for your structure and keep the ventilation openings clear. A removable screen gives you flexibility on cloudy days and in early spring, when crops may benefit from the available light. See our guide to greenhouse glazing if you are comparing permanent options for a new greenhouse.
Adjust watering and plant spacing
Check the growing medium in pots near the door and along the sunniest wall. A small pot can dry out while a larger container still has plenty of moisture. Water according to what you find at root level, and leave space between crowded plants so air can move through the foliage. Move trays out of the path of opening windows as the plants get taller.
On your next warm day, record the morning temperature, open the vents early and check the afternoon maximum. Repeat after adjusting one thing, such as opening a low vent or adding shade. Changing one variable at a time makes it easier to tell which step helped.