A greenhouse is built to trap heat. That is great in a cool spring, but it becomes a problem once summer arrives. On a still, sunny day the air inside can climb well past what most plants can handle.

The good news is that heat is easy to manage with the right setup. Ventilation, shading and a little water on the floor do most of the work. This guide explains how each one helps and how to plan for them in a home greenhouse.

Why summer heat is a problem

Plants under glass or plastic cannot escape the sun. Heat builds through the day, and humidity often climbs with it. The Royal Horticultural Society notes that damage can occur once temperatures build up, typically above 27°C. A thermometer that records the highest and lowest readings each day is the simplest way to see what is really happening inside.

Heat is only part of the story. The NSW Department of Primary Industries says ventilation is needed for both temperature and humidity management. Poor air circulation slows plant activity and can lead to humidity and disease problems. Stale air can also run low in carbon dioxide, which affects plant growth.

Roof vents do the heavy lifting

Most greenhouses in Australia are cooled with passive roof vents. These let hot air rise and escape without any power. The department explains this in its guide to ventilation in greenhouses. Its advice is written for commercial growers, but the principles apply to a backyard structure too.

Roof vents beat side vents by a wide margin. The department says roof vents can be up to five times more effective than side wall vents, depending on the design. Hot air collects up near the ridge. A vent placed up high lets it leave first.

How much vent area you need

Vent capacity is measured as a share of floor area. A greenhouse with 30% roof ventilation has 0.3 square metres of open vent for every square metre of floor. NSW DPI suggests most of Australia needs at least 25%, and up to 40% is desirable. It also says it is better to have more venting capacity than you need.

Small kit greenhouses often fall short here. The RHS points out that small greenhouses need a higher share of ridge ventilation, yet it is seldom provided. The door helps make up some of the gap. If you are shopping for a greenhouse for small areas, count the roof vents before you buy.

Automatic vent openers

Automatic openers are handy if you are out during the day. Many use wax that expands as it warms and pushes the vent open. The RHS notes they respond slowly, so doors and side vents should be there as a backup. On a hot morning, open things up early instead of waiting for the opener to catch up.

When fans make sense

Passive venting has limits on low structures. NSW DPI says it becomes truly effective above about 3.5 metres in height, and lower structures need forced cooling. Most backyard greenhouses are much lower than that. A fan can make a real difference in a compact build.

The department suggests at least 30 air changes an hour, and ideally 60 in hot Australian conditions. That is one full change of air every minute. Exhaust fans placed higher up cool better than fans set low. Pulling air along the full length of the structure stops hot pockets from lingering in the middle.

Fans do have downsides. They cost money to run and can be noisy, which may bother neighbours. The department also says fans need regular cleaning and maintenance to keep working properly. Check how efficient a fan is before you buy.

Gentle air movement matters even when the vents are open. NSW DPI recommends air movement of 0.2 to 0.7 metres per second inside the greenhouse. A small circulating fan can help where natural venting is limited. It keeps air moving around the plants instead of sitting still.

Shading to cut the heat

Ventilation works best with some shade on top. The RHS says shading is usually needed from mid spring to early autumn. Use only as much as you need, because shading also cuts the light plants use to grow. Its advice is to use the least shading that keeps temperatures below about 25 to 27°C.

There are a few ways to add shade. The RHS guide to greenhouse ventilation and shading compares the main options. Each one has its own cost and level of fuss. The list below sums them up.

• External blinds give the most cooling because they stop sunlight before it reaches the glass, but they cost more and can get in the way of vents.
• Internal blinds cool less, since the sun still passes through the glazing, but they are easier to automate.
• Shade netting is cheaper and easy to replace, and it works better fitted outside than inside.
• Shading paint goes on the outside of the glass in spring and comes off in early autumn, and it suits glass better than polycarbonate or acrylic.

Blinds and netting have a winter use too. Drawn on cold nights, they reduce heat loss. Sun loving succulents often need no shading at all. Group plants by their needs where you can.

Damping down and watering

Damping down is an old greenhouse trick. In bright, sunny weather, wet the floor, paths and other hard surfaces inside. This lowers the temperature and raises humidity. The RHS suggests doing it at least three times a day.

Plants also drink more in the heat, so watering needs to keep up. A timer and drippers take the guesswork out of hot days when you are away. The same planning that goes into an irrigation system for your yard works on a smaller scale under cover. Check the lines after each heatwave to catch blocked drippers.

Pots matter too. Small containers dry out quickly on hot days, so check them often. Moving lightweight garden pots into the shadiest corner is much easier than lugging heavy ones around.

Planning a cooler greenhouse

If you are building or buying, think about cooling from the start. Choose a design with generous roof vents and a good sized door. Leave room for a fan and a power point if the structure is low. Shade cloth fittings are easier to add before the plants go in.

Materials affect heat and how long the structure lasts. Frames, benches and flooring all play a part over many summers. Our guide to sustainable materials for your garden covers choices that hold up. A little thought now saves a lot of fuss in January.

Frequently asked questions

What temperature is too hot for a greenhouse?

The RHS says damage can occur once temperatures build up beyond about 27°C. It suggests shading that keeps the air below about 25 to 27°C. Some plants cope with more heat than others, so watch how yours respond.

Are roof vents better than side vents?

Yes, in most designs. NSW DPI says roof vents can be up to five times more effective than side wall vents. Side vents and doors still add to the airflow.

Do I need a fan in a small greenhouse?

Often, yes. NSW DPI says passive ventilation becomes truly effective above about 3.5 metres, and lower structures need forced cooling. Most home greenhouses are lower than that, so a fan is worth considering.

When should shading go up and come down?

The RHS suggests shading from mid spring to early autumn. Reduce it gradually as the weather cools and remove it once ventilation alone controls the heat. In Australia, mid spring falls in October and early autumn in March.