How to Cool Your Greenhouse: Be a Master of Heat Transfer
Plants need sunlight to grow, but it will turn your greenhouse into a solar oven if you’re not careful. Here’s how to deal.
Earlier this year, I had a vivid revelation. As I laid in bed all day on a Monday in early March, exhausted from my weekend gardening binge, I realized something profound: It wasn’t cold outside anymore!
Flowers were in full bloom, spinach was bolting, my greenhouse had hit 100°F (38°C) the week before with the fan running, and the water in my aquaponic system threatened to break 70°F (21°C).
What do we do with our cold-climate greenhouse growing systems in times like these? We’ve got to think about how to cool them as it gets hotter outside.
Four Ways to Cool Down a Greenhouse
On Earth, heat is transferred in four ways: conduction, convection, mass transfer, and radiation. These are the theoretical principles we use to remove heat from our greenhouses, a.k.a. cool them down. The following is a brief discussion of each method.
Conduction: In the conduction method of heat transfer, heat moves through solid surfaces, or moves between them. To experiment with conduction, go find a friend, 6 inches of bare metal wire and a fire. Next, each of you grabs an end of the wire, holding the middle over the fire. Whoever lets go first pays for the beer.
In this example, heat travels through the wire, then between the wire and your finger. The greater the difference in temperature in or between the surfaces, the greater the heat transfer. Also, some materials conduct heat better than others. Most people don’t use conduction to cool greenhouses because it would take too much wire.
Convection: In the convection method of heat transfer, fluids absorb heat from a solid. To see for yourself, stick a fan next to your electric stove. Turn the stove on high and the fan on low. If the air feels warm (air is a fluid), you’ve experienced convection, and created a fire hazard.
The greater the difference in temperature between the solid and the fluid, the more heat will transfer. Also, the greater the surface area, the greater the heat transfer. Greenhouse cooling maximizes convection.
Mass Transfer: Mass transfer pretty much speaks for itself. To give it a try, when you get back from some hard exercise, take off your clothes and put them in the freezer. Guess what? You’re now naked. Now drink some cold water. The clothes you took off were hot, while the water you drank was cold.
Replacing your hot clothes with cold water cools you off. Now put your frozen clothes on. That’s more mass transfer cooling-off. The more clothes you take off and the more water you drink, the more heat transfers.
If your spouse finds your clothes, tell them you were doing a science experiment! Mass transfer is used to cool greenhouses by taking in cooler outside air and expelling hot greenhouse air. In aquaponics, we do it by adding ice cubes to our water, or by changing out the water periodically.
Radiation: Radiation is a bit trickier, as it involves transferring heat without any touching involved. To see what I mean, start a fire in your fireplace. Put on a blindfold, stand in the middle of the room and spin around until you’re totally lost. Point to the fireplace and take off your blindfold. How did you know?
It’s not because the air was warmer in that direction. I mean, it was, but not by much. It’s because the fire radiated heat particles at you. In fact, everyone and everything radiates heat particles back and forth all the time. FYI, telling someone, “Hey, you, over there, I just radiated some heat particles at you,” is not a good pick-up line.
Radiation is a big deal. The average surface temperature of a room makes a bigger difference in how warm or cold you feel than the temperature of the air does. It’s the same reason we feel warm in sunlight and cool in shade. In greenhouse cooling, radiation is your enemy. Plants need it to grow, but it’ll turn your greenhouse into a solar oven. Here’s how to best handle it.
Putting it all Together: Controlling Greenhouse Temperatures
In cold climates, we put up greenhouses to trap radiation. The plastic covering allows radiant heat from the sun to pass through, heating up the air molecules and objects inside the greenhouse while evaporating water. At nighttime in the winter, the warm objects release this energy and the water condenses, keeping the greenhouse warm enough for the plants inside to survive.
In the summer, a greenhouse still traps radiant heat from the sun, storing it in the air, in objects, and in evaporating water. But in the summer, this is the problem rather than the goal. With my greenhouse still sealed up for winter in March, a string of warm, sunny days drove the temperature to 140°F (60°C).
This killed my tomatoes and all my other plants except basil and hot peppers, which thrived. Radiation is powerful. Because of this, controlling radiation serves as our first line of defense against an overheating greenhouse. To control radiation, we must aim to:
- Keep it out of the greenhouse altogether
- Keep it off the things we want to stay cool
- Make sure it bounces back up to the sky
Keep It Out: Most commercial greenhouses make use of shade cloth in the summer. Shade cloth comes in knitted and woven varieties, and blocks 30-90 per cent of sunlight. If you grow only ferns and moss, go for the 90 per cent option.
Most other plants need sunlight, so you should choose the 40-50 per cent blockage option for most greenhouse applications. I use 40 per cent because my greenhouse doesn’t have full sun to begin with. White shade cloth works best, though I mostly see black being used, so perhaps the color doesn’t matter all that much.
Keep It Off: Now that you’re blocking 40-50 per cent of the sun’s radiation, you’re well on your way to having an effective greenhouse cooling strategy. For that remaining 50-60 per cent of sunlight, our next goal is to keep the sun from heating up the things we want to keep cool, including plants, reservoirs, and grow beds.
We can protect reservoirs and grow beds simply by covering them up. Many folks leave their nutrient reservoirs and grow beds open in the summer, which is a bad idea because the grow media, walls and water will absorb sunlight and heat up your system. Rigid foam insulation works well over grow beds, just cut some holes in it for your plants.
Leave it there in winter for its insulating value. Note that covering your grow beds and reservoirs also prevents evaporation—another strategy for keeping cool. Unfortunately, you can’t do much to keep the heat off of your plants. I would not recommend painting them white. Why not? No good reason, just heard it doesn’t work well. Maybe it affects their taste or something.
Other important items to keep in mind include objects with the potential to store heat. For example, those water-filled, black-metal barrels designed to absorb heat in the winter will continue to absorb heat in the summer.
Bounce It Back: Covering your system’s components helps prevent heat gain, but covering them with radiant-barrier types of materials is even better. A radiant barrier basically consists of anything that reflects radiation, such as foil, reflective paint (usually light-colored) and the moon. A sheet of reflective Mylar or foil-faced insulation works especially well as a radiant barrier. These solutions also work well for covering reservoirs and grow beds, as discussed earlier.
The bare earth, or your cement floor, performs a lot of thermal mass storage and can also be covered. With a radiant barrier covering your thermal mass storage, the radiation particles that enter your greenhouse magically reflect back up to the sky, freeing up your thermal mass storage to store coolness from the nighttime and release it during a hot day.
To manage radiation in your greenhouse this summer, remember these three rules: keep it out, keep it off, and bounce it back.