Cool Down Your Greenhouse This Summer

On a sunny winter day, your greenhouse can feel like a magical refuge inside, where the signs of life provide a welcome diversion from the cold weather ouside.  

On a bright, summer day, your greenhouse can feel like an overcrowded sauna on the planet Mercury.

By design, your greenhouse allows sunlight to pass through and then retains that energy inside in the form of heat. This is what it is supposed to do, of course. But your greenhouse doesn’t have to be demoted to a tool shed just because it’s summertime. With thoughtful planning, and a few simple steps, you can cool your greenhouse’s environment so that it’s productive all year long. 

Shading Your Greenhouse
An excellent way to shade your greenhouse in the summer months is to site it where it will lie in the shade of a deciduous tree for part, or even most of the day - preferably during the afternoon, when daytime temperatures peak. This isn’t always possible of course, and many a hardworking greenhouse only dreams about the luxury of a shade tree while earning its keep out in an open field. If you are able to site your greenhouse near a shade tree, remember that your plants will still require a minimum of four hours of sunlight. 

Polyethylene shade cloth is another effective, low cost way to cool your greenhouse environment, and is sold in densities that block anywhere from 25% to 70% or more of light transmission. Greenhouse growers in warmer climates will almost always benefit from shade cloth designed to block more light. While shade cloth can be secured to either the inside or the outside of the greenhouse glazing, it cools the greenhouse environment more effectively when placed on the outside where it can prevent heat from passing inside the glazing. Some professionals even position the shade cloth just above the glazing, with a small space of a few inches between the cloth and exterior of the greenhouse. This design allows air to flow between, and help vent away heat. Remember to securely fasten exterior mounted  shade cloth so that it stays in place during extreme weather.

Ventilating Your Greenhouse
While ventilation systems vary, the idea is the same : replace hot air leaving the greenhouse with new air from outside. Because heat rises, exhaust vents are almost always placed at the apex of the greenhouse roof, or high on an end wall, where trapped heat can easily escape and rise into the atmosphere.

Intake vents are best placed near the floor, which allows fresh air from outside to pass over plants, helping to prevent fungal diseases and mildew from damaging the plants. Size recommendations vary, but at a minimum, exhaust vents should be at least 10% of the Greenhouse’s floor area. 20% or even 30% is better.

Remember to screen vents placed near the floor with hardware cloth to keep out wildlife. Exhaust vents can be fitted with automated solar openers that open ad close vents based on the temperature inside the greenhouse. In winter it may be necessary to remove these to avoid venting supplemental heat.

Evaporative Cooling

Evaporative cooling systems use the heat in the air to evaporate water from leaves and other wetted surfaces, and can cool the greenhouse as much as 10-20° F below the temperature outside the greenhouse. Portable, self contained evaporative cooling units also called swamp units, are common used with smaller greenhouses.

The pad and fan evaporative cooling system has been used by commercial greenhouse growers for decades, and relies on a system that circulates water through a network of cellulose pads. Air drawn through the wetted pads by an intake fan placed at the opposite end wall, becomes saturated, and cools the air inside the greenhouse.