Growing in a year-round greenhouse often means contending with heat. For most crops, photosynthesis slows when temperatures exceed 95 F. As the dog-days of summer hit, we explore the most energy-efficient ways of cooling a greenhouse.
1. Keep heat out (Shade Cloth)
Best for: all climates
The first is to reduce light and heat gain to only what is necessary for growth. While light requirements vary by crop, most commercial greenhouse growers shoot for a Daily Light Integral (a measure light useful for growth) of 12 – 20.
The Daily Light Integral maps from Purdue University show outdoor light levels across the US. Keep in mind that siting and shading factors will significantly reduce light inside the greenhouse. If you already have a greenhouse, you can more accurately measure indoor light levels with a light meter, like the DLI Scout 100.
Once you know target light levels, you can select a shade fabric. Fabrics range from 10 to 100% shading factor. 100% shade cloth is also known as a black-out or light deprivation fabric, commonly used in commercial cannabis greenhouses. Ensure that your shade system is automated, or easily removable so it can be taken down in the winter. We recommend using UV-stabilized fabrics, which increase longevity.
For best results, install the shade cloth on the exterior of the greenhouse roof, to keep the heat out of the greenhouse. Shade cloth under the roof still helps, but is less effective because the heat has already entered the structure. In places with high winds, expect to replace your shade cloth every few years.
Best for: all climates, except extremely hot
Ventilating the greenhouse is the easiest and cheapest way to cool a greenhouse, and should be used whenever outdoor air temperatures allow. Methods can be active (using fans) or passive.
Passive ventilation requires operable vents (both intake and outtake vents). Passive vents don’t require electricity, and are an excellent tactic for efficient cooling, but one that must be designed into the greenhouse when you choose the structure.
Active ventilation uses exhaust fans and intake vents. A qualified greenhouse designer can help customize fans for your cooling load. A common rule of thumb is to size fans they can exchange the entire volume of greenhouse air at least once per minute, though this varies by climate.
Ventilation should be used when possible. In very hot climates this may only be at night, necessitating additional cooling methods.
3. Use Evaporation (Misters and Wet Walls)
Best for: dry climates
The cooling effect of evaporation has been used for millennia. As water changes phases from a liquid to vapor, it absorbs heat from the air. Evaporative coolers, misting systems and wet walls take advantage of this process to efficiently cool greenhouses.
Wet walls are the most common systems in commercial greenhouses. Large fans blow air over a wet pad on one wall of the greenhouse. Residential greenhouses can employ the same methods using use portable evaporative coolers often called swamp coolers. Misting systems cool the air by spraying a light mist into the greenhouse which quickly evaporates.
In all cases, these systems can cool by many degrees using a small amount of energy. Their primary limitation is that they only work well in dry climates. Their performance decreases when there is more water vapor in the outdoor air (something you can see in a performance charts of evaporative coolers). However, technology is expanding to enable wet walls systems for humid climates as well. Florida growers will rejoice.