The Complete Handbook of Greenhouses

Chapter 1: Understanding Greenhouses

A greenhouse is a structure designed to trap heat and manage light while protecting plants from weather, pests, and uncontrolled temperature fluctuations. It is not primarily a shelter or a decoration. Its function is to create and sustain a controlled environment—one in which you deliberately adjust temperature, humidity, light, and air movement to produce plants faster, longer, or out of season than would be possible outdoors. Understanding what a greenhouse actually does, and how its core systems work together, is the foundation for every decision that follows: what type to buy, how to install it, how to operate it, and how to fix it when something fails.

A greenhouse works by letting light in while trapping heat and water vapor inside. Sunlight passes through the glazing (the transparent or translucent covering—glass, polycarbonate, or plastic film). That light warms the soil, plants, benches, and structural materials inside. The warm surfaces then emit infrared radiation (heat), which cannot easily escape through the glazing because the glazing is largely opaque to infrared wavelengths. The glass or plastic acts as a one-way barrier: visible light gets in, but radiant heat gets trapped. This is the greenhouse effect, named after the structure itself. On a sunny day, even in winter, the interior can become significantly warmer than the outside air—sometimes 20 to 40 degrees Fahrenheit warmer, depending on glazing type, mass inside, and outside wind speed. On a cold night with no sun, the trapped heat radiates away through the glazing